Umzch high fidelity nikolai sukhova 1989 reviews. Umzch vv with microcontroller control system
UMZCH VVS-2011 version Ultimate
Amplifier Specifications:
Large power: 150W / 8 ohm
High linearity: 0.0002 - 0.0003% (at 20 kHz 100 W / 4 ohms)
Complete set of service nodes:
Maintaining zero constant voltage
AC wire resistance compensator
Overcurrent protection
Output DC voltage protection
Smooth start
Electrical diagram
A participant in many popular projects LepekhinV (Vladimir Lepekhin) was involved in the layout of printed circuit boards. It turned out very well).
VVS-2011 amplifier board
Start-up protective device
VVS-2011 AC amplifier protection board
The board of the VLF VVS-2011 amplifier was developed for tunnel blowing (parallel to the radiator). Installation of transistors UN (voltage amplifier) \u200b\u200band VC (output stage) is somewhat difficult, because assembly / disassembly has to be done with a screwdriver through holes in the PCB with a diameter of about 6 mm. When access is open, the projection of the transistors does not fall under the PCB, it is much more convenient. I had to modify the board a little.
Amplifier board
VVS-2011 amplifier wiring diagram
In the new PCB, I did not take into account one point - this is the convenience of setting the protection on the amplifier board
C25 \u003d 0.1 nF, R42 * \u003d 820 ohms and R41 \u003d 1 kΩ. All elements of the smd are located on the soldering side, which is very inconvenient when setting up, because it will be necessary to unscrew and fasten the PP fastening bolts on the racks and transistors to the radiators several times.
Sentence: R42 * 820 Ohm consists of two SMD resistors located in parallel, from here a suggestion: we solder one SMD resistor immediately, the other output resistor is soldered with a canopy to VT10, one terminal to the base, the other to the emitter, we select to a suitable one. We picked it up, we change the output to smd, for clarity.
The high fidelity audio frequency power amplifier (UMZCH), developed in 1989 by Nikolai Sukhov, can rightfully be called legendary. It was developed using a professional approach based on knowledge and experience in the field of analog circuitry. As a result, the parameters of this amplifier turned out to be so high that even today this design has not lost its relevance. This article describes a slightly improved version of the amplifier. Improvements come down to the use of a new element base and the use of a microcontroller control system.
The power amplifier (PA) is an integral part of any sound reproducing complex. There are many descriptions of the design of such amplifiers available. But in the overwhelming majority of cases, even with very good characteristics, there is a complete lack of service amenities. But now, when microcontrollers have become widespread, it is not difficult to create a sufficiently perfect control system. At the same time, a homemade device in terms of functional saturation may not be inferior to the best branded samples. A variant of the UMZCH VV with a microcontroller control system is shown in Fig. one:
Figure: 1. Exterior of the amplifier.
The original UMZCH VV circuit has sufficient parameters so that the amplifier is not a dominant source of nonlinearity of the sound reproducing path in the entire range of output powers. Therefore, further improvement of the characteristics no longer gives noticeable advantages.
At least, the sound quality of different phonograms differs much more than the sound quality of amplifiers. On this topic, you can quote from the magazine "Audio": " There are obvious aural differences in categories such as speakers, microphones, LP pickups, listening rooms, studio spaces, concert halls, and especially the studio and recording equipment configurations used by various recording companies. If you want to hear subtle differences in the soundstage, compare John Eargle's recordings on Delos with Jack Renner's recordings on Telarc, not preamps. Or if you want to hear subtle differences in transitions, compare Studio dmp's jazz recordings to Chesky's jazz recordings, not two interconnects.»
Despite this fact, Hi-End lovers do not stop searching for the "right" sound, which affects, among other things, the UM. In fact, the PA is an example of a very simple linear path. The current level of development of circuitry makes it possible to provide sufficiently high parameters for such a device so that the introduced distortions become invisible. Therefore, in practice, any two modern, non-eccentrically designed PAs sound the same. On the contrary, if the PA has some special, specific sound, it says only one thing: the distortions introduced by such a PA are great and well noticeable.
This does not mean that it is very easy to design a high-quality PA. There are many subtleties, both schematic and constructive plan. But all these subtleties have long been known to serious manufacturers of PAs, and gross errors in the designs of modern PAs usually do not occur. Exceptions are expensive Hi-End amplifiers, which are often very illiterate designed. Even if the distortions introduced by the PA are pleasing to the ear (as fans of tube amplifiers claim), this has nothing to do with high fidelity of sound reproduction.
In addition to the traditional requirements for broadband and good linearity, a high-quality PA has a number of additional requirements. Sometimes you can hear that a 20-35 W amplifier power is sufficient for home use. If we are talking about average power, then this statement is true. But a real music signal can have a peak power level of 10 to 20 times the average. Therefore, in order to obtain an undistorted reproduction of such a signal at an average power of 20 W, it is necessary to have a PA power of about 200 W. For example, here is the conclusion of the expert judgment for the amplifier described in: “ The only caveat was the insufficient volume of the sound of large percussion instruments, which is explained by the insufficient output power of the amplifier (120 watts peak at a load of 4 ohms).»
Loudspeaker systems (AC) are a complex load and have a very complex impedance versus frequency. At some frequencies, it may be 3-4 times less than the nominal value. The amplifier should be able to work without distortion for such a low-impedance load. For example, if the nominal impedance of the speaker system is 4 ohms, then the PA should normally operate on a 1 ohm load. This requires very high output currents, which must be taken into account when designing a PA. The described amplifier meets these requirements.
Recently, the topic of the optimal output impedance of an amplifier from the point of view of minimizing speaker distortion has been discussed quite often. However, this topic is relevant only when designing active speakers. Passive speaker crossovers are designed with negligible output impedance from the signal source. If the PA has a high output impedance, then the frequency response of such speakers will be greatly distorted. Therefore, nothing else remains but to provide a low output impedance for the PA.
It can be noted that the new developments of the AM are mainly on the path of reducing the cost, improving the manufacturability of the design, increasing the output power, increasing the efficiency, and improving the consumer qualities. This article focuses on the service functions that are implemented thanks to the microcontroller control system.
The amplifier is made in a MIDI format, its dimensions are 348x180x270 mm, and its weight is about 20 kg. The built-in microcontroller allows you to control the amplifier using an IR remote control (common with the pre-amplifier). In addition, the microcontroller measures and displays the average and quasi-peak output power, heatsink temperatures, implements a timer shutdown and handles emergency situations. The amplifier protection system, as well as the power on and off control, are implemented with the participation of a microcontroller. The amplifier has a separate standby power supply, which allows it to be in "STANDBY" mode when the main power supplies are turned off.
The described amplifier is called NSM (National Sound Machines), model PA-9000, since the name of the device is part of its design and must be present. The implemented set of service functions in some cases may turn out to be redundant, for such situations a "minimalist" version of the amplifier (model PA-2020) has been developed, which has only a power switch and a two-color LED on the front panel, and the built-in microcontroller only controls the process of turning on and off the power. complements the protection system and provides remote control of the "STANDBY" mode.
All controls and indication of the amplifier are located on the front panel. Its appearance and the purpose of the controls are shown in Fig. 2:
Figure: 2. Front panel of the amplifier.
| 1 - LED for switching on external consumers EXT | 9 - "minus" button |
| 2 - LED for switching on the standby power supply DUTY | 10 - button for indication of peak power PEAK |
| 3 - button for switching to standby mode STANDBY | 11 - TIMER indication button |
| 4 - button for complete power off POWER | 12 - temperature indication button° C |
| 5 - LED for turning on the main power supply MAIN | 13 - plus button |
| 6 - LED of normal operating mode OPERATE | 14 - LED of the alarm of the left channel FAIL L |
| 7 - LED for switching on the load LOAD | 15 - LED fault of the right channel FAIL R |
| 8 - display |
POWER button provides complete disconnection of the amplifier from the network. Physically, this button disconnects from the network only the standby power supply, so it can be designed for a small current. The main power supplies are switched on using a relay, the windings of which are powered from a standby source. Therefore, when the POWER button is off, all amplifier circuits are guaranteed to be de-energized.
When the POWER button is turned on, the amplifier turns on completely. The switch-on process is as follows: the standby source is switched on immediately, as evidenced by the "DUTY" standby power-on LED. After a while, which is required to reset the microcontroller, power is turned on to external sockets and the "EXT" LED is lit. Then the "MAIN" LED lights up, and the first stage of turning on the main sources takes place. Initially, the main transformers are connected through limiting resistors, which prevent the initial inrush current due to discharged filter capacitors. The capacitors are gradually charged, and when the measured supply voltage reaches the set threshold, the limiting resistors are removed from the circuit. The OPERATE LED lights up. If during the allotted time the supply voltage has not reached the set threshold, then the process of turning on the amplifier is interrupted and the alarm indication turns on. If the switching on of the main sources was successful, then the microcontroller checks the status of the protection system. In the absence of emergency situations, the microcontroller allows the load relay to be turned on and the "LOAD" LED lights up.
STANDBY button manages the standby mode. A short press of the button puts the amplifier into standby mode or, conversely, turns on the amplifier. In practice, it may be necessary to turn on external sockets, leaving the PA in standby mode. This is required, for example, when listening to phonograms on stereo phones or when dubbing without sound control. External sockets can be independently switched on / off by long (until beep) pressing the "STANDBY" button. The option when the PA is turned on and the sockets are turned off does not make sense, therefore it is not implemented.
The front panel contains a 4-digit digital display and 5 display control buttons. The display can operate in the following modes (fig.3a):
- disabled
- indication of average output power [W]
- quasi-peak output power indication
- timer status indication [M]
- display of temperature of radiators [° C]
Figure: 3. Display options.
PEAK button turns on the display of output power and switches between average / quasi-peak power modes. In the output power indication mode, the display lights up "W", and for quasi-peak power - also "PEAK". The output power is indicated in watts with a resolution of 0.1 watts. The measurement is carried out by multiplying the current and voltage across the load, therefore the readings are valid for any permissible value of the load resistance. Holding down the PEAK button until the beep turns off the display. Turning off the display, as well as switching it between different display modes, occurs smoothly (one image "flows" into another). This effect is implemented in software.
TIMER button displays the current status of the timer, while the letter "M" lights up. The timer allows you to set the time interval after which the amplifier goes into standby mode and external sockets are turned off. It should be noted that when using this function, other components of the complex must be able to turn off the power on the fly. For a tuner and a CD-player, this is usually acceptable, but with some cassette decks, when the power is turned off, the CVL may not go into the "STOP" mode. For these decks, turning off the power during playback or recording is not permitted. However, such decks are extremely rare among branded devices. Conversely, most decks have a Timer switch that has 3 positions: Off, Record, and Play, allowing you to instantly turn on playback or recording mode by simply applying power. You can also turn off these modes by simply removing the power supply. The amplifier timer can be programmed for the following intervals (fig. 3b): 5, 15, 30, 45, 60, 90 and 120 minutes. If the timer is not in use, it must be set to OFF. It is in this state immediately after turning on the power.
The timer interval is set buttons "+" and "-" in timer indication mode. If the timer is on, the “TIMER” LED is always lit on the display, and turning on the timer indication shows the real current state, i.e. how many minutes are left until shutdown. In such a situation, the interval can be extended by pressing the "+" button.
"° C" button turns on the display of the temperature of the radiators, the symbol "° C" lights up. Each radiator has a separate thermometer, but the display shows the maximum temperature value. These thermometers are also used to control the fan and to thermally protect the amplifier's output transistors.
For alarm indication there are two LEDs on the front panel: "FAIL LEFT" and "FAIL RIGHT". When the protection is triggered, the corresponding LED lights up in one of the PA channels, and the letter name of the cause of the accident is indicated on the display (Fig. 3c). In this case, the amplifier goes into standby mode. The amplifier implements the following types of protection:
- overcurrent protection of the output stage
- output DC component protection
- power supply failure protection
- protection against mains voltage failure
- overheating protection of output transistors
It reacts to the constant component of the output voltage of the PA, greater than 2 V. It protects the speaker, it is also implemented in hardware. Indicated on the display as "dcF".
Reacts to a drop in the supply voltage of any arm below a specified level. A significant violation of the symmetry of the supply voltages can cause the appearance of a constant component at the output of the PA, which is dangerous for the AC. Indicated on the display as "UF".
Reacts to the loss of several periods of the mains voltage in a row. The purpose of this protection is to disconnect the load before the supply voltage drops and the transient starts. It is implemented in hardware, the microcontroller only reads its state. Indicated on the display as "prF".
overheat protection output transistors are implemented in software, it uses information from thermometers that are installed on the radiators. Indicated on the display as "tF".
UM has the ability remote control... Since there are not many control buttons required, the same remote control is used as to control the pre-amplifier. This remote control operates in the RC-5 standard and has three buttons specifically designed to operate the PA. The STANDBY button completely duplicates the analogous button on the front panel. The "DISPLAY" button allows you to switch the display mode around the ring (Fig. 3a). Holding the DISPLAY button until a beep turns off the display. The "MODE" button allows you to change the time interval of the timer (Fig. 3b), i.e. it replaces the "+" and "-" buttons.
On back panel amplifier (Fig. 4) installed sockets designed to power other components of the complex. These sockets have independent disconnection, which allows the entire complex to be de-energized from the remote control.
Figure: 4. Rear panel of the amplifier.
As noted earlier, the described amplifier is based on the UMZCH VV circuit of Nikolai Sukhov, which is described in. The basic principles of building a high fidelity UM are set out in Schematic diagram main amplifier board is shown in Fig. 5.
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Figure: 5. Schematic diagram of the main board of the amplifier.
The amplifier has been slightly modified compared to the original design. These changes are not fundamental and basically represent a transition to a newer element base.
Changed quiescent current temperature stabilization circuit... In the original design, along with the output transistors, a transistor was installed on the radiators - a temperature sensor, which set the bias voltage of the output stage. In this case, only the temperature of the output transistors was taken into account. But the temperature of the pre-terminal transistors, due to the rather high power dissipated on them, also increased significantly during operation. Due to the fact that these transistors were installed on small separate heatsinks, their temperature could fluctuate quite sharply, for example, as a result of changes in power dissipation or even due to external air currents. This led to the same sharp fluctuations in the quiescent current. And any other element of the PA can get quite hot during operation, since there are heat sources in the same case (radiators of output transistors, transformers, etc.). This also applies to the very first composite emitter follower transistors, which did not have radiators at all. As a result, the quiescent current could increase several times when the PA was heated. The solution to this problem was proposed by Alexey Belov.
Usually, for temperature stabilization of the quiescent current of the output stage of the PA, the following circuit is used (Fig.6a):
Figure: 6. Scheme of temperature stabilization of the quiescent current.
The bias voltage is applied to points A and B. It is allocated on a two-pole, which consists of a transistor VT1 and resistors R1, R2. The initial bias voltage is set by R2. Transistor VT1 is usually fixed on a radiator common with VT6, VT7. Stabilization is carried out as follows: when the transistors VT6, VT7 are heated, the base-emitter drop decreases, which, at a fixed bias voltage, leads to an increase in the quiescent current. But together with these transistors, VT1 also heats up, which causes a decrease in the voltage drop across the two-terminal, i.e. reduction of the quiescent current. The disadvantage of such a scheme is that the temperature of the transitions of the remaining transistors included in the composite emitter follower is not taken into account. To take this into account, the junction temperature of all transistors must be known. The easiest way is to make it the same. To do this, it is enough to install all the transistors included in the composite emitter follower on a common radiator. At the same time, to obtain a quiescent current that does not depend on temperature, the bias voltage of the composite emitter follower must have a temperature coefficient the same as that of six pn junctions connected in series. Approximately, we can assume that the forward voltage drop across the p-n junction decreases linearly with a K factor of approximately 2.3 mV / ° C. For a composite emitter follower, this coefficient is 6 * K. To provide such a temperature coefficient of the bias voltage is the task of a two-terminal device, which is connected between points A and B. The two-terminal device shown in Fig. 6a has a temperature coefficient equal to (1 + R2 / R1) * K. When the resistor R2 adjusts the quiescent current, the temperature coefficient also changes, which is not entirely correct. The simplest practical solution is the circuit shown in Fig. 6b. In this circuit, the temperature coefficient is (1 + R3 / R1) * K, and the initial quiescent current is set by the position of the slider of the resistor R2. The voltage drop across the resistor R2, which is shunted by the diode, can be considered almost constant. Therefore, adjusting the initial closed-circuit current does not affect the temperature coefficient. With such a circuit, when the PA is heated, the quiescent current changes by no more than 10-20%. In order for all composite emitter follower transistors to be placed on a common heat sink, they must have a package suitable for mounting on a heat sink (transistors in TO-92 packages are not suitable). Therefore, the UM uses other types of transistors, at the same time more modern ones.
In the amplifier circuit (Fig. 5), a two-terminal block for temperature stabilization of the quiescent current is shunted by a capacitor C12. This capacitor is optional, although it also does no harm. The fact is that between the bases of the transistors of the composite emitter follower, it is necessary to provide a bias voltage, which should be constant for the selected quiescent current and not depend on the amplified signal. In short, the variable component of the voltage across the two-pole, as well as across the resistors R26 and R29 (Fig. 5) should be zero. Therefore, all these elements can be bridged with capacitors. But due to the low dynamic resistance of the two-terminal network, as well as the low resistance values \u200b\u200bof these resistors, the presence of shunting capacities has a very weak effect. Therefore, these capacities are not necessary, especially since for shunting R26 and R29 their ratings should be quite large (about 1 μF and 10 μF, respectively).
Output transistors PAs are replaced by transistors KT8101A, KT8102A, which have a higher cutoff frequency of the current transfer coefficient. Powerful transistors have a rather pronounced effect of a drop in the current transfer coefficient with an increase in the collector current. This effect is extremely undesirable for the PA, since here the transistors have to work at high output currents. Modulation of the current transfer ratio leads to a significant degradation of the linearity of the amplifier output stage. To reduce the influence of this effect, the parallel connection of two transistors is used in the output stage (and this is the minimum that you can afford).
When transistors are connected in parallel, separate emitter resistors are used to reduce the influence of the spread of their parameters and equalize the operating currents. For normal operation of the overcurrent protection system, a circuit has been added to isolate the maximum voltage value on the VD9 - VD12 diodes (Fig. 5), since now it is necessary to remove the drop not from two, but from four emitter resistors.
Other transistors the composite emitter follower is KT850A, KT851A (TO-220 package) and KT940A, KT9115A (TO-126 package). In the quiescent current stabilization circuit, a composite transistor KT973A (TO-126 package) is used.
Made and replaced OU to more modern ones. The main op-amp U1 has been replaced by the AD744, which has improved performance and good linearity. Op-amp U2, which operates in the circuit for maintaining the zero potential at the output of the UMZCH, has been replaced by OP177, which has a low zero offset (no more than 15 μV). This eliminated the need for a bias trimmer. It should be noted that due to the peculiarities of the AD744 circuitry, the op-amp U2 must provide an output voltage close to the supply voltage (pin 8 of the AD744 op-amp is only two pn junctions away from pin 4 in terms of constant voltage). Therefore, not all types of precision op amps will work. As a last resort, you can use a pull-up resistor from the output of the op amp to –15 V. Op amp U3, which works in the AC lead impedance compensation circuit, has been replaced by the AD711. The parameters of this op-amp are not so critical, so a cheap op-amp with sufficient speed and fairly low zero offset was chosen.
Resistor dividers R49 - R51, R52 - R54 and R47, R48 are added to the circuit, which are used to remove current and voltage signals for the power measurement circuit.
Implementation changed earthen chains... Since every amplifier channel is now fully assembled on a single board, there is no need for multiple ground wires to be connected at a single point on the chassis. A special PCB topology provides star wiring for earth circuits. The ground star is connected with a single conductor to the common terminal of the power supply. It should be noted that this topology is only suitable with completely separate power supplies for the left and right channels.
In the original amplifier circuit, the AC feedback loop covers and relay contactsthat connect the load. This measure is taken to reduce the influence of non-linearity of contacts. However, this may cause problems with the operation of the DC component. The fact is that when the amplifier is turned on, the power is supplied before the load relay turns on. At this time, a signal may be present at the input of the PA, and the gain of the amplifier due to the broken feedback loop is very high. In this mode, the PA limits the signal, and the offset voltage compensation circuit is generally unable to maintain a zero DC component at the PA output. Therefore, even before connecting the load, it may be discovered that a constant component is present at the output of the PA, and then the protection system will work. It is very easy to eliminate this effect if you use a relay with changeover contacts.
Normally closed contacts should close the feedback loop in the same way as normally open contacts. In this case, when the relay is activated, the feedback is broken only for a very short time, during which all the relay contacts are open. During this time, the relatively inertial protection on the constant component does not have time to work. In fig. 7 shows the relay switching process taken by a digital oscilloscope. As you can see, 4 ms after the voltage is applied to the relay coil, the normally closed contacts open. After about another 3 ms, the normally open contacts close (with noticeable bounce, which lasts about 0.7 ms). Thus, the contacts are in "flight" for about 3 ms, it is at this time that the feedback will be broken.
Figure: 7. The process of switching the relay AJS13113.
Protection circuit completely revised (fig. 8). It is now located on the main board. Thus, each channel has its own independent circuit. This is somewhat redundant, but each main board is completely autonomous and is a complete mono amplifier. Some of the protective functions are carried by the microcontroller, but to increase reliability, a sufficient set of them is implemented in hardware. In principle, an amplifier board can work without a microcontroller at all. Since the PA has a separate standby power supply, the protection circuit is powered from it (+ 12V level). This makes the behavior of the protection circuit more predictable when one of the main power supplies fails.
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Figure: 8. Amplifier protection circuit.
Overcurrent protection includes a trigger assembled on transistors VT3, VT4 (Fig. 5), which turns on when the transistor VT13 is opened. VT13 receives a signal from the current sensor and opens when the current reaches the value set using the trimmer R30. The trigger turns off the current generators VT5, VT6, which turns off all transistors of the composite emitter follower. Zero voltage at the output is maintained in this mode by means of resistor R27 (Fig. 5). In addition, the state of the flip-flop is read through the VD13, R63 chain (Fig. 8), and when it turns on, a low logic level is set at the inputs of the logic element U4D. The VT24 transistor provides an open collector output for the IOF (I Out Fail) signal, which is polled by the microcontroller.
DC protection implemented on transistors VT19 - VT22 and logic elements U4B, U4A. The signal from the amplifier output through the divider R57, R59 is fed to the low-pass filter R58C23 with a cutoff frequency of about 0.1 Hz, which separates the DC component of the signal. If a constant component of positive polarity appears, then the VT19 transistor, connected according to the OE circuit, opens. It, in turn, opens the transistor VT22, and a high logic level appears at the inputs of the logic element U4B. If a constant component of negative polarity appears, then the VT21 transistor, turned on with ON, opens. This asymmetry is a necessary measure associated with the unipolar power supply of the protection circuit. In order to increase the current transfer ratio, cascode switching of transistors VT21, VT20 (OB - OK) was applied. Further, as in the first case, the VT22 transistor opens, etc. The output of the logic element U4A is connected to the transistor VT23, which provides an open collector output for the DCF (DC Fail) signal.
Mains power failure protection contains an auxiliary rectifier (fig. 13) VD1, VD2 (VD3, VD4), which has a smoothing filter with a very small time constant. If several periods of the mains voltage drop out in a row, the output voltage of the rectifier drops, and a low logic level is set at the inputs of the logic element U4C (Fig. 8).
Logic signals from the three protection circuits described above are fed to the "OR" element of U5C, at the output of which a low logic level is formed in case of operation of any of the circuits. At the same time, capacitor C24 is discharged through the VD17 diode, and a low logic level appears at the inputs of the logic element U5B (also at the output of U5A). This leads to the closing of the transistor VT27 and the disconnection of the relay K1. The R69C24 chain provides a certain minimum delay at power-up in case the microcontroller for some reason does not generate the initial delay. The VT25 transistor provides an open collector output for an OKL (OK Left) or OKR (OK Right) signal. The microcontroller can prevent the relay from turning on. For this, a VT26 transistor is installed. This capability is necessary to implement software overheating protection, software relay turn-on delay, and to synchronize the operation of left and right channel protection systems.
Interaction of the microcontroller with the hardware protection circuit the following: when the amplifier is turned on, after the supply voltage has reached the nominal value, the microcontroller polls the OKL and OKR hardware protection readiness signals. All this time, the switching on of the relay is prohibited by the microcontroller by maintaining the ENB (Enable) signal in the state of a high logic level. As soon as the microcontroller receives ready signals, it generates a time delay and allows the relay to turn on. During the operation of the amplifier, the microcontroller constantly monitors the ready signal. In case of loss of such a signal for one of the channels, the microcontroller removes the ENB signal, thus turning off the relay in both channels. It then polls the protection status signals to identify the channel and the type of protection.
overheat protection implemented entirely in software. In case of overheating of the radiators, the microcontroller removes the ENB signal, which causes the load relay to turn off. To measure the temperature, a Dallas DS1820 thermometer is attached to each of the radiators. The protection is triggered when the radiators reach a temperature of 59.8 ° C. A little earlier, at a temperature of 55.0 ° C, a preliminary overheating message appears on the display - the temperature of the radiators is automatically displayed. The amplifier is switched on again automatically when the radiators cool down to 35.0 ° C. Switching on at a higher temperature of the radiators is possible only manually.
To improve the cooling conditions for the elements inside the amplifier housing, a small-sized fanwhich is located on the back panel. A fan with a brushless DC motor with a rated supply voltage of 12 V is used to cool the computer processor. Since the fan generates some noise that can be noticeable during pauses, a rather complex control algorithm is used. At a radiator temperature of 45.0 ° C, the fan starts working, and when the radiators cool down to 35.0 ° C, the fan turns off. If the output power is less than 2 W, the fan operation is prohibited so that its noise is not noticeable. To prevent periodic switching on and off of the fan, when the output power fluctuates near the threshold value, the software limited the minimum time for switching off the fan to 10 seconds. At a radiator temperature of 55.0 ° C and higher, the fan runs without shutdowns, since this temperature is close to emergency. If the fan turns on while the amplifier is operating, then upon entering the "STANDBY" mode, if the temperature of the radiators is higher than 35.0 ° C, the fan continues to work even at zero output power. This allows the amplifier to cool quickly.
Power supply failure protection also implemented entirely in software. The microcontroller uses an ADC to monitor the supply voltages of both amplifier channels. This voltage is supplied to the processor from the main boards through resistors R55, R56 (Fig. 8).
The main power supplies are switched on in steps. This is necessary for the reason that the load of the rectifiers is completely discharged filter capacitors, and with a sharp turn on, a strong current surge will occur. This surge poses a hazard to the rectifier diodes and may blow the fuses. Therefore, when the amplifier is turned on, relay K2 is first closed (Fig. 12), and the transformers are connected to the network through limiting resistors R1 and R2. At this time, the threshold for the measured supply voltages is programmed to be ± 38 V. If this voltage threshold is not reached within the set time, the switch-on process is interrupted. This can be the case if the current draw of the amplifier circuit is significantly increased (amplifier is damaged). In this case, the “UF” power supply failure indication turns on.
If the ± 38 V threshold is reached, then relay K3 (Fig. 12) is activated, which excludes the resistors from the primary circuits of the main transformers. Then the threshold is reduced to ± 20 V, and the microcontroller continues to monitor the supply voltages. If during the operation of the amplifier, the supply voltage drops below ± 20 V, the protection is activated and the amplifier is turned off. Lowering the threshold in normal operation is necessary so that when the supply voltage “dips” under load, a false operation of the protection does not occur.
Schematic diagram processor board is shown in Fig. 9. The processor is based on the Atmel AT89C51 U1 microcontroller, which operates at a clock frequency of 12 MHz. To increase the reliability of the system, the U2 supervisor is used, which has a built-in watchdog timer and a power monitor. To reset the watchdog timer, a separate WD line is used, on which a periodic signal is generated by the software. The program is structured in such a way that this signal will be present only if the timer interrupt handler and the main program loop are executed. Otherwise, the watchdog timer will restart the microcontroller.
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Figure: 9. Schematic diagram of the processor board.
The display is connected to the processor using an 8-bit bus (connectors XP4 - XP6). To strobe the registers of the display board, signals C0..C4 are used, which are generated by the U4 address decoder. Register U3 is the latch of the low byte of the address, only bits A0, A1, A2 are used. The high byte of the address is not used at all, which made it possible to free port P2 for other purposes.
When you press the control buttons, beeps are generated programmatically. For this, the BPR line is used, to which the VT1 transistor switch is connected, loaded on the HA1 dynamic emitter.
The main boards of the left and right channels are connected to the processor board using XP1 and XP2 connectors, respectively. These connectors provide IOF and DC protection status signals to the processor at the output of the DCF amplifier. These signals are common to the left and right channels, and their combination is possible thanks to the outputs of the open collector protection circuit. The protection readiness signals OKL and OKR are separate per channel so that the processor can identify the channel in which the protection scheme has been triggered. The ENB signal, which is sent from the processor to the protection system, allows the load relay to turn on. This signal is common for the two channels, which automatically synchronizes the operation of the two relays.
The TRR and TRL lines are used to read the thermometers installed on the right and left channel radiators, respectively. The temperature measured by thermometers can be indicated on the display if the corresponding display mode is on. The maximum temperature value of the two for the left and right channels is displayed. The measured value is also used for software implementation of overheating protection.
Additionally, XP1 and XP2 have WUR, WIR, WUL and WIL signals, which are used by the output power measurement circuit.
The processor board is powered from the standby source through the XP3 connector. For power supply, 4 levels are used: ± 15 V, +12 V and +5 V. The levels of ± 15 V are disabled when switching to standby mode, and the remaining levels are always present. Consumption from levels +5 V and +12 V in standby mode is minimized due to software shutdown of main consumers. In addition, several control logic signals are sent to the standby power supply through this connector: PEN - controls the standby power supply, REX - turns on the relay of external sockets, RP1 and RP2 - turn on the main power supply relay, FAN - turns on the fan. The protection circuits located on the main boards are powered from the processor board at the +12 V level, and the display board is powered by the +5 V level.
A 12-bit U6 AD7896 ADC from Analog Devices is used to measure the output power and to monitor the supply voltages. One ADC channel is not enough, therefore the U5 switch is used at the input (it would be even better to use an 8-channel ADC, for example, of the AD7888 type). Data is read from the ADC in serial form. To do this, the SDATA (serial data) and SCLK (clock) lines are used. The conversion process is started by the software signal START. REF195 (U7) is used as a reference source and at the same time a voltage regulator for the ADC. Since the ± 15 V supply voltage is disconnected in standby mode, all logic signals are connected to the ADC through resistors R9 - R11, which limit possible current surges when switching to standby mode and back.
Of the eight switch inputs, six are used: two for power measurements, and four for monitoring supply voltages. The desired channel is selected using the address lines AX0, AX1, AX2.
Consider power measurement circuit left channel. The applied circuit provides a multiplication of the load current and voltage, therefore the load impedance is automatically taken into account and the readings always correspond to the real active power in the load. Through resistor dividers R49 - R54, located on the main board (Fig. 5), the voltage from the current sensors (emitter resistors of the output transistors) goes to the differential amplifier U8A (Fig. 9), which separates the current signal. From the output of U8A through the trimmer resistor R17, the signal is fed to the Y input of the analog multiplier U9 of the K525PS2 type. The voltage signal is simply removed from the divider and fed to the X input of the analog multiplier. At the output of the multiplier, a low-pass filter R18C13 is installed, which selects a signal proportional to the quasi-peak output power with an integration time of about 10 ms. This signal goes to one of the switch inputs, then to the ADC. Diode VD1 protects the switch input from negative voltage.
In order to compensate for the initial zero offset of the multipliers, when the amplifier is turned on (when the load relay is not yet turned on, and the output power is zero), an auto-calibration process takes place. The measured offset voltage during further operation is subtracted from the ADC readings.
The power in the left and right channels is measured separately, and the maximum value for the channels is displayed. Since the indicator should display both the quasi-peak and average output power, as well as the indicated values \u200b\u200bshould be easy to read, the values \u200b\u200bmeasured with the ADC are subjected to software processing. The time characteristics of the power meter are characterized by the integration time and the return time. For a quasi-peak power meter, the integration time is set by the hardware filtering chain and is approximately 10 ms. The average power meter differs only in increased integration time, which is implemented in software. The average power is calculated using a 256-point moving average. The return travel time in both cases is programmed. For ease of reading, this time should be relatively long. In this case, the reverse movement of the indicator is realized by subtracting 1/16 of the current power code once every 20 ms. In addition, during the display, the peak values \u200b\u200bare held for 1.4 seconds. Since too frequent updating of the indicator readings is not accepted well, the update occurs every 320 ms. In order not to miss the next peak and display it synchronously with the input signal, when a peak is detected, an extraordinary update of readings occurs.
As mentioned above, the PA uses a common with a pre-amplifier remote controlwhich works in the RC-5 standard. Remote control receiver type SFH-506 is located on the display board. From the output of the photodetector, the signal goes to the input SER (INT1) of the microcontroller. Decoding of the RC-5 code is carried out by software. The number of the used system is 0AH, the STANDBY button has a code of 0CH, the DISPLAY button is 21H, the MODE button is 20H. If necessary, these codes can be easily changed, since a lookup table is used, which can be found at the end of the source code of the microcontroller program.
On display board (Fig. 10) two two-digit seven-segment displays HG1 and HG2 of the LTD6610E type are installed. They are controlled by parallel registers U1 - U4. Dynamic indication is not used, as this can cause an increased level of interference.
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Figure: 10. Schematic diagram of the indication board.
The U5 register is used to control the LEDs. A limiting resistor is connected in series with each segment and with each LED. The OC inputs of all registers are combined and connected to the PEN signal of the microcontroller. During reset and initialization of registers, this signal is in the state of a high logic level. This prevents the indication from accidentally lighting up during transients.
The display board also has control buttons SB1 - SB6. They are connected to the data bus lines and to the RET return line. Diodes VD1 - VD6 prevent short circuiting of data lines when two or more buttons are pressed simultaneously. When scanning the keyboard, the microcontroller uses port P0 as a simple output port, forming a running zero on its lines. The RET line is polled at the same time. This is how the code of the pressed button is determined.
An integral remote control photodetector U6 is installed next to the indicators under a common protective glass. The signal from the photodetector output through the XP6 connector is fed to the SER (INT1) microcontroller input.
Duty source (Fig. 11) provides 4 levels at the output: +5 V, +12 V and ± 15 V. Levels of ± 15 V are disabled in standby mode. The source uses a small toroidal transformer wound on a 50x20x25 mm core. The standby transformer has a large power reserve, and the number of turns per volt is chosen more than the calculated one. Thanks to these measures, the transformer practically does not heat up, which increases its reliability (after all, it must work continuously throughout the entire life of the amplifier). Winding data and wire diameter are shown in the diagram. Voltage stabilizers have no special features. The stabilizer chips U1 and U2 are mounted on a small common heatsink. To turn off the ± 15 V levels, switches are used on transistors VT1 - VT4, which are controlled by the PEN signal coming from the processor board.
Figure: 11. Schematic diagram of the standby power supply board.
In addition to voltage stabilizers, on the board of the standby power supply, keys on transistors VT5 - VT12 are installed to control the relay and the fan. Since the ports of MCS-51 family microcontrollers are in the state of a high logic level during the "Reset" signal, all executive devices must be turned on at a low level. Otherwise, there will be false positives when the power is turned on or when the watchdog timer is triggered. For this reason, single npn transistors with OE or ULN2003 driver chips and the like cannot be used as keys.
Relays, fuses and limiting resistors are located on relay board (fig. 12). All mains wires are connected via screw terminals. Each main transformer, standby transformer and external receptacle box are fused separately. For safety reasons, the external sockets are disconnected by two sets of K1 relay contacts, which break both wires. The main transformers are tapped from the middle of the primary winding. This tap can be used to provide 110 V to power other components in the complex. Devices that meet the American standard are somewhat cheaper than multisystem ones, so they are sometimes found on our territory. There are points on the relay board where 110 V can be removed, but this voltage is not used in the basic version.
Figure: 12. Schematic diagram of the relay board.
Block diagram on amplifier chassis shown in fig. 13. Bridge rectifiers assembled on diodes VD5 - VD12 of type KD2997A are connected to the secondary windings of the main transformers T1 and T2. Filter capacitors with a total capacity of more than 100,000 μF are connected to the output of the rectifiers. This high capacitance is necessary in order to obtain low ripple and improve the amplifier's ability to reproduce pulsed signals. From the filter capacitors, a supply voltage of ± 45 V is supplied to the main boards of the amplifier. Additionally, there are low-power rectifiers assembled on diodes VD1 - VD4, the output voltage of which is filtered with a relatively short time constant by capacitors C1 and C2. Through the resistors R1 and R2, the output voltage of these auxiliary rectifiers is supplied to the protection circuits, which are assembled on the main boards of the amplifier. If several half-periods of the mains voltage fail, the output voltage of the auxiliary rectifiers drops, which is detected by the protection circuits, and the load relays are disconnected. At this time, the output voltage of the main rectifiers is still quite large due to large capacitors, so the transient process in the amplifier does not start when the load is connected.
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Figure: 13. Connection diagram of the amplifier blocks.
For the power amplifier, the design and arrangement is no less important than circuitry. The main problem is that efficient heat dissipation is required for the output transistors. With a natural way of cooling, this translates into massive radiators, which become almost the main structural elements. The common arrangement, when the rear wall serves as a radiator at the same time, is not suitable, since then there is no room at the back for installing the necessary terminals and connectors. Therefore, in the described PA, an arrangement with lateral radiators was chosen (Fig. 14):
Figure: 14. General layout of the amplifier.
The radiators are slightly raised (this can be clearly seen in Fig. 4), which ensures their better cooling. The main boards of the amplifier are fixed parallel to the radiators. This minimizes the length of the conductors between the board and the output transistors. Another dimensional elements of the amplifier are network transformers. In this case, two toroidal transformers are used, which are installed on top of each other in a common cylindrical screen. This shield takes up a significant portion of the internal volume of the amplifier enclosure. The main rectifiers are mounted on a common heat sink, which is located vertically behind the transformer shield. The filter capacitors are located underneath the amplifier chassis and are covered by a tray. The relay board is also located there. The standby power supply is fixed on a special bracket near the rear panel. The processor and display boards are housed in a thicker front panel, which has a box section.
During the development of the amplifier design, great attention was paid to the manufacturability of the design and ease of access to any unit. More details on the amplifier layout can be found in Fig. 15 and 18:
Figure: 15. Arrangement of the assembled amplifier units.
The main body of the amplifier is aluminum alloy chassis D16T 4mm thick (4 in Fig. 18). Attached to the chassis radiators (1 in Fig. 18) which are milled from an aluminum plate or cast. The required area of \u200b\u200bthe radiators strongly depends on the operating conditions of the amplifier, but it should not be less than 2000 cm 2. To facilitate access to the amplifier boards, the heatsinks are fixed to the chassis with hinges (10 in Figure 18), which allows the heatsinks to be folded back. The rear panel is split into three parts so that the wires of the input and output connectors do not interfere with this (Fig. 4). The middle section is fixed with a bracket to the chassis, and the two side sections are fixed to the radiators. The connectors are installed on the sides of the panel, which fold out along with the radiators. Thus, the assembled heat sink is a mono PA that is connected only with power wires and a flat control cable. In fig. 18 for clarity, the radiators are only partially folded back, and the rear panel is not disassembled.
Main amplifier boards they are also fixed to the radiators with hinges (12 in Fig. 18), which allows them to be folded back, gaining access to the soldering side. The axis of rotation of the board runs along the line of holes for connecting the wires of the output transistors. This made it possible to practically not increase the length of these wires while simultaneously being able to fold the board. The upper mounting points of the boards are ordinary threaded posts 15mm high. Left and right single sided main boards are wired mirrored (fig. 16), which allowed to optimize the connections. Naturally, the mirroring of the topology is not complete, since elements are used that cannot simply be mirrored (microcircuits and relays). The figure gives an approximate idea of \u200b\u200bthe topology of the boards, the topology of all boards is available in the archive (see the Download section) as files in the PCAD 4.5 format.
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Figure: 16. The layout of the main boards of the amplifier.
Each radiator 1 (Fig. 17) has a smooth surface 2, which is processed after blackening. Nine transistors 4 are installed on it through ceramic spacers 2.
Figure: 17. Design of radiators:
Studies have shown that mica, and even more so modern elastic gaskets, do not have sufficient thermal conductivity. The best material for insulating gaskets is BeO based ceramics. However, for transistors in plastic cases, such spacers are almost never found. Quite good results were obtained by making spacers from substrates of hybrid microcircuits. This is a pink ceramic (unfortunately, the material is not exactly known, most likely something based on Al 2 O 3). To compare the thermal conductivity of different gaskets, a stand was assembled in which two identical transistors in a TO-220 package were fixed to the radiator: one directly, the other through the investigated gasket. The base current for both transistors was the same. The transistor on the gasket dissipated power of about 20W, and the other transistor did not dissipate power (no voltage was applied to the collector). The difference between the FE drops for two transistors was measured, and from this difference the difference between the transition temperatures was calculated. Thermal transfer paste was used for all pads, without which the results were worse and unstable. The comparison results are presented in the table:
The output transistors are pressed down with pads 5, the rest of the transistors are fastened with screws. This is not very convenient, since drilling of ceramic gaskets is required, which can only be done with the help of diamond drills, and even then with great difficulty.
Thermometer 9 is installed next to the transistors. Experience has shown that when attaching DS1820 thermometers to their body, you cannot exert much pressure, otherwise the readings will be distorted, and very significantly (it is better to glue the thermometers with glue with high thermal conductivity).
Board 6 is fixed under the transistors on the heat sink. There are no conductors on the reverse side of this board, so it can be mounted directly to the heat sink surface. The leads of all transistors are soldered to the pads on the top side of the board. Connections of the board with the main board are made with short wires that are soldered into hollow rivets 7. To prevent the rivets from closing to the radiator, a recess 8 is made in it.
Basic toroidal transformers (7 in Fig. 18) are installed on top of each other through elastic pads. To reduce interference from the side of transformers to other equipment (a cassette deck, for example), it is recommended to place the transformers in a screen made of annealed steel with a thickness of at least 1.5 mm. The screen is a steel cylinder and two covers, tightened with a hairpin. To avoid the appearance of a short-circuited loop, the top cover has a dielectric sleeve. However, if it is supposed to operate the PA at high average power, then ventilation holes should be provided in the screen or the screen should be completely abandoned. It would seem that for mutual compensation of the leakage fields of transformers, it is enough to simply turn on their primary windings in antiphase. But in practice, this measure is very ineffective. The stray field of a toroidal transformer, with its apparent axial symmetry, has a very complex spatial distribution. Therefore, the polarity reversal of one of the primary windings leads to a weakening of the stray field at one point in space, but to an amplification at another. In addition, the configuration of the stray field is highly dependent on the transformer load.
Figure: 18. The main components of the amplifier:
| 1 - radiators | 12 - board fixing loop |
| 2 - main amplifier boards | 13 - board fastening rack |
| 3 - platform on the radiator for installing transistors | 14 - control cable connector (from the processor board) |
| 4 - bearing plate | 15 - wire from the output of add. rectifier |
| 5 - carrier plate of the front panel | 16 - standby transformer in the screen |
| 6 - box-section front panel | 17 - board on duty power supply |
| 7 - main transformers in the screen | 18 - heat sink of voltage stabilizers |
| 8 - radiator of rectifier diodes | 19 - control wires of the relay unit |
| 9 - power supply to the boards | 20 - back panel |
| 10 - fastening radiators on hinges | 21 - output terminals |
| 11 - radiator mounting bracket | 22 - input connectors |
The UM power transformer has very stringent requirements. This is due to the fact that it is loaded on a rectifier with very large filter capacitors. This leads to the fact that the current consumed from the secondary winding of the transformer is pulsed in nature, and the value of the current in the pulse is many times higher than the average consumed current. To keep transformer losses low, the windings must have very low resistance. In other words, the transformer must be designed for significantly more power than the average consumed from it. In the described amplifier, two toroidal transformers are used, each of which is wound on a 110x60x40 mm core made of E-380 steel tape. Primary windings contain 2x440
UMZCH VV with microcontroller control system
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Victor Zhukovsky, Krasnoarmeisk, Donetsk region
UMZCH BB-2010 is a new development from the well-known line of amplifiers UMZCH BB (high fidelity) [1; 2; 5]. A number of used technical solutions were influenced by the work of SI Ageev. ...
The amplifier provides Kr of the order of 0.001% at a frequency of 20 kHz at Pout \u003d 150 W at a load of 8 Ohm, a small signal frequency band at a level of -3 dB - 0 Hz ... 800 kHz, an output voltage slew rate of -100 V / μs, a signal-to-noise ratio and signal / background -120 dB.
Thanks to the use of an op-amp operating in a light mode, as well as the use of only stages with OK and OB, covered by deep local OOS, in the voltage amplifier, the UMZCH BB is highly linear even before the general OOS is covered. In the very first high-fidelity amplifier, back in 1985, solutions were applied that had until then only been used in measuring technology: the DC modes are supported by a separate service unit, to reduce the level of interface distortions, the transition resistance of the contact group of the AC switching relay is covered by a common negative feedback, and a special unit effectively compensates for the influence on these distortions of the resistance of the AC cables. The tradition has been preserved in the UMZCH VV-2010, at the same time the general OOS also covers the resistance of the output low-pass filter.
In the vast majority of other UMZCH designs, both professional and amateur, many of these solutions are still absent. At the same time, the high technical characteristics and audiophile advantages of the UMZCH BB are achieved by simple circuitry solutions and a minimum of active elements. In fact, this is a relatively simple amplifier: one channel can be assembled without haste in a couple of days, and the adjustment consists only in setting the required quiescent current of the output transistors. Especially for novice radio amateurs, a method has been developed for a node-by-node, stage-by-stage performance check and adjustment, using which you can guaranteedly localize the places of possible errors and prevent their possible consequences even before the complete assembly of the UMZCH. For all possible questions about this or similar amplifiers, there are detailed explanations, both on paper and on the Internet.
At the input of the amplifier, a high-pass filter R1C1 with a cutoff frequency of 1.6 Hz is provided, Fig. 1. But the efficiency of the mode stabilization device allows the amplifier to work with an input signal containing up to 400 mV DC voltage. Therefore, C1 is excluded, which realizes the age-old audiophile dream of a path without capacitors © and significantly improves the sound of the amplifier.
The capacitance of the capacitor C2 of the input low-pass filter R2C2 is chosen so that the cutoff frequency of the input low-pass filter, taking into account the preamplifier output impedance of 500 Ohm -1 kOhm, was within the range from 120 to 200 kHz. A frequency correction circuit R3R5C3 is placed at the input of op-amp DA1, limiting the band of harmonics and interference received through the feedback circuit from the output side of the UMZCH, with a band of 215 kHz at a level of -3 dB and increasing the stability of the amplifier. This circuit allows you to reduce the difference signal above the cutoff frequency of the circuit and thereby eliminate unnecessary overloading of the voltage amplifier by signals of high-frequency interference, interference and harmonics, eliminating the possibility of dynamic intermodulation distortion (TIM; DIM).
Next, the signal is fed to the input of a low-noise operational amplifier with field-effect transistors at the input of DA1. Many "claims" to the UMZCH BB are presented by opponents regarding the use of an op-amp at the input, allegedly degrading the sound quality and "stealing virtual depth" of the sound. In this regard, it is necessary to pay attention to some quite obvious features of the operation of the OS in the UMZCH explosive.
Operational amplifiers, pre-amplifiers, post-DAC op amps are forced to develop several volts of output voltage. Since the gain of the op amps is small and ranges from 500 to 2,000 times at 20 kHz, this indicates their operation with a relatively high voltage of the difference signal - from several hundred microvolts at low frequencies to several millivolts at 20 kHz and a high probability of introducing intermodulation distortion by the input stage of the op amp. The output voltage of these op amps is equal to the output voltage of the last stage of voltage amplification, usually performed according to the scheme with an OE. The output voltage of several volts indicates the operation of this stage with rather large input and output voltages, and as a result, it introduces distortions into the amplified signal. The op-amp is loaded on the resistance of the parallel-connected OOS circuit and the load, which is sometimes several kilo-ohms, which requires up to several milliamps from the output follower of the amplifier. Therefore, the changes in the current of the output follower of the IC, the output stages of which consume a current of no more than 2 mA, are quite significant, which also indicates that they introduce distortions into the amplified signal. We see that the input stage, voltage amplification stage and the op-amp output stage can introduce distortion.
But the high-fidelity amplifier circuitry, due to the high gain and input resistance of the transistor part of the voltage amplifier, provides very gentle operating conditions for the DA1 op-amp. Judge for yourself. Even in the UMZCH that has developed a nominal output voltage of 50 V, the differential input stage of the op-amp operates with differential signals of voltage from 12 μV at frequencies of 500 Hz to 500 μV at a frequency of 20 kHz. The ratio of the high input overload capacity of the differential cascade based on field-effect transistors and the scanty voltage of the difference signal ensures high linearity of the signal amplification. The op-amp output voltage does not exceed 300 mV. which indicates the low input voltage of the voltage amplification stage with a common emitter from the operational amplifier - up to 60 μV - and the linear mode of its operation. The output stage of the op-amp delivers about 100 kOhm to the load from the side of the VT2 base an alternating current of no more than 3 μA. Consequently, the output stage of the op-amp also operates in an extremely lightweight mode, practically at idle. On a real musical signal, the voltages and currents most of the time are an order of magnitude less than the given values.
From a comparison of the voltages of the difference and output signals, as well as the load current, it can be seen that, in general, the operational amplifier in the UMZCH BB operates hundreds of times lighter, and therefore in a linear mode than the op-amp mode of preamplifiers and post-DAC op-amps of CD-players that serve as sources signal for UMZCH with any OOS depth, as well as without it at all. Consequently, the same op-amp will introduce much less distortion in the UMZCH BB than in a single inclusion.
Rarely, there is an opinion that the distortion introduced by the cascade ambiguously depends on the voltage of the input signal. This is mistake. The dependence of the manifestation of the nonlinearity of the cascade on the voltage of the input signal may obey one or another law, but it is always unambiguous: an increase in this voltage never leads to a decrease in the introduced distortion, but only to an increase.
It is known that the level of distortion products at a given frequency decreases in proportion to the depth of negative feedback for that frequency. The no-load gain, before the coverage of the OOS amplifier, at low frequencies, due to the smallness of the input signal, cannot be measured. According to calculations, the no-load amplification developed before the OOS coverage makes it possible to achieve the OOS depth of 104 dB at frequencies up to 500 Hz. Measurements for frequencies starting from 10 kHz show that the OOS depth at a frequency of 10 kHz reaches 80 dB, at a frequency of 20 kHz - 72 dB, at a frequency of 50 kHz - 62 dB and 40 dB - at a frequency of 200 kHz. Figure 2 shows the amplitude-frequency characteristics of the UMZCH VV-2010 and, for comparison, similar in complexity to the UMZCH of Leonid Zuev.
High gain before OOS coverage is the main feature of HV amplifier circuitry. Since the goal of all circuitry tricks is to achieve high linearity and high gain for maintaining deep feedback in the widest possible frequency band, this means that circuitry methods for improving the parameters of amplifiers are exhausted by such structures. A further reduction in distortion can be provided only by constructive measures aimed at reducing the interference of the output stage harmonics to the input circuits, especially to the inverting input circuit, the gain from which is maximum.
Another feature of the UMZCH BB circuitry is the current control of the output stage of the voltage amplifier. The input op-amp controls the voltage-current conversion stage, performed with OK and OB, and the resulting current is subtracted from the quiescent current of the stage, performed according to the OB circuit.
The use of a linearizing resistor R17 with a resistance of 1 kOhm in a differential cascade VT1, VT2 on transistors of different structures with series power supply increases the linearity of the conversion of the output voltage of the op-amp DA1 into the collector current VT2 by creating a local feedback with a depth of 40 dB. This can be seen from comparing the sum of the emitter's own resistances VT1, VT2 - about 5 Ohm each - with resistance R17, or the sum of thermal voltages VT1, VT2 - about 50 mV - with a voltage drop across the resistance R17, which is 5.2 - 5.6 V ...
Amplifiers built according to the circuitry under consideration have a sharp, 40 dB per decade of frequency, a decrease in gain above the frequency of 13 ... 16 kHz. The error signal, which is a product of distortion, at frequencies above 20 kHz is two to three orders of magnitude less than the useful audio signal. This makes it possible to convert the excess linearity at these frequencies of the differential cascade VT1, VT2 into an increase in the gain of the transistor part of the VN. Due to minor changes in the current of the differential cascade VT1, VT2 with the amplification of weak signals, its linearity does not significantly deteriorate with a decrease in the depth of the local OOS, but the operation of op-amp DA1, on the operating mode of which at these frequencies the linearity of the entire amplifier depends, the gain margin will facilitate, since all voltages, determining the distortion introduced by the operational amplifier, starting from the difference signal to the output signal, decrease in proportion to the gain in gain at a given frequency.
The phase advance correction circuits R18C13 and R19C16 were optimized in the simulator in order to reduce the op-amp differential voltage to frequencies of several megahertz. It was possible to increase the gain of the UMZCH VV-2010 in comparison with the UMZCH VV-2008 at frequencies of the order of several hundred kilohertz. The gain in gain was 4 dB at 200 kHz, 6 at 300 kHz, 8.6 at 500 kHz, 10.5 dB at 800 kHz, 11 dB at 1 MHz, and 10 to 12 dB at frequencies above 2 MHz. This can be seen from the simulation results, Fig. 3, where the lower curve refers to the frequency response of the lead correction circuit UMZCH VV-2008, and the upper one - UMZCH VV-2010.
VD7 protects the emitter junction VT1 from reverse voltage arising from the flow of recharge currents C13, C16 in the mode of limiting the output signal of the UMZCH by voltage and the resulting limiting voltages with a high rate of change at the output of the op-amp DA1.
The output stage of the voltage amplifier is made on a transistor VT3, connected according to a circuit with a common base, which excludes the penetration of the signal from the output circuits of the stage into the input circuits and increases its stability. A cascade with OB, loaded on a current generator on a VT5 transistor and the input resistance of the output stage, develops a high stable gain - up to 13,000 ... 15,000 times. The choice of the resistance of the resistor R24 \u200b\u200bis half the resistance of the resistor R26 guarantees the equality of the quiescent currents VT1, VT2 and VT3, VT5. R24, R26 provide local OOS that reduce the effect of the Earley effect - the change in p21e depending on the collector voltage and increase the initial linearity of the amplifier by 40 dB and 46 dB, respectively. The power supply of the UN with a separate voltage, modulo 15 V higher than the voltage of the output stages, eliminates the effect of quasi-saturation of transistors VT3, VT5, which manifests itself in a decrease in p21e when the collector-base voltage drops below 7 V.
The three-stage output follower is assembled on bipolar transistors and does not require special comments. Do not try to fight with entropy © by saving on the quiescent current of the output transistors. It should not be less than 250 mA; in the author's version - 320 mA.
Before the activation of the relay for turning on the AC K1, the amplifier is covered by OOS1, implemented by turning on the divider R6R4. The accuracy of the resistance R6 and the consistency of these resistances in different channels is not essential, but to maintain the stability of the amplifier, it is important that the resistance of R6 is not much lower than the sum of the resistances R8 and R70. By triggering the relay K1, OOS1 is turned off and the OOS2 circuit formed by R8R70C44 and R4 enters into operation, and covering the contact group K1.1, where R70C44 excludes the output low-pass filter R71L1 R72C47 from the OOS circuit at frequencies above 33 kHz. Frequency-dependent OOS R7C10 forms a drop in the frequency response of the UMZCH to the output low-pass filter at a frequency of 800 kHz at a level of -3 dB and provides a margin in OOS depth above this frequency. The drop in frequency response at the AC terminals above 280 kHz in terms of -3 dB is provided by the joint action of R7C10 and the output low-pass filter R71L1 -R72C47.
The resonant properties of the loudspeakers lead to the emission of damped sound vibrations by the diffuser, overtones after impulse exposure and generation of its own voltage when the turns of the loudspeaker coil cross the magnetic field lines in the gap of the magnetic system. The damping coefficient shows how large the amplitude of the oscillations of the diffuser is and how quickly they damp when the AC is loaded as a generator on the impedance of the UMZCH. This coefficient is equal to the ratio of the AC resistance to the sum of the output resistance of the UMZCH, the contact resistance of the contact group of the AC switching relay, the resistance of the output low-pass filter, usually wound with an insufficient diameter wire, the contact resistance of the AC cable clamps and the resistance of the AC cables itself.
In addition, the impedance of loudspeakers is non-linear. The flow of distorted currents through the wires of AC cables creates a voltage drop with a high degree of harmonic distortion, also subtracted from the amplifier's undistorted output voltage. Therefore, the signal at the AC terminals is distorted much more than at the output of the UMZCH. These are the so-called interface distortions.
To reduce these distortions, compensation for all components of the amplifier output impedance is applied. The intrinsic output resistance of the UMZCH together with the contact resistance of the relay contacts and the resistance of the wire of the inductor of the output low-pass filter is reduced by the action of a deep general OOS taken from the right terminal of L1. In addition, by connecting the right terminal R70 to the "hot" AC terminal, you can easily compensate for the transition resistance of the AC cable clamp and the resistance of one of the AC wires, without fear of generating UMZCH due to phase shifts in the wires covered by the OOS.
The node for compensating the resistance of the AC wires is made in the form of an inverting amplifier with Ky \u003d -2 on the op-amp DA2, R10, C4, R11 and R9. The input voltage for this amplifier is the voltage drop across the "cold" ("earth") AC wire. Since its resistance is equal to the resistance of the "hot" wire of the AC cable, to compensate for the resistance of both wires, it is enough to double the voltage on the "cold" wire, invert it and through the resistor R9 with a resistance equal to the sum of the resistances R8 and R70 of the OOS circuit, apply it to the inverting input of the op-amp DA1 ... Then the output voltage of the UMZCH will increase by the sum of the voltage drops on the AC wires, which is tantamount to eliminating the effect of their resistance on the damping coefficient and the level of interface distortion at the AC terminals. Compensation for the drop on the resistance of the AC wires of the nonlinear component of the back-emf of loudspeakers is especially needed at the low frequencies of the audio range. The signal voltage at the HF loudspeaker is limited by a resistor and capacitor connected in series with it. Their complex impedance is much greater than the resistance of the wires of the AC cable, so compensation for this resistance at HF \u200b\u200bis meaningless. Based on this, the R11C4 integrating circuit limits the operating frequency band of the compensator to 22 kHz.
It should be especially noted: the resistance of the "hot" wire of the AC cable can be compensated by covering its general OOS by connecting the right terminal R70 with a special wire to the "hot" AC terminal. In this case, only the resistance of the "cold" AC wire needs to be compensated, and the gain of the wire resistance compensator must be reduced to the value of Ku \u003d -1 by choosing the resistance of the resistor R10 equal to the resistance of the resistor R11.
The overcurrent protection unit prevents damage to the output transistors during short circuits in the load. Resistors R53 - R56 and R57 - R60 serve as a current sensor, which is quite enough. The flow of the amplifier output current through these resistors creates a voltage drop that is applied to the divider R41R42. A voltage with a value greater than the threshold opens the transistor VT10, and its collector current opens the VT8 trigger cell VT8VT9. This cell goes into a steady state with open transistors and shunts the HL1VD8 circuit, reducing the current through the zener diode to zero and blocking VT3. Discharging C21 with a small base current VT3 can take several milliseconds. After the trigger cell is triggered, the voltage on the lower plate of C23, charged with the voltage on the LED HL1 to 1.6 V, rises from a level of -7.2 V from the positive power rail of the UN to the level of -1.2 V 1, the voltage on the top plate of this capacitor also rises by 5 V. C21 is quickly discharged through the resistor R30 to C23, the transistor VT3 is locked. In the meantime, VT6 opens and through R33, R36 opens VT7. VT7 shunts the Zener diode VD9, discharges capacitor C22 through R31 and locks the transistor VT5. Not receiving a bias voltage, the output stage transistors are also turned off.
Restoring the initial state of the trigger and turning on the UMZCH is done by pressing the SA1 "Reset protection" button. C27 is charged with the VT9 collector current and bypasses the VT8 base circuit, locking the trigger cell. If by this moment the emergency situation has been eliminated and VT10 is locked, the cell goes into a state with stably closed transistors. VT6, VT7 are closed, reference voltage is supplied to the bases VT3, VT5 and the amplifier enters the operating mode. If a short circuit in the UMZCH load continues, the protection is triggered again, even if the capacitor C27 is connected to SA1. The protection works so efficiently that during the work on adjusting the correction, the amplifier was de-energized several times for small re-soldering ... by touching the non-inverting input. The emerging self-excitation led to an increase in the current of the output transistors, and the protection turned off the amplifier. Although this crude method should not be generally suggested, due to the overcurrent protection it did not harm the output transistors.
The work of the compensator of the resistance of the AC cables.
The efficiency of the UMZCH VV-2008 compensator was tested by the old audiophile method, by ear, by switching the compensator input between the compensating wire and the common wire of the amplifier. The improvement in sound was clearly noticeable, and the future owner was eager to get an amplifier, so no measurements of the effect of the compensator were carried out. The advantages of the "cable stripper" circuit were so obvious that the "compensator + integrator" configuration was adopted as a standard unit for installation in all amplifiers under development.
It's amazing how much unnecessary controversy has flared up on the Internet about the usefulness / unnecessaryness of cable resistance compensation. As usual, those to whom the extremely simple cable stripper circuit seemed complicated and incomprehensible, the costs for it were exorbitant, and the installation was laborious © especially insisted on listening to a nonlinear signal. There were even suggestions that, since so much money is spent on the amplifier itself, it is a sin to save on a saint, but you need to go the best, glamorous way that all civilized mankind walks and ... get normal, human © super-expensive cables made of precious metals. To my great surprise, the statements of highly respected specialists about the uselessness of the compensation unit at home, including those specialists who successfully use this unit in their amplifiers, added fuel to the fire. It is very regrettable that many fellow radio amateurs reacted with distrust to the reports of improving the sound quality at low and mid frequencies with the inclusion of a compensator, they did their best to avoid this simple way of improving the work of the UMZCH, than robbed themselves.
A little research has been done to document the truth. A number of frequencies were fed from the GZ-118 generator to the UMZCH VV-2010 in the vicinity of the resonant frequency of the AC, the voltage was monitored by an S1-117 oscilloscope, and Kr at the AC terminals was measured by INI S6-8, Fig. 4. 
Resistor R1 is installed to avoid pickup at the input of the compensator during its switching between the control and common wire. The experiment used common and commonly available AC cables 3 m long and 6 sq. M. mm, as well as the GIGA FS Il speaker system with a frequency range of 25-22.000 Hz, a nominal impedance of 8 ohms and a nominal power of 90 W from Acoustic Kingdom.
Unfortunately, the circuitry of the harmonic signal amplifiers from the C6-8 composition provides for the use of high-capacity oxide capacitors in the OOS circuits. This leads to the influence of low-frequency noise of these capacitors on the resolution of the device at low frequencies, as a result of which its resolution at low frequencies deteriorates. When measuring Kr signal with a frequency of 25 Hz from GZ-118 directly to C6-8, the readings of the device dance around the value of 0.02%. It is not possible to bypass this limitation using the notch filter of the GZ-118 generator in the case of measuring the efficiency of the compensator, since a number of discrete values \u200b\u200bof the tuning frequencies of the 2T filter is limited at LF values \u200b\u200bof 20.60, 120, 200 Hz and does not allow us to measure Kr at the frequencies of interest to us. Therefore, reluctantly, the level of 0.02% was accepted as zero, the reference.
At a frequency of 20 Hz with a voltage at the AC terminals of 3 V peak, which corresponds to an output power of 0.56 W at a load of 8 ohms, Kr was 0.02% with the compensator turned on and 0.06% after turning it off. At a voltage of 10 V amps, which corresponds to an output power of 6.25 W, the Kr value is 0.02% and 0.08%, respectively, at a voltage of 20 V amps and a power of 25 W - 0.016% and 0.11%, and at a voltage of 30 In amplitude and power of 56 W - 0.02% and 0.13%.
Knowing the lightweight attitude of the manufacturers of imported equipment to the values \u200b\u200bof the inscriptions regarding the power, and also remembering the wonderful, after the adoption of Western standards, the transformation of the 35AC-1 speaker system with a subwoofer power of 30 W into the S-90, long-term power of more than 56 W was not supplied to the AC.
At a frequency of 25 Hz with a power of 25 W Kr was 0.02% and 0.12% with the compensation unit turned on / off, and with a power of 56 W - 0.02% and 0.15%.
At the same time, the necessity and effectiveness of the coverage of the output low-pass filter of the general OOS was checked. At a frequency of 25 Hz at a power of 56 W and connected in series to one of the wires of the AC output RL-RC low-pass filter, similar to that installed in the super-linear UMZCH, Kr with the compensator turned off reaches 0.18%. At a frequency of 30 Hz at a power of 56 W Kr 0.02% and 0.06% with the compensation unit on / off. At a frequency of 35 Hz at a power of 56 W Kr 0.02% and 0.04% with the compensation unit on / off. At frequencies of 40 and 90 Hz at a power of 56 W Kr 0.02% and 0.04% with the compensation unit on / off, and at a frequency of 60 Hz -0.02% and 0.06%.
The conclusions are clear. The presence of nonlinear signal distortions at the AC terminals is observed. Deterioration of the linearity of the signal at the AC terminals with its inclusion through the uncompensated, not covered by the OOS resistance of the low-pass filter, containing 70 cm of a relatively thin wire, is clearly recorded. The dependence of the distortion level on the power supplied to the AC suggests that it depends on the ratio of the signal power to the nominal power of the speaker woofers. Distortion is most pronounced at frequencies near the resonant frequency. The back-EMF generated by the speakers in response to the sound signal is shunted by the sum of the output resistance of the UMZCH and the resistance of the wires of the AC cable, so the level of distortion at the AC terminals directly depends on the resistance of these wires and the output resistance of the amplifier.
The cone of a poorly damped subwoofer itself emits noise and, in addition, this loudspeaker generates a wide tail of THD and intermodulation products that the midrange loudspeaker reproduces. This explains the deterioration in the sound at midrange.
Despite the assumption of a zero Kr level of 0.02% adopted due to the imperfection of ISI, the effect of the cable resistance compensator on signal distortions at the AC terminals is clearly and unambiguously noted. It can be stated that the conclusions made after listening to the operation of the compensation unit on a musical signal and the results of instrumental measurements can be stated.
The improvement, clearly audible when the cable stripper is turned on, can be explained by the fact that when the distortion at the AC terminals disappears, the mid-range loudspeaker stops reproducing all this dirt. Apparently, therefore, by reducing or eliminating the reproduction of distortions by a mid-frequency loudspeaker, a two-cable circuit for switching on the speaker, the so-called. "Bi-wiring", when the LF and MF-HF links are connected by different cables, has an advantage in sound compared to a single-cable scheme. However, since in a two-cable circuit the distorted signal at the terminals of the AC low-frequency section does not disappear anywhere, this circuit loses to the version with a compositor in terms of the coefficient of damping of free vibrations of the woofer cone.
Physics cannot be fooled, and for decent sound it is not enough to get brilliant performance at the amplifier output with an active load, but it is also necessary not to lose linearity after the signal is delivered to the speaker terminals. As part of a good amplifier, a compensator made according to one scheme or another is absolutely necessary.
Integrator.
We also tested the efficiency and the possibility of reducing the error of the integrator on the DA3. In UMZCH BB with op-amp TL071, the output constant voltage is within 6 ... 9 mV and it was not possible to reduce this voltage by including an additional resistor in the non-inverting input circuit.
The effect of low-frequency noise, typical for op amps with a DC input, due to deep OOS coverage through the R16R13C5C6 frequency-dependent circuit, manifests itself in the form of an instability of the output voltage of several millivolts, or -60 dB relative to the output voltage at rated output power, at frequencies below 1 Hz not reproducible speakers.
On the Internet, it was mentioned about the low resistance of the protective diodes VD1 ... VD4, which supposedly introduces an error in the operation of the integrator due to the formation of a divider (R16 + R13) / R VD2 | VD4 . . To check the reverse resistance of the protective diodes, a circuit was assembled in Fig. 6. Here OA DA1, connected according to the inverting amplifier circuit, is covered by OOS through R2, its output voltage is proportional to the current in the circuit of the tested diode VD2 and the protective resistor R2 with a coefficient of 1 mV / nA, and the resistance of the R2VD2 circuit - with a coefficient of 1 mV / 15 GΩ. To exclude the influence of the op-amp's additive errors - bias voltage and input current - on the results of diode leakage current measurements, it is only necessary to calculate the difference between the op-amp's own voltage measured without the tested diode and the op-amp output voltage after its installation. In practice, the difference in the output voltages of the op-amp of several millivolts gives the value of the reverse resistance of the diode on the order of ten to fifteen gigaohms at a reverse voltage of 15 V. Obviously, the leakage current will not increase with a decrease in the voltage across the diode to a level of several millivolts, which is characteristic of the differential voltage of the op-amp integrator and compensator ...
But the photoelectric effect inherent in diodes placed in a glass case really leads to a significant change in the output voltage of the UMZCH. When they were illuminated with a 60 W incandescent lamp from a distance of 20 cm, the constant voltage at the UMZCH output increased to 20 ... 3O mV. Although it is unlikely that a similar level of illumination can be observed inside the amplifier case, a drop of paint applied to these diodes eliminated the dependence of the UMZCH modes on illumination. According to the simulation results, the drop in the frequency response of the UMZCH is not observed even at a frequency of 1 millihertz. But you should not decrease the time constant R16R13C5C6. The phases of alternating voltages at the outputs of the integrator and the compensator are opposite, and with a decrease in the capacitance of the capacitors or the resistance of the integrator resistors, an increase in its output voltage can worsen the compensation of the AC cable resistance.
Comparison of the sound of amplifiers. The sound of the assembled amplifier was compared with the sound of several foreign industrial amplifiers. The source was a Cambridge Audio CD-player, the Radiotekhnika UP-001 preamplifier was used to drive and adjust the sound level of the terminal UMZCH, the Sugden A21a and NAD C352 used standard controls.
The first to check the legendary, shocking and devilishly expensive English UMZCH "Sugden A21a", operating in class A with an output power of 25 watts. What is noteworthy, in the accompanying documentation on the VCL, the British considered it a blessing not to indicate the level of nonlinear distortions. Say, it's not about distortions, but about spirituality. "Sugden А21а\u003e" lost to UMZCH VV-2010 with comparable power both in level and clarity, confidence, nobility of sounding at low frequencies. This is not surprising, given the peculiarities of its circuitry: just a two-stage quasi-symmetric output follower on transistors of the same structure, assembled according to the circuitry of the 70s of the last century with a relatively high output resistance and an electrolytic capacitor connected at the output that further increases the total output impedance - this is the last the solution itself degrades the sound of any amplifiers at low and mid frequencies. At mid and high frequencies, the UMZCH BB showed higher detail, transparency and excellent stage development, when singers, instruments could be clearly localized in sound. By the way, speaking of the correlation between objective measurement data and subjective impressions of sound: in one of the magazine articles of Sugden's competitors, its Kr was determined at 0.03% at a frequency of 10 kHz.
The next was also the English amplifier NAD C352. The general impression was the same: the pronounced "bucket" sound of the Englishman on the bass did not leave him any chance, while the work of the UMZCH BB was recognized as impeccable. Unlike NADa, whose sound was associated with thick bushes, wool, cotton wool, the sound of BB-2010 at mid and high frequencies made it possible to clearly distinguish the voices of performers in the general choir and instruments in the orchestra. In the work of NAD C352, the effect of better audibility of a more vociferous performer, a louder instrument was clearly expressed. As the owner of the amplifier himself put it, in the sound of the UMZCH BB, the vocalists did not "shout" to each other, and the violin did not fight in the power of sound with a guitar or a trumpet, but all the instruments peacefully and harmoniously "were friends" in the overall sound image of the melody. At high frequencies, UMZCH VV-2010, according to figuratively thinking audiophiles, sounds like "as if drawing a sound with a thin-thin brush." These effects can be attributed to the difference in intermodulation distortion of the amplifiers.
The sound of the UMZCH Rotel RB 981 was similar to the sound of the NAD C352, except for better performance at low frequencies, yet the UMZCH VV-2010 in the clarity of AC control at low frequencies, as well as transparency, delicacy of sound at mid and high frequencies remained unrivaled.
The most interesting thing in terms of understanding the way of thinking of audiophiles was the general opinion that, despite the superiority over these three UMZCH, they bring “warmth” to the sound, which makes it more pleasant, and the UMZCH BB works smoothly, “it is neutral to the sound”.
The Japanese Dual CV1460 lost in sound immediately after switching on in the most obvious way for everyone, and they did not waste time on listening to it in detail. Its Kr was in the range of 0.04 ... 0.07% at low power.
The main impressions from the comparison of amplifiers in the main features were completely identical: the UMZCH BB was ahead of them in sound unconditionally and unambiguously. Therefore, further testing was deemed unnecessary. As a result, friendship won, everyone got what they wanted: for a warm, intimate sound - Sugden, NAD and Rotel, and to hear the director recorded on a disc - UMZCH VV-2010.
Personally, I like the UMZCH of high fidelity with its light, clean, impeccable, noble sound, it playfully reproduce passages of any complexity. As my acquaintance, an audiophile with great experience, put it, he works out the sounds of drums at low frequencies without options, like a press, in the middle it sounds as if it does not exist, and at high frequencies it seems to draw a sound with a thin brush. For me, the non-stressing sound of the UMZCH BB is associated with the ease of operation of the cascades.
Literature
1. Sukhov I. UMZCH of high fidelity. Radio, 1989, No. 6, pp. 55-57; No. 7, pp. 57-61.
2. Ridiko L. UMZCH BB on a modern element base with a microcontroller control system. "Radiohobby", 2001, No. 5, pp. 52-57; No. 6, pp. 50-54; 2002, No. 2, pp. 53-56.
3. Ageev S. Superlinear UMZCH with deep OOS "Radio", 1999, No. 10 ... 12; "Radio", 2000, No. 1; 2; 4 ... 6; 9 ... 11.
4. Zuev. L. UMZCH with parallel OOS. "Radio", 2005, No. 2, p. 14.
5. Zhukovsky V. Why do you need the speed of the UMZCH (or "UMZCH VV-2008"). “Radiohobby”, 2008, No. 1, pp. 55-59; No. 2, pp. 49-55.
UMZCH VVS-2011 version Ultimate
UMZCH VVS-2011 version Ultimate author of the scheme Viktor Zhukovsky Krasnoarmeysk
Amplifier Specifications:
1. Large power: 150W / 8 ohm,
2. High linearity - 0.000.2 ... 0.000.3% at 20 kHz 100 W / 4 Ohm,
Complete set of service nodes:
1. Maintaining zero constant voltage,
2. Resistance compensator of AC wires,
3. Current protection,
4. Protection against constant voltage output,
5. Smooth start.
UMZCH VVS2011 scheme
A participant in many popular projects LepekhinV (Vladimir Lepekhin) was involved in the layout of printed circuit boards. It turned out very well).
UMZCH-VVS2011 board
Amplifier board ULF VVS-2011 was designed for tunnel blowing (parallel to the radiator). Installation of transistors UN (voltage amplifier) \u200b\u200band VC (output stage) is somewhat difficult, because mounting / dismantling has to be done with a screwdriver through holes in the PCB with a diameter of about 6 mm. When the access is open, the projection of the transistors does not fall under the PCB, it is much more convenient. I had to modify the board a little.
I did not take into account one point in the new PP Is the convenience of setting up protection on the amplifier board:
C25 0.1n, R42 * 820 Ohm and R41 1k, all SMD elements are located on the soldering side, which is very inconvenient when setting up, because it will be necessary to unscrew and fasten the PP fastening bolts on the racks and transistors to the radiators several times. Sentence: R42 * 820 consists of two SMD resistors located in parallel, from here a proposal: we solder one SMD resistor immediately, the other output resistor is soldered with a canopy to VT10, one terminal to the base, the other to the emitter, we select to a suitable one. We picked it up, we change the output to smd, for clarity:
UMZCH BB-2010 is a new development from the well-known line of amplifiers UMZCH BB (high fidelity). A number of used technical solutions were influenced by the work of Ageev.
Specifications:
Harmonic distortion at 20,000 Hz: 0.001% (150 W / 8 ohms)
-3 dB small signal bandwidth: 0 - 800,000 Hz
Slew rate of output voltage: 100 V / μs
Signal to noise ratio and signal to background ratio: 120dB
Electrical diagram of the Air Force-2010
Thanks to the use of an op-amp operating in a light mode, as well as the use of only stages with OK and OB, covered by deep local OOS, in the voltage amplifier, the UMZCH BB is highly linear even before the general OOS is covered. In the very first high-fidelity amplifier, back in 1985, solutions were applied that had until then only been used in measuring technology: the DC modes are supported by a separate service unit, to reduce the level of interface distortions, the transition resistance of the contact group of the AC switching relay is covered by a common negative feedback, and a special unit effectively compensates for the influence on these distortions of the resistance of the AC cables. The tradition has been preserved in the UMZCH VV-2010, at the same time the general OOS also covers the resistance of the output low-pass filter.
In the vast majority of other UMZCH designs, both professional and amateur, many of these solutions are still absent. At the same time, the high technical characteristics and audiophile advantages of the UMZCH BB are achieved by simple circuitry solutions and a minimum of active elements. In fact, this is a relatively simple amplifier: one channel can be assembled without haste in a couple of days, and the adjustment consists only in setting the required quiescent current of the output transistors. Especially for novice radio amateurs, a method has been developed for a node-by-node, stage-by-stage performance check and adjustment, using which you can guaranteedly localize the places of possible errors and prevent their possible consequences even before the complete assembly of the UMZCH. For all possible questions about this or similar amplifiers, there are detailed explanations, both on paper and on the Internet.
At the input of the amplifier, a high-pass filter R1C1 with a cutoff frequency of 1.6 Hz is provided, Fig. 1. But the efficiency of the mode stabilization device allows the amplifier to work with an input signal containing up to 400 mV DC voltage. Therefore, C1 is excluded, which realizes the age-old audiophile dream of a circuit without capacitors and significantly improves the sound of the amplifier.
The capacitance of the capacitor C2 of the input low-pass filter R2C2 is chosen so that the cutoff frequency of the input low-pass filter, taking into account the preamplifier output impedance of 500 Ohm -1 kOhm, was within the range from 120 to 200 kHz. A frequency correction circuit R3R5C3 is placed at the input of op-amp DA1, limiting the band of harmonics and interference received through the feedback circuit from the output side of the UMZCH, with a band of 215 kHz at a level of -3 dB and increasing the stability of the amplifier. This circuit allows you to reduce the difference signal above the cutoff frequency of the circuit and thereby eliminate unnecessary overloading of the voltage amplifier by signals of high-frequency interference, interference and harmonics, eliminating the possibility of dynamic intermodulation distortion (TIM; DIM).
Next, the signal is fed to the input of a low-noise operational amplifier with field-effect transistors at the input of DA1. Many "claims" to the UMZCH BB are presented by opponents regarding the use of an op-amp at the input, allegedly degrading the sound quality and "stealing virtual depth" of the sound. In this regard, it is necessary to pay attention to some quite obvious features of the operation of the OS in the UMZCH explosive.
Op-amps, pre-amplifiers, post-DAC op-amps are forced to develop several volts of output voltage. Since the gain of the op-amp is small and ranges from 500 to 2000 times at 20 kHz, this indicates their operation with a relatively high voltage of the difference signal - from several hundred microvolts at low frequencies to several millivolts at 20 kHz and a high probability of introducing intermodulation distortions by the input stage of the op-amp. The output voltage of these op amps is equal to the output voltage of the last stage of voltage amplification, usually performed according to the scheme with an OE. The output voltage of several volts indicates the operation of this stage with rather large input and output voltages, and as a result, it introduces distortions into the amplified signal. The op-amp is loaded on the resistance of the parallel connected OOS circuit and the load, which is sometimes several kilo-ohms, which requires up to several milliamperes from the output follower of the amplifier. Therefore, the changes in the current of the output follower of the IC, the output stages of which consume no more than 2 mA, are quite significant, which also indicates that they introduce distortions into the amplified signal. We see that the input stage, voltage amplification stage and the op-amp output stage can introduce distortion.
But the high-fidelity amplifier circuitry, due to the high gain and input resistance of the transistor part of the voltage amplifier, provides very gentle operating conditions for the DA1 op-amp. Judge for yourself. Even in the UMZCH that has developed a nominal output voltage of 50 V, the differential input stage of the op-amp operates with differential signals of voltage from 12 μV at frequencies of 500 Hz to 500 μV at a frequency of 20 kHz. The ratio of the high input overload capacity of the differential cascade based on field-effect transistors and the scanty voltage of the difference signal ensures high linearity of the signal amplification. The op-amp output voltage does not exceed 300 mV. which indicates a low input voltage of the voltage amplification stage with a common emitter from the operational amplifier - up to 60 μV - and a linear mode of its operation. The output stage of the op-amp delivers about 100 kOhm to the load from the side of the VT2 base an alternating current of no more than 3 μA. Consequently, the output stage of the op-amp also operates in an extremely lightweight mode, practically at idle. On a real musical signal, the voltages and currents most of the time are an order of magnitude less than the given values.
From a comparison of the voltages of the difference and output signals, as well as the load current, it can be seen that, in general, the operational amplifier in the UMZCH BB operates hundreds of times lighter, and therefore in a linear mode than the op-amp mode of preamplifiers and post-DAC op-amps of CD-players that serve as sources signal for UMZCH with any OOS depth, as well as without it at all. Consequently, the same op-amp will introduce much less distortion in the UMZCH BB than in a single inclusion.
Rarely, there is an opinion that the distortion introduced by the cascade ambiguously depends on the voltage of the input signal. This is mistake. The dependence of the manifestation of the nonlinearity of the cascade on the voltage of the input signal may obey one or another law, but it is always unambiguous: an increase in this voltage never leads to a decrease in the introduced distortion, but only to an increase.
It is known that the level of distortion products at a given frequency decreases in proportion to the depth of negative feedback for that frequency. The no-load gain, before the coverage of the OOS amplifier, at low frequencies, due to the smallness of the input signal, cannot be measured. According to calculations, the no-load amplification developed before the OOS coverage makes it possible to achieve the OOS depth of 104 dB at frequencies up to 500 Hz. Measurements for frequencies starting from 10 kHz show that the OOS depth at a frequency of 10 kHz reaches 80 dB, at a frequency of 20 kHz - 72 dB, at a frequency of 50 kHz - 62 dB and 40 dB - at a frequency of 200 kHz. Figure 2 shows the amplitude-frequency characteristics of the UMZCH VV-2010 and, for comparison, similar in complexity.
High gain before NFB coverage is the main feature of HV amplifier circuitry. Since the goal of all circuitry tricks is to achieve high linearity and high gain for maintaining deep feedback in the widest possible frequency band, this means that circuitry methods for improving the parameters of amplifiers are exhausted by such structures. A further reduction in distortion can be provided only by constructive measures aimed at reducing the interference of the output stage harmonics to the input circuits, especially to the inverting input circuit, the gain from which is maximum.
Another feature of the UMZCH BB circuitry is the current control of the output stage of the voltage amplifier. The input op-amp controls the voltage-current conversion stage, performed with OK and OB, and the resulting current is subtracted from the quiescent current of the stage, performed according to the OB circuit.
The use of a linearizing resistor R17 with a resistance of 1 kOhm in a differential cascade VT1, VT2 on transistors of different structures with series power supply increases the linearity of the conversion of the output voltage of the op-amp DA1 into the collector current VT2 by creating a local feedback with a depth of 40 dB. This can be seen from comparing the sum of the emitter's own resistances VT1, VT2 - about 5 ohms each - with resistance R17, or the sum of thermal voltages VT1, VT2 - about 50 mV - with a voltage drop across the resistance R17, which is 5.2 - 5.6 V ...
Amplifiers built according to the circuitry under consideration have a sharp, 40 dB per decade of frequency, a decrease in gain above the frequency of 13 ... 16 kHz. The error signal, which is a product of distortion, at frequencies above 20 kHz is two to three orders of magnitude less than the useful audio signal. This makes it possible to convert the excess linearity at these frequencies of the differential cascade VT1, VT2 into an increase in the gain of the transistor part of the VN. Due to minor changes in the current of the differential cascade VT1, VT2 with the amplification of weak signals, its linearity does not significantly deteriorate with a decrease in the depth of the local OOS, but the operation of op-amp DA1, on the operating mode of which at these frequencies the linearity of the entire amplifier depends, the gain margin will facilitate, since all voltages, determining the distortion introduced by the operational amplifier, starting from the difference signal to the output signal, decrease in proportion to the gain in gain at a given frequency.
The phase advance correction circuits R18C13 and R19C16 were optimized in the simulator in order to reduce the op-amp differential voltage to frequencies of several megahertz. It was possible to increase the gain of the UMZCH VV-2010 in comparison with the UMZCH VV-2008 at frequencies of the order of several hundred kilohertz. The gain in gain was 4 dB at 200 kHz, 6 at 300 kHz, 8.6 at 500 kHz, 10.5 dB at 800 kHz, 11 dB at 1 MHz and 10 to 12 dB at frequencies above 2 MHz. This can be seen from the simulation results, Fig. 3, where the lower curve refers to the frequency response of the lead correction circuit UMZCH VV-2008, and the upper one - UMZCH VV-2010.
VD7 protects the emitter junction VT1 from reverse voltage arising from the flow of recharge currents C13, C16 in the mode of limiting the output signal of the UMZCH by voltage and the resulting limiting voltages with a high rate of change at the output of the op-amp DA1.
The output stage of the voltage amplifier is made on a transistor VT3, connected according to a circuit with a common base, which excludes the penetration of the signal from the output circuits of the stage into the input circuits and increases its stability. A stage with OB, loaded on a current generator on a transistor VT5 and the input resistance of the output stage, develops a high stable gain - up to 13,000 ... 15,000 times. The choice of the resistance of the resistor R24 \u200b\u200bis half the resistance of the resistor R26 guarantees the equality of the quiescent currents VT1, VT2 and VT3, VT5. R24, R26 provide local OOS that reduce the effect of the Earley effect - the change in p21e depending on the collector voltage and increase the initial linearity of the amplifier by 40 dB and 46 dB, respectively. The power supply of the UN with a separate voltage, modulo 15 V higher than the voltage of the output stages, eliminates the effect of quasi-saturation of transistors VT3, VT5, which manifests itself in a decrease in p21e when the collector-base voltage drops below 7 V.
The three-stage output follower is assembled on bipolar transistors and does not require special comments. Don't try to fight entropy by saving on the quiescent current of the output transistors. It should not be less than 250 mA; in the author's version - 320 mA.
Before the activation of the relay for turning on the AC K1, the amplifier is covered by OOS1, implemented by turning on the divider R6R4. The accuracy of the resistance R6 and the consistency of these resistances in different channels is not essential, but to maintain the stability of the amplifier, it is important that the resistance of R6 is not much lower than the sum of the resistances R8 and R70. By triggering the relay K1, OOS1 is turned off and the OOS2 circuit formed by R8R70C44 and R4 enters into operation, and covering the contact group K1.1, where R70C44 excludes the output low-pass filter R71L1 R72C47 from the OOS circuit at frequencies above 33 kHz. Frequency-dependent OOS R7C10 forms a drop in the frequency response of the UMZCH to the output low-pass filter at a frequency of 800 kHz at a level of -3 dB and provides a margin in OOS depth above this frequency. The drop in frequency response at the AC terminals above 280 kHz in terms of -3 dB is provided by the joint action of R7C10 and the output low-pass filter R71L1 -R72C47.
The resonant properties of the loudspeakers lead to the emission of damped sound vibrations by the diffuser, overtones after impulse exposure and generation of its own voltage when the turns of the loudspeaker coil cross the magnetic field lines in the gap of the magnetic system. The damping coefficient shows how large the amplitude of the oscillations of the diffuser is and how quickly they damp when the AC is loaded as a generator on the impedance of the UMZCH. This coefficient is equal to the ratio of the AC resistance to the sum of the output resistance of the UMZCH, the contact resistance of the contact group of the AC switching relay, the resistance of the output low-pass filter, usually wound with an insufficient diameter wire, the contact resistance of the AC cable clamps and the resistance of the AC cables itself.
In addition, the impedance of loudspeakers is non-linear. The flow of distorted currents through the wires of AC cables creates a voltage drop with a high degree of harmonic distortion, also subtracted from the amplifier's undistorted output voltage. Therefore, the signal at the AC terminals is distorted much more than at the output of the UMZCH. These are the so-called interface distortions.
To reduce these distortions, compensation for all components of the amplifier output impedance is applied. The intrinsic output resistance of the UMZCH together with the contact resistance of the relay contacts and the resistance of the wire of the inductor of the output low-pass filter is reduced by the action of a deep general OOS taken from the right terminal of L1. In addition, by connecting the right terminal R70 to the "hot" AC terminal, you can easily compensate for the transition resistance of the AC cable clamp and the resistance of one of the AC wires, without fear of generating UMZCH due to phase shifts in the wires covered by the OOS.
The node for compensating the resistance of the AC wires is made in the form of an inverting amplifier with Ky \u003d -2 on the op-amp DA2, R10, C4, R11 and R9. The input voltage for this amplifier is the voltage drop across the "cold" ("earth") AC wire. Since its resistance is equal to the resistance of the "hot" wire of the AC cable, to compensate for the resistance of both wires, it is enough to double the voltage on the "cold" wire, invert it and through the resistor R9 with a resistance equal to the sum of the resistances R8 and R70 of the OOS circuit, apply it to the inverting input of the op-amp DA1 ... Then the output voltage of the UMZCH will increase by the sum of the voltage drops on the AC wires, which is tantamount to eliminating the effect of their resistance on the damping coefficient and the level of interface distortion at the AC terminals. Compensation for the drop on the resistance of the AC wires of the nonlinear component of the back-emf of loudspeakers is especially needed at the low frequencies of the audio range. The signal voltage at the HF loudspeaker is limited by a resistor and capacitor connected in series with it. Their complex impedance is much greater than the resistance of the wires of the AC cable, so compensation for this resistance at HF \u200b\u200bis meaningless. Based on this, the R11C4 integrating circuit limits the operating frequency band of the compensator to 22 kHz.
It should be especially noted: the resistance of the "hot" wire of the AC cable can be compensated by covering its general OOS by connecting the right terminal R70 with a special wire to the "hot" AC terminal. In this case, only the resistance of the "cold" AC wire needs to be compensated, and the gain of the wire resistance compensator must be reduced to the value of Ku \u003d -1 by choosing the resistance of the resistor R10 equal to the resistance of the resistor R11.
The overcurrent protection unit prevents damage to the output transistors during short circuits in the load. Resistors R53 - R56 and R57 - R60 serve as a current sensor, which is quite enough. The flow of the amplifier output current through these resistors creates a voltage drop that is applied to the divider R41R42. A voltage with a value greater than the threshold opens the transistor VT10, and its collector current opens the VT8 trigger cell VT8VT9. This cell goes into a steady state with open transistors and shunts the HL1VD8 circuit, reducing the current through the zener diode to zero and blocking VT3. Discharging C21 with a small base current VT3 can take several milliseconds. After the trigger cell is triggered, the voltage on the lower plate of C23, charged with the voltage on the LED HL1 to 1.6 V, rises from a level of -7.2 V from the positive power supply rail of the UN to a level of -1.2 B1, the voltage on the upper plate of this capacitor also increases by 5 V. C21 is quickly discharged through the resistor R30 to C23, the transistor VT3 is locked. In the meantime, VT6 opens and through R33, R36 opens VT7. VT7 shunts the Zener diode VD9, discharges capacitor C22 through R31 and locks the transistor VT5. Not receiving a bias voltage, the output stage transistors are also turned off.
Restoring the initial state of the trigger and turning on the UMZCH is done by pressing the SA1 "Reset protection" button. C27 is charged with the VT9 collector current and bypasses the VT8 base circuit, locking the trigger cell. If by this moment the emergency situation has been eliminated and VT10 is locked, the cell goes into a state with stably closed transistors. VT6, VT7 are closed, reference voltage is supplied to the bases VT3, VT5 and the amplifier enters the operating mode. If a short circuit in the UMZCH load continues, the protection is triggered again, even if the capacitor C27 is connected to SA1. The protection works so effectively that during the work on adjusting the correction, the amplifier was de-energized several times for small soldering by touching the non-inverting input. The emerging self-excitation led to an increase in the current of the output transistors, and the protection turned off the amplifier. Although this crude method should not be generally suggested, due to the overcurrent protection it did not harm the output transistors.
The work of the compensator of the resistance of the AC cables
The efficiency of the UMZCH VV-2008 compensator was tested by the old audiophile method, by ear, by switching the compensator input between the compensating wire and the common wire of the amplifier. The improvement in sound was clearly noticeable, and the future owner was eager to get an amplifier, so no measurements of the effect of the compensator were carried out. The advantages of the "cable stripper" circuit were so obvious that the "compensator + integrator" configuration was adopted as a standard unit for installation in all amplifiers under development.
It's amazing how much unnecessary controversy has flared up on the Internet about the usefulness / unnecessaryness of cable resistance compensation. As usual, those to whom the extremely simple cable stripper circuit seemed complicated and incomprehensible, the costs for it were exorbitant, and the installation was laborious © especially insisted on listening to a nonlinear signal. There were even suggestions that, since so much money is spent on the amplifier itself, it is a sin to save on a saint, but you need to go the best, glamorous way that all civilized mankind walks and ... get normal, human © super-expensive cables made of precious metals. To my great surprise, the statements of highly respected specialists about the uselessness of the compensation unit at home, including those specialists who successfully use this unit in their amplifiers, added fuel to the fire. It is very regrettable that many fellow radio amateurs reacted with distrust to the reports of improving the sound quality at low and mid frequencies with the inclusion of a compensator, they did their best to avoid this simple way of improving the work of the UMZCH, than robbed themselves.
A little research has been done to document the truth. A number of frequencies were fed from the GZ-118 generator to the UMZCH VV-2010 in the vicinity of the resonant frequency of the AC, the voltage was monitored by an S1-117 oscilloscope, and Kr at the AC terminals was measured by INI S6-8, Fig. 4. Checking the effectiveness of the resistance of the wires Resistor R1 is installed in order to avoid pickups at the input of the compensator during its switching between the control and common wire. The experiment used common and commonly available AC cables 3 m long and 6 sq. M. mm, as well as the GIGA FS Il speaker system with a frequency range of 25-22000 Hz, a nominal impedance of 8 ohms and a nominal power of 90 W from Acoustic Kingdom.
Unfortunately, the circuitry of the harmonic signal amplifiers from the C6-8 composition provides for the use of high-capacity oxide capacitors in the OOS circuits. This leads to the influence of low-frequency noise of these capacitors on the resolution of the device at low frequencies, as a result of which its resolution at low frequencies deteriorates. When measuring Kr signal with a frequency of 25 Hz from GZ-118 directly to C6-8, the readings of the device dance around the value of 0.02%. It is not possible to bypass this limitation using the notch filter of the GZ-118 generator in the case of measuring the efficiency of the compensator, since a number of discrete values \u200b\u200bof the tuning frequencies of the 2T filter is limited at low frequencies to 20, 60, 120, 200 Hz and does not allow us to measure Kr at the frequencies of interest to us. Therefore, reluctantly, the level of 0.02% was accepted as zero, the reference.
At a frequency of 20 Hz with a voltage at the AC terminals of 3 V peak, which corresponds to an output power of 0.56 W at a load of 8 ohms, Kr was 0.02% with the compensator turned on and 0.06% after turning it off. At a voltage of 10 V amps, which corresponds to an output power of 6.25 W, the Kr value is 0.02% and 0.08%, respectively, at a voltage of 20 V amps and a power of 25 W - 0.016% and 0.11%, and at a voltage of 30 In amplitude and power of 56 W - 0.02% and 0.13%.
Knowing the lightweight attitude of the manufacturers of imported equipment to the values \u200b\u200bof the inscriptions regarding the power, and also remembering the miraculous, after the adoption of Western standards, the transformation of a speaker system with a subwoofer power of 30 W into, long-term power of more than 56 W was not supplied to the AC.
At a frequency of 25 Hz at a power of 25 W Kr was 0.02% and 0.12% with the compensation unit on / off, and at a power of 56 W - 0.02% and 0.15%.
At the same time, the necessity and effectiveness of the coverage of the output low-pass filter of the general OOS was checked. At a frequency of 25 Hz at a power of 56 W and connected in series to one of the wires of the AC output RL-RC low-pass filter, similar to that installed in the super-linear UMZCH, Kr with the compensator turned off reaches 0.18%. At a frequency of 30 Hz at a power of 56 W Kr 0.02% and 0.06% with the compensation unit on / off. At a frequency of 35 Hz at a power of 56 W Kr 0.02% and 0.04% with the compensation unit on / off. At frequencies of 40 and 90 Hz at a power of 56 W Kr 0.02% and 0.04% with the compensation unit on / off, and at a frequency of 60 Hz -0.02% and 0.06%.
The conclusions are clear. The presence of nonlinear signal distortions at the AC terminals is observed. Deterioration of the linearity of the signal at the AC terminals with its inclusion through the uncompensated, not covered by the OOS resistance of the low-pass filter, containing 70 cm of a relatively thin wire, is clearly recorded. The dependence of the distortion level on the power supplied to the AC suggests that it depends on the ratio of the signal power to the nominal power of the speaker woofers. Distortion is most pronounced at frequencies near the resonant frequency. The back-EMF generated by the speakers in response to the sound signal is shunted by the sum of the output resistance of the UMZCH and the resistance of the wires of the AC cable, so the level of distortion at the AC terminals directly depends on the resistance of these wires and the output resistance of the amplifier.
The cone of a poorly damped subwoofer itself emits noise and, in addition, this loudspeaker generates a wide tail of THD and intermodulation products that the midrange loudspeaker reproduces. This explains the deterioration in the sound at midrange.
Despite the assumption of a zero Kr level of 0.02% adopted due to the imperfection of the ISI, the influence of the cable resistance compensator on signal distortions on the AC is clearly and unambiguously noted. It can be stated that the conclusions made after listening to the operation of the compensation unit on a musical signal and the results of instrumental measurements can be stated.
The improvement, clearly audible when the cable stripper is turned on, can be explained by the fact that when the distortion at the AC terminals disappears, the mid-range loudspeaker stops reproducing all this dirt. Apparently, therefore, by reducing or eliminating the reproduction of distortions by a mid-frequency loudspeaker, a two-cable circuit for switching on the speaker, the so-called. "Bi-wiring", when the LF and MF-HF links are connected by different cables, has an advantage in sound compared to a single-cable scheme. However, since in a two-cable circuit the distorted signal at the terminals of the AC low-frequency section does not disappear anywhere, this circuit loses to the version with a compositor in terms of the coefficient of damping of free vibrations of the woofer cone.
Physics cannot be fooled, and for decent sound it is not enough to get brilliant performance at the amplifier output with an active load, but it is also necessary not to lose linearity after the signal is delivered to the speaker terminals. As part of a good amplifier, a compensator made according to one scheme or another is absolutely necessary.
Integrator
We also tested the efficiency and the possibility of reducing the error of the integrator on the DA3. In UMZCH BB with op-amp TL071, the output constant voltage is within 6 ... 9 mV and it was not possible to reduce this voltage by including an additional resistor in the non-inverting input circuit.
The effect of low-frequency noise, typical for op amps with a DC input, due to deep OOS coverage through the R16R13C5C6 frequency-dependent circuit, manifests itself in the form of an instability of the output voltage of several millivolts, or -60 dB relative to the output voltage at rated output power, at frequencies below 1 Hz not reproducible speakers.
On the Internet, it was mentioned about the low resistance of the protective diodes VD1 ... VD4, which supposedly introduces an error in the operation of the integrator due to the formation of the divider (R16 + R13) / R VD2 | VD4 .. To check the reverse resistance of the protective diodes, a circuit was assembled in Fig. 6. Here OA DA1, connected according to the inverting amplifier circuit, is covered by OOS through R2, its output voltage is proportional to the current in the circuit of the tested diode VD2 and the protective resistor R2 with a coefficient of 1 mV / nA, and the resistance of the R2VD2 circuit - with a coefficient of 1 mV / 15 GΩ ... To exclude the influence of the op-amp's additive errors - bias voltage and input current - on the results of diode leakage current measurements, it is only necessary to calculate the difference between the op-amp's own voltage measured without the tested diode and the op-amp output voltage after its installation. In practice, the difference in the output voltages of the op-amp of several millivolts gives the value of the reverse resistance of the diode on the order of ten to fifteen gigaohms at a reverse voltage of 15 V. Obviously, the leakage current will not increase with a decrease in the voltage across the diode to a level of several millivolts, characteristic of the differential voltage of the op-amp integrator and compensator ...
But the photoelectric effect inherent in diodes placed in a glass case really leads to a significant change in the output voltage of the UMZCH. When they were illuminated with a 60 W incandescent lamp from a distance of 20 cm, the constant voltage at the UMZCH output increased to 20 ... 3O mV. Although it is unlikely that a similar level of illumination can be observed inside the amplifier case, a drop of paint applied to these diodes eliminated the dependence of the UMZCH modes on illumination. According to the simulation results, the drop in the frequency response of the UMZCH is not observed even at a frequency of 1 millihertz. But you should not decrease the time constant R16R13C5C6. The phases of alternating voltages at the outputs of the integrator and the compensator are opposite, and with a decrease in the capacitance of the capacitors or the resistance of the integrator resistors, an increase in its output voltage can worsen the compensation of the AC cable resistance.
Comparison of the sound of amplifiers. The sound of the assembled amplifier was compared with the sound of several foreign industrial amplifiers. The source was a Cambridge Audio CD-player, a preamplifier was used to drive and adjust the sound level of the terminal UMZCH, the Sugden A21a and NAD C352 used standard controls.
The first to check the legendary, shocking and devilishly expensive English UMZCH "Sugden A21a", operating in class A with an output power of 25 watts. What is noteworthy, in the accompanying documentation on the VCL, the British considered it a blessing not to indicate the level of nonlinear distortions. Say, it's not about distortions, but about spirituality. "Sugden А21а\u003e" lost to UMZCH VV-2010 with comparable power both in level and clarity, confidence, nobility of sounding at low frequencies. This is not surprising, given the peculiarities of its circuitry: just a two-stage quasi-symmetric output follower on transistors of the same structure, assembled according to the circuitry of the 70s of the last century with a relatively high output resistance and an electrolytic capacitor connected at the output that further increases the total output impedance - this is the last the solution itself degrades the sound of any amplifiers at low and mid frequencies. At mid and high frequencies, the UMZCH BB showed higher detail, transparency and excellent stage development, when singers, instruments could be clearly localized in sound. By the way, speaking of the correlation between objective measurement data and subjective impressions of sound: in one of the magazine articles of Sugden's competitors, its Kr was determined at 0.03% at a frequency of 10 kHz.
The next was also the English amplifier NAD C352. The general impression was the same: the pronounced "bucket" sound of the Englishman on the bass did not leave him any chance, while the work of the UMZCH BB was recognized as impeccable. Unlike NADa, whose sound was associated with thick bushes, wool, cotton wool, the sound of BB-2010 at mid and high frequencies made it possible to clearly distinguish the voices of performers in the general choir and instruments in the orchestra. In the work of NAD C352, the effect of better audibility of a more vociferous performer, a louder instrument was clearly expressed. As the owner of the amplifier himself put it, in the sound of the UMZCH BB, the vocalists did not "shout" to each other, and the violin did not fight in the power of sound with a guitar or a trumpet, but all the instruments peacefully and harmoniously "were friends" in the overall sound image of the melody. At high frequencies, UMZCH VV-2010, according to figuratively thinking audiophiles, sounds like "as if drawing a sound with a thin-thin brush." These effects can be attributed to the difference in intermodulation distortion of the amplifiers.
The sound of the UMZCH Rotel RB 981 was similar to the sound of the NAD C352, except for better performance at low frequencies, yet the UMZCH VV-2010 in the clarity of AC control at low frequencies, as well as transparency, delicacy of sound at mid and high frequencies remained unrivaled.
The most interesting thing in terms of understanding the way of thinking of audiophiles was the general opinion that, despite the superiority over these three UMZCH, they bring “warmth” to the sound, which makes it more pleasant, and the UMZCH BB works smoothly, “it is neutral to the sound”.
The Japanese Dual CV1460 lost in sound immediately after switching on in the most obvious way for everyone, and they did not waste time on listening to it in detail. Its Kr was in the range of 0.04 ... 0.07% at low power.
The main impressions from the comparison of amplifiers in the main features were completely identical: the UMZCH BB was ahead of them in sound unconditionally and unambiguously. Therefore, further testing was deemed unnecessary. As a result, friendship won, everyone got what they wanted: for a warm, intimate sound - Sugden, NAD and Rotel, and to hear the director recorded on a disc - UMZCH VV-2010.
Personally, I like the UMZCH of high fidelity with its light, clean, impeccable, noble sound, it playfully reproduce passages of any complexity. As my acquaintance, an audiophile with great experience, put it, he works out the sounds of drums at low frequencies without options, like a press, in the middle it sounds as if it does not exist, and at high frequencies it seems to draw a sound with a thin brush. For me, the non-stressing sound of the UMZCH BB is associated with the ease of operation of the cascades.
