Long ago, two years ago, I bought an old soviet speaker 35GD-1. Despite its initially poor condition, I repaired it, painted it a nice blue, and even made a plywood box for it. A large box with two bass reflexes greatly improved its acoustic qualities. There remains a case for good amplifier that will download this column. I decided to do it differently than most people do - buy a ready-made D-class amplifier from China and install it. I decided to make an amplifier myself, but not some generally accepted one on the TDA7294 microcircuit, and indeed not on a microcircuit at all, and not even the legendary Lanzar, but a very rare amplifier on field effect transistors... Yes, and there is very little information on the network about amplifiers on field workers, so it became interesting what it is and how it sounds.

Assembly

This amplifier has 4 pairs of output transistors. 1 pair - 100 watts of output power, 2 pairs - 200 watts, 3 - 300 watts and 4, respectively, 400 watts. I don't need all 400 watts yet, but I decided to put all 4 pairs in order to distribute the heat and reduce the power dissipated by each transistor.

The diagram looks like this:

On the diagram, exactly those component denominations are signed, which are installed by me, the diagram is checked and works properly. I am attaching the printed circuit board. Lay6 board.

Attention! All power tracks must be tinned with a thick layer of solder, since a very large current will flow through them. We solder carefully, without snot, wash the flux. Power transistors must be installed on a heat sink. The advantage of this design is that the transistors can not be isolated from the radiator, but sculpted all on one. Agree, this greatly saves mica heat-conducting pads, because 8 transistors would take 8 pieces of them (surprising, but true)! The radiator is the common drain of all 8 transistors and the audio output of the amplifier, so when installing in the case, do not forget to somehow isolate it from the case. Despite the absence of the need to install mica gaskets between the flanges of the transistors and the radiator, this place must be smeared with thermal grease.

Attention! It is better to check everything right away before installing the transistors on the radiator. If you screw the transistors to the radiator, and there are any snot or non-soldered contacts on the board, it will be unpleasant to unscrew the transistors again and get smeared with thermal grease. So check everything at once.

Bipolar Transistors: T1 - BD139, T2 - BD140. It also needs to be screwed to the radiator. They don't get very hot, but they still get warm. They also need not be isolated from heat sinks.

So, we proceed directly to the assembly. The parts are located on the board as follows:

Now I am attaching a photo of the different stages of the amplifier assembly. First, cut out a piece of PCB to fit the board.

Then we overlay the image of the board on the textolite and drill holes for the radio components. We sand and degrease. We take a permanent marker, stock up on a fair amount of patience and draw paths (I don’t know how to do LUT, so I suffer).

We arm ourselves with a soldering iron, take flux, solder and tinker.

We wash off the remnants of the flux, take a multimeter and call for a short circuit between the tracks where it should not be. If everything is normal, we proceed to the installation of the parts.
Possible replacements.
First, I'll attach a parts list:
C1 = 1u
C2, C3 = 820p
C4, C5 = 470u
C6, C7 = 1u
C8, C9 = 1000u
C10, C11 = 220n

D1, D2 = 15V
D3, D4 = 1N4148

OP1 = KR54UD1A

R1, R32 = 47k
R2 = 1k
R3 = 2k
R4 = 2k
R5 = 5k
R6, R7 = 33
R8, R9 = 820
R10-R17 = 39
R18, R19 = 220
R20, R21 = 22k
R22, R23 = 2.7k
R24-R31 = 0.22

T1 = BD139
T2 = BD140
T3 = IRFP9240
T4 = IRFP240
T5 = IRFP9240
T6 = IRFP240
T7 = IRFP9240
T8 = IRFP240
T9 = IRFP9240
T10 = IRFP240

The first step is to replace the operational amplifier with any other, even imported, with the same pinout. Capacitor C3 is needed to suppress self-excitation of the amplifier. You can put more, which I did later. Any Zener diodes for 15 V and with a power of 1 W. Resistors R22, R23 can be set based on the calculation R = (Upit.-15) / Ist., Where Upit. - supply voltage, Ist. - stabilization current of the zener diode. Resistors R2, R32 are responsible for the gain. With these ratings, it is somewhere between 30 - 33. Capacitors C8, C9 - filter capacities - can be set from 560 to 2200 uF with a voltage not lower than Usup. * 1.2 so as not to exploit them at the limit of possibilities. Transistors T1, T2 - any complementary pair of average power, with a current of 1 A, for example, our KT814-815, KT816-817 or imported BD136-135, BD138-137, 2SC4793-2SA1837. Source resistors R24-R31 can be set at 2 W, albeit undesirable, with a resistance of 0.1 to 0.33 ohms. It is not advisable to change the power keys, although you can also IRF640-IRF9640 or IRF630-IRF9630; it is possible for transistors with similar passed currents, gate capacities and, of course, the same pin layout, although if you solder on the wires, this does not matter. There seems to be nothing more to change here.

First launch and setup.

We start the amplifier for the first time through a safety lamp to break the 220 V network. Be sure to short the input to ground and do not connect the load. At the moment of switching on, the lamp should flash and go out, and go out completely: the spiral should not glow at all. We turn it on, hold it for 20 seconds, then turn it off. We check if there is anything heating up (although if the lamp is off, it is unlikely that anything is heating up). If nothing really heats up, turn it on again and measure the constant voltage at the output: it should be in the range of 50 - 70 mV. For example, I have 61.5 mV. If everything is within the normal range, we connect the load, give a signal to the input and listen to music. There should be no interference, extraneous hums, etc. If none of this is present, proceed to setting up.

Setting up the whole thing is extremely simple. It is only necessary to set the quiescent current of the output transistors by rotating the trimming resistor slider. It should be approximately 60 - 70 mA for each transistor. This is done in the same way as on Lanzar. The quiescent current is calculated according to the formula I = Upfall / R, where Upfall. - voltage drop across one of the resistors R24 - R31, and R is the resistance of this very resistor. From this formula, we derive the voltage drop across the resistor required to set such a quiescent current. Upad. = I * R. For example, in my case it is = 0.07 * 0.22 = somewhere 15 mV. The quiescent current is set on a “warm” amplifier, that is, the radiator must be warm, the amplifier must play for a few minutes. The amplifier warmed up, disconnect the load, short-circuit the input to the common one, take the multimeter and carry out the previously described operation.

Characteristics and features:

Supply voltage - 30-80 V
Working temperature - up to 100-120 degrees.
Load resistance - 2-8 Ohm
Amplifier power - 400 W / 4 Ohm
SOI - 0.02-0.04% at a power of 350-380 W
Gain - 30-33
Frequency Response Range - 5-100000 Hz

It is worth dwelling on the last point in more detail. Using this amplifier with noisy tone blocks such as the TDA1524 may result in the amplifier's seemingly unreasonable power consumption. In fact, this amplifier reproduces interference frequencies that are inaudible to our ears. It may seem that this is self-excitation, but most likely it is precisely interference. Here it is worth distinguishing between interference that is not audible to the ear from real self-excitation. I ran into this problem myself. Originally used as a pre-amplifier, the TL071 opamp. This is a very good high-frequency imported op-amp with low-noise field-effect transistor output. It can operate at frequencies up to 4 MHz - this is more than enough for reproducing interference frequencies and for self-excitation. What to do? One good person, thank him very much, advised me to replace the opamp with another, less sensitive and reproducing a smaller frequency range, which simply cannot work at the self-excitation frequency. Therefore, I bought our domestic KR544UD1A, installed it and ... nothing has changed. All this prompted me to think that the variable resistors of the timbre block are making noise. The resistor motors make a little “rustling” noise, which causes interference. I removed the timbre block and the noise disappeared. So it's not self-agitation. With this amplifier, you need to put a low-noise passive timbre block and a transistor preamplifier in order to avoid the above.

A few words about installation errors:
In order to improve the readability of the circuits, consider a power amplifier with two pairs of terminal field-effect transistors and a power supply of ± 45 V.
As the first error, let's try to "solder" the Zener diodes VD1 and VD2 with the wrong polarity (the correct connection is shown in Figure 11). The stress map will take the form shown in Figure 12.

Figure 11 The pinout of the BZX84C15 zener diodes (however, the pinout is the same on the diodes).

Figure 12 Voltage map of the power amplifier with incorrect installation of the zener diodes VD1 and VD2.

These zener diodes are needed to form the supply voltage of the operational amplifier and are selected at 15 V solely because this voltage is optimal for this operational amplifier. The amplifier also retains its performance without loss of quality when using nearby ratings - 12 V, 13 V, 18 V (but not more than 18 V). In case of incorrect installation, instead of the prescribed supply voltage, the op-amp receives only a drop voltage n-p junction stabilitrons. The rest current is regulated normally, a small constant voltage is present at the output of the amplifier, there is no output signal.
It is also possible that the VD3 and VD4 diodes are not correctly installed. In this case, the quiescent current is limited only by the values ​​of the resistors R5, R6 and can reach a critical value. There will be a signal at the output of the amplifier, but a rather rapid heating of the terminal transistors will definitely lead to their overheating and the output of the amplifier. The voltage and current map for this error is shown in Figures 13 and 14.

Figure 13 Voltage map of the amplifier with incorrect installation of thermal stabilization diodes.

Figure 14 Amplifier current map with incorrect installation of thermal stabilization diodes.

The next popular wiring mistake can be incorrect wiring of the penultimate stage transistors (drivers). In this case, the voltage map of the amplifier takes the form shown in Figure 15. In this case, the terminal transistors are completely closed and there is no sound at the amplifier output, and the constant voltage level is as close to zero as possible.

Figure 15 Voltage map with incorrect mounting of the driver stage transistors.

Further, the most dangerous mistake is that the transistors of the driver stage are beguiled in places, and the pinout is also beguiled, as a result of which the one attached to the terminals of the transistors VT1 and VT2 is correct and they work in the mode of emitter followers. In this case, the current through the terminal stage depends on the position of the trimming resistor slider and can be from 10 to 15 A, which in any case will cause an overload of the power supply and rapid heating of the terminal transistors. Figure 16 shows the currents at the middle position of the trimmer.

Figure 16 Current map with incorrect installation of the driver stage transistors, the pinout is also confused.

It is unlikely that it will be possible to solder the outputs of the terminal field-effect transistors IRFP240 - IRFP9240 "vice versa", but it turns out to swap them quite often. In this case, the diodes installed in the transistors are obtained in a difficult situation - the voltage applied to them has a polarity corresponding to their minimum resistance, which causes the maximum consumption from the power supply and how quickly they burn out depends more on luck than on the laws of physics.
Fireworks on the board can happen for another reason - 1.3 W zener diodes are on sale in a housing the same as that of 1N4007 diodes, so before installing zener diodes in the board, if they are in a black housing, you should take a closer look at the inscriptions on the housing. When installing instead of zener diodes, the supply voltage of the operational amplifier is limited only by the values ​​of the resistors R3 and R4 and the current consumption of the operational amplifier itself. In any case, the resulting voltage value is much higher than the maximum supply voltage for a given op-amp, which leads to its failure, sometimes with the removal of a part of the op-amp itself, and then a constant voltage may appear at its output, close to the amplifier's supply voltage, which will entail the appearance of a constant voltage at the output of the power amplifier itself. As a rule, the final stage in this case remains operational.
And finally, a few words about the values ​​of the resistors R3 and R4, which depend on the supply voltage of the amplifier. 2.7 kOhm is the most universal, however, when the amplifier is powered with a voltage of ± 80 V (only for an 8 Ohm load), these resistors will dissipate about 1.5 W, so it must be replaced with a 5.6 kOhm or 6.2 kOhm resistor, which will reduce the heat output to 0.7 W.

EK B BD135; BD137

GIS IRF240 - IRF9240

This amplifier deservedly found its fans and began to acquire new versions. First of all, the circuit for generating the bias voltage of the first transistor stage has undergone a change. In addition, overload protection was introduced into the circuit.
As a result of improvements, the schematic diagram of a power amplifier with field-effect transistors at the output has acquired the following form:

INCREASE

PCB options are shown in graphical format (needs to be scaled)

Appearance the resulting modification of the power amplifier is shown in the photos below:

It remains to splash a fly in the ointment into this barrel of honey ...
The fact is that the field-effect transistors IRFP240 and IRFP9240 used in the amplifier were discontinued by the developer International Rectifier (IR), which paid more attention to the quality of the products. The main problem with these transistors is that they were designed for use in power supplies, but they turned out to be quite suitable for sound amplification equipment. The increased attention to the quality of the manufactured components on the part of the International Rectifier made it possible, without making a selection of transistors, to connect several transistors in parallel without worrying about the differences in the characteristics of the transistors - the spread did not exceed 2%, which is quite acceptable.
Today, the IRFP240 and IRFP9240 transistors are manufactured by Vishay Siliconix, which is not so sensitive to the manufactured products and the parameters of the transistors have become suitable only for power supplies - the spread of the "gain box" of transistors of one batch exceeds 15%. This excludes parallel connection without preliminary selection, and the number of tested transistors for a choice of 4 is equal to several tens of copies.
In this regard, before assembling this amplifier, first of all, you should find out which company the transistors can be obtained from. If Vishay Siliconix is ​​on sale in your stores, then it is strongly recommended that you refuse to assemble this power amplifier - you run the risk of spending quite a lot and not achieving anything.
However, the work on the development of "VERSION 2" of this power amplifier and the lack of decent and not expensive field-effect transistors for the output stage made me think a little about the future of this circuitry. As a result, VERSION 3 was modeled, which uses a bipolar pair from TOSHIBA - 2SA1943 - 2SC5200 instead of field-effect transistors IRFP240 - IRFP9240 from Vishay Siliconix, which are still of quite decent quality today.
The schematic diagram of the new version of the amplifier has incorporated the modifications of "VERSION 2" and has undergone changes in the output stage, making it possible to abandon the use of field-effect transistors. The schematic diagram is shown below:

Schematic diagram using field-effect transistors as repeaters ENLARGE

V this option field-effect transistors have survived, but they are used as voltage followers, which significantly relieves the driver stage. A small positive connection is introduced into the protection system to avoid excitation of the power amplifier at the protection operation limit.
Printed circuit board in the development process, indicative results real measurement and workable printed circuit board will appear at the end of November, but for now you can offer the THD measurement graph obtained by MICROCAP. You can read more about this program.

This device allows you to connect a dynamic microphone, electric guitar and other signal sources with high output impedance to your computer sound card. The device does not introduce frequency distortions in the audio frequency range, as well as distortions associated with the nonlinearity of the amplifying device, since it is built according to the source follower circuit.

In other words, if you care even a little about the quality of the recorded sound, you have a good sound card and an expensive microphone, then this device is what you need.

A little about the scheme. The device starts working if a mono jack is inserted into the J1 connector, or, scientifically, a 6.35 mm (1/4 inch) plug. In this case, through the jack, the minus contact of the power supply battery is closed to the minus power supply and the device starts to work. Also, with the second contact of this plug, the input signal is fed to the resistor R1, which provides a high input resistance of the device. Capacitor C2 performs frequency correction by cutting frequencies above the audio range. Resistors R2-R4 provide the necessary bias at the gate of the field-effect transistor.

In this design, a KP303 field-effect transistor with an E index is used. When using a transistor with a different index, it may be necessary to reduce the values ​​of the resistors R3 and R4. Resistor R5 is the load of the amplifier stage, from which sound signal removed by capacitor C5 and fed through resistor R7 to the input sound card computer.

The VD1 diode in the circuit performs the function of foolproofing against accidental polarity reversal, since the design features of the "Krona" battery connector do not exclude this possibility. It is better to use a germanium diode, since the voltage drop across it will be less. But this is not at all critical, it can be replaced with any low-power silicon diode, for example KD521, KD522, 1N4148, etc.

The device is assembled on a board made of single-layer PCB with dimensions of 47x26mm. The board tracing in the Dip Trace program will be shown below. But you can do without making a board, and collect everything on a universal circuit board(this is the one with a bunch of holes) the same size.

The body of the device is made of a single-layer PCB for complete shielding of the amplifier.

The dimensions of its parts are as follows:
- side walls 60x50 mm - 2 pieces
- front wall 50x30 mm - 1 piece
- back wall 46x30 mm - 1 piece. The size of 46 mm is not critical, it can vary from 50 mm to 35 mm. It all depends on how you want to install the battery.
- bottom and intermediate walls 55x30 mm

The walls of the case are soldered together with solder. The foil on all walls should be inside the case. Try not to overheat the PCB, as the foil can peel off easily.

First of all, all the walls are soldered together, except for the back. Then holes are drilled for a jack connector with a diameter of 10 mm, a hole for power wires, about 3 mm in diameter and the same in the back wall for a shielded wire with a minijack.

Also, at the point of attachment of the back wall, a brace of thick copper wire is soldered into which the bottom of the back wall will be inserted.

After that, you will need to glue the connector for the "Crown". By the way, you can take it from the already spent crown, as I always do. This connector is glued with hot glue to the back of the front wall. It is important that none of the connector pins touch the case foil.

After that, the power wires and the third wire connecting the housing foil and the circuit “ground” are soldered to the circuit. A shielded output wire is also soldered, the circuit is installed in the case and the back wall is sealed at the top of the sides.

Below are schematic diagrams and articles on the topic "ULF on field-effect transistors" on the site on radio electronics and radio hobby site.

What is "ULF on field effect transistors" and where is it used, schematic diagrams homemade devices which relate to the term "ULF on field effect transistors".

An electronic schematic diagram of a simple high quality amplifier power AF for 20 watts, made entirely on transistors, at the output - field-effect transistors KP904. Simple and powerful amplifier low frequency with an output stage on field-effect transistors KP912. The maximum output power is 65 watts. A schematic diagram of a broadband AF power amplifier (UMZCH), made according to a symmetrical scheme on KP904 field-effect transistors, is presented. In amateur radio practice, the AF power amplifier (UMZCH), made according to a symmetrical scheme, has become widespread. Complementary bipolar transistors of its input stage are connected according to the scheme of a push-pull differential amplifier, and the next one according to the scheme ... Schematic diagram of a power amplifier with MOS transistors in the output stage, the power is about 12W. The diagram is shown in the following figure. Its main technical characteristics ... The class AB audio power amplifier described in this article uses a pair of complementary MOSFETs in the output stage. This feature makes it possible to improve performance in comparison with the equivalent output stage on bipolar ... The construction of audio frequency power amplifiers (UMZCH) on field-effect transistors attracts developers with the possibility of achieving "tube" softness of sound (the current-voltage characteristics of field-effect transistors are very similar to those of vacuum tubes) ... Karel Barton built his High-End UMZCH on field-effect transistors with a hexagonal structure (HEXFET from International Rectifier). The input stages are made on discrete bipolar transistors using symmetric differential cascode circuitry ... Endre Pireta's "field" UMZCH is noticeably simple, but also meets the standards of high-quality sound reproduction. The input stage was originally solved (without the usual differential amplifiers) - this is a push-pull complementary stage ... in a conventional resistive amplifier stage with an OE and a blocking capacitor, theoretically ... UMZCH circuit, developed by Matt Tucker. The first differential stage is made on bipolar transistors Q1Q5 according to typical scheme with a current mirror Q7Q8 in the load, and the voltage amplification stage - on Q9Q13 with an OE and a load on the Q6Q2 current generator ... This design is a development upgrade. Principled UMZCH scheme on MOSFET transistors (200W). All the main parts of the amplifier - transformer, radiators ... Several schematic diagrams high-quality UMZCH on field-effect transistors, attracting with their simplicity and technical characteristics... The use of field-effect transistors in a power amplifier can significantly improve sound quality with a general simplification of the circuit ...

Old but golden

Old but golden

Amplifier circuitry has already gone through a spiral in its development and now we are witnessing a "tube renaissance". In accordance with the laws of dialectics, which were so persistently hammered into us, a "transistor renaissance" should follow. The very fact of this is inevitable, for lamps, for all their beauty, are very inconvenient. Even at home. But transistor amplifiers have accumulated their disadvantages ...
The reason for the "transistor" sound was explained back in the mid-70s - deep feedback. It creates two problems at once. The first is transient intermodulation distortion (TIM distortion) in the amplifier itself, caused by signal delay in the feedback loop. There is only one way to deal with this - by increasing the speed and gain of the original amplifier (without feedback), which is fraught with a serious complication of the circuit. The result is difficult to predict: whether it will be, or not.
The second problem is that deep feedback greatly reduces the amplifier's output impedance. And this, for most loudspeakers, is fraught with the appearance of those very intermodulation distortions directly in the dynamic heads. The reason is that when the coil is moved in the gap of the magnetic system, its inductance changes significantly, so the impedance of the head also changes. With a low output impedance of the amplifier, this leads to additional changes in the current through the coil, which generates unpleasant overtones, which are mistaken for amplifier distortion. This can also explain the paradoxical fact that with an arbitrary choice of speakers and amplifiers, one set “sounds” and the other “does not sound”.

the secret of tube sound =
high output impedance of the amplifier
+ shallow feedback .
However, similar results can be achieved with transistor amplifiers... All the schemes given below are united by one thing - unconventional and now forgotten "asymmetric" and "incorrect" circuitry. However, is it as bad as it is presented? For example, a bass reflex with a transformer is a real Hi-End! (Fig. 1) And the phase inverter with shared load (Fig. 2) is borrowed from the tube circuitry ...
fig. 1

fig. 2

fig. 3

These schemes are now undeservedly forgotten. But in vain. On their basis, using a modern element base, you can create simple amplifiers with a very high quality sound. In any case, what I happened to collect and listen to sounded worthy - soft and "tasty". The depth of feedback in all circuits is small, there is local OOS, and the output impedance is significant. There is also no general OOS for direct current.

However, the above schemes work in the class B, therefore, they are characterized by "switching" distortions. To eliminate them, it is necessary to operate the output stage in a "pure" class A... And such a scheme also appeared. The author of the scheme is J.L. Linsley Hood. The first mentions in domestic sources date back to the second half of the 70s.


fig. 4

The main disadvantage of class amplifiers A, limiting the area of ​​their application - a large quiescent current. However, there is another way to eliminate switching distortions - the use of germanium transistors. Their advantage is low distortion in the mode B. (Someday I will write a saga dedicated to Germany.) Another question is that it is not easy to find these transistors now, and the choice is limited. When repeating the following constructions, it must be remembered that the thermal resistance of germanium transistors is low, so there is no need to save on radiators for the output stage.

fig. 5

This diagram shows an interesting symbiosis of germanium transistors with a field one. The sound quality, despite the more than modest characteristics, is very good. To refresh the impressions of a quarter of a century ago, I was not too lazy to assemble the structure on a model, slightly modernizing it to match the modern denominations of parts. The MP37 transistor can be replaced with silicon KT315, since when establishing it, you will still have to select the resistance of the resistor R1. When working with a load of 8 ohms, the power will increase to about 3.5 W, the capacity of the capacitor C3 will have to be increased to 1000 μF. And to work with a 4 ohm load, you will have to reduce the supply voltage to 15 volts so as not to exceed the maximum power dissipation of the output stage transistors. Since there is no general DC feedback, thermal stability is sufficient only for home use.

The following two schemes have interesting feature... The transistors of the AC output stage are connected in a common emitter circuit, therefore they require a small excitation voltage. The traditional voltage boost is also not required. However, for direct current, they are connected according to a common collector circuit, so a "floating" power supply, not connected to ground, is used to power the output stage. Therefore, a separate power supply must be used for the output stage of each channel. In the case of switching voltage converters, this is not a problem. The power supply for the prestages can be shared. The DC and AC OOS circuits are separated, which, in combination with the quiescent current stabilization circuit, guarantees high thermal stability at a shallow AC OOS depth. For MF / HF channels this is an excellent circuit.

fig. 6


fig. 7 Author: A.I.Shikhatov (compilation and comments) 1999-2000
Published: collection "Designs and circuits for reading with a soldering iron" M. Solon-R, 2001, p.19-26.

• Schemes 1,2,3,5 were published in the magazine "Radio".
• Scheme 4 borrowed from the collection
  V.A. Vasiliev "Foreign amateur radio designs"M.Radio and communication, 1982, p. 14 ... 16
• Schemes 6 and 7 are borrowed from the collection
  J. Bozdeh "Construction additional devices to tape recorders "(translated from Czech) M. Energoizdat 1981, p. 148,175
• Details about the mechanism of intermodulation distortion: Should the UMZCH have a low output impedance?

Table of contents

UMZCH on field-effect transistors

UMZCH on field-effect transistors

The use of field-effect transistors in a power amplifier can significantly improve the sound quality with a general simplification of the circuit. The transfer characteristic of field-effect transistors is close to linear or quadratic, therefore there are practically no even harmonics in the spectrum of the output signal, in addition, there is a rapid drop in the amplitude of higher harmonics (as in tube amplifiers). This allows the use of a shallow negative feedback or abandon it altogether. After conquering the expanses of "home" Hi-Fi, field-effect transistors began an attack on car audio. The published circuits were originally intended for home systems, but maybe someone dares to apply the ideas embedded in them in the car ...


fig. 1
This scheme is already considered a classic one. In it, the output stage, operating in the AB mode, is made on MOS transistors, and the preliminary stages are on bipolar ones. The amplifier provides fairly high performance, but to further improve the sound quality, bipolar transistors should be completely excluded from the circuit (next picture).


fig. 2
After all the reserves for improving the sound quality have been exhausted, there is only one thing left - a single-ended output stage in "pure" class A. The current consumed by the preliminary stages from a higher voltage source in both this and the previous circuit is minimal.


fig. 3
The output stage with a transformer is a complete analogue of tube circuits. This is for a snack ... The integral current source CR039 sets the operating mode of the output stage.


fig. 4
However, a broadband output transformer is a rather complicated assembly to manufacture. An elegant solution - a current source in the drain circuit - proposed by the company