Modification of 6AC-2 to work with a non-standard amplifier

A. MARCHENKO, Uman, Cherkasy region, Ukraine

It often happens that the service life of acoustic systems (AS) significantly exceeds the life of a radio receiving or amplifying equipment, equipped with an external speaker. The author proposes to modify a two-way speaker of medium power to work with another industrial or amateur UMZCH.

Each loudspeaker of the 6AC-2 acoustic system (photo in Fig. 1) for the radio "Melodiya-101 stereo" and their modifications (104, 105), the electrophones "Melodiya-103" produced by the Riga Radio Plant named after Popov, consists of two direct radiation drivers installed in a housing made of 10 mm thick plywood. The loudspeaker heads are mounted in the housing coaxially relative to each other on a plastic base (without the front baffle board). The advantages of such an arrangement of heads are a flat, without dips, directional diagram, which cannot be achieved with spaced, even closely spaced heads. But on the other hand, all designs of such loudspeakers have one common drawback - due to the interference of sound waves emitted simultaneously by several heads, the total AFC of the sound pressure in the zone of their joint action acquires a number of local peaks and dips at medium and high frequencies. To eliminate this phenomenon, it is necessary to increase the slope of the frequency response of the crossover filters (which significantly complicates their design) or to bring the radiation centers of the heads closer together. The absence of a front reflective board entails certain losses due to the diffraction of sound waves - a type of distortion that affects the sound quality of any speaker system. This type of distortion manifests itself in the frequency band 100 ... 800 Hz and is a smooth decrease in the acoustic pressure created by the acoustic system below a certain frequency (for 6AC-2, this calculated frequency is 732 Hz). The actually measured value of the loss is 3 ... 4 dB.

To damp the moving system of the woofer at frequencies close to the frequency of its mechanical resonance, the entire free volume of the 6AC-2 speaker box is filled with cotton wool. In addition, the 10GD-34 head used in the AC is small in size. All this determined the low sensitivity of 6AC-2 and caused a drop in its frequency response in the low-frequency region (Fig. 2).

The specified features of the speaker system are compensated for in the frequency response of the ultrasonic frequency response of the low-frequency tract "Melodiya". To equalize the characteristics of the sound pressure of the speaker system, the radio amplifier has a significant rise in the low frequency region (at a frequency of 60 Hz - about 14 dB). The necessary increase in sound pressure provides
with an increase in the output power of the UMZCH (the maximum output power of the "Melody-101 stereo" radio is about 15 ... 20 W).

From the above, it follows that the 6AC-2 and the Melodiya amplifier are designed to work together. To connect the speaker to a non-standard signal source, you need to modify it.

Among the shortcomings, one should also note the vibration of the walls of the case and a significant unevenness of the amplitude-frequency response in the mid-frequency region. The latter is due to the fact that the 10GD-34 low-frequency head, which also performs the function of a mid-frequency emitter, has a sharp drop in the frequency response of the sound pressure from 4.5 kHz (Fig. 3). The high-frequency head ZGD-2 (its AFC is shown in Fig. 4) is connected through a first-order filter with a cutoff frequency of 10 kHz. It is quite simple to equalize the frequency response of the sound pressure at mid frequencies - to lower the cutoff frequency of the filter. In a similar speaker M. Korzinin installed a filter with a cutoff frequency of 4 kHz. However, in this case, the frequency of the main resonance of the HF head (4.5 kHz) is higher than the cutoff frequency, which is undesirable. The ZGD-2 head operating at the frequency of the main resonance cannot provide full-fledged high-quality sound. In addition, the proposed filter is laborious to manufacture (winding two inductors).

There is an easier option. To do this, the isolation capacitor for the HF head is installed with a capacity of 8.8 μF (film capacitors connected in parallel: two - 3.3 μF each and one - 2.2 μF with a nominal voltage of 400 V). Since such a refinement will bring the cutoff frequency very close to the frequency of the main resonance of the HF head, the latter is shunted with a 5.4 Ohm resistor. Such a shunt equalizes the frequency characteristics of the sensitivity of the heads, the electrical resistance and, most importantly, dampens the resonances of the HF head, including the main one. In this case, the HF head is connected in antiphase relative to the LF head (Fig. 5). In the described design, two series-connected 2.7 Ohm resistors are used. The total capacitance of the filter capacitors is calculated using an online calculator. When
this takes into account the total DC resistance of the 16 ohm head and 5.4 ohm shunt resistor. Thus, the ZGD-2 head will operate already at 4.5 kHz, providing full-fledged sound of the speakers in the entire 34 band. It should be noted that one of the design features of the ZGD-2 is the presence of a silk dome diffuser, which provides a relatively flat frequency response of sound pressure in band 1.5 ... 18 kHz.

To equalize the sound pressure of the acoustic system in the low-frequency region, a phase inverter (FI) is added to the speaker, following the example of I. Smirnov. The author used a plastic pipe with an inner diameter of 50 and a length of 100 mm. This decision is correct, since heads with a total Q-factor of no more than 0.6 are suitable for the manufacture of speakers with FI (for 10GD-34 it is equal to 0.45). The tuning frequency of such a FI is 90 Hz. This value is unacceptable, because the resonant frequency of the 10GD-34 head in open space is 80 Hz, to which it is easiest to tune the FI, and the optimal frequency of its tuning for this head is 35 Hz.

By experience, I have established the best frequency for tuning the FI - 55 Hz. Calculate the dimensions of the FI port using the computer program BassPort. Based on the calculations obtained, a hole with a diameter of 35 mm is cut out in the rear wall of the speaker cabinet (with a center spaced 50 ... 60 mm from the side surfaces of the housing) and a piece of cardboard tube with an inner diameter of 32 and a length of 130 mm is glued into it. On the back wall, a 50x50 mm board is also installed with filter elements mounted on the foil surface of the printed conductors. Felt 10 ... 15 mm thick or foam rubber is glued to the remaining free spaces. The filler (cotton wool) is removed.

The joints of the walls are sealed, and the walls themselves are covered from the inside with a self-adhesive vibroplast 1.5 mm thick or linoleum, and the LF head is provided with a soft mount (to reduce the vibrations of the walls of the case). On the back wall, four rubber supports in the form of a truncated cone with a base of 30 and a height of 15 mm are mounted.

After such a simple and simple revision of 6АС-2, objective indicators of measurements of the frequency response of sound pressure (Fig. 6) and subjective examination of listening to music programs note a significant improvement in its sound quality. A measuring microphone, a PC, and the RightMark Audio Analyzer 6.2.4 program were used to test the heads and the acoustic system.

Two loudspeakers of the AU have, following the example of R. Kunafin, dynamic heads up. Operation of the speaker system is allowed with an UMZCH with an output power of up to 25 W per channel, which is quite enough for sounding a room with a volume of up to 60 m.

As an afterword, you can add some advice for those who think that flipping bookshelf speaker cabinets is inappropriate.

In this case, the rubber mounts should naturally be on the bottom surface of the housing. To overcome the acoustic effect of different diffraction of radiation in the frequency band (baffle-step), an LR-circuit can be added to the crossover circuit, which is included in the break of the crossover wire at point A (see Fig. 5). This circuit corrects the rise in direct emission frequencies above 600 Hz by 6 dB. The resistance of the correction resistor and the inductance of the coil can be selected based on subjective impressions or the results of acoustic measurements.

LITERATURE

1. Deryabin V. Transistor stereo radios of the first and higher classes. - M .: Communication, 1979.

2. Alekseev I. About distortions of frequency characteristics of small-sized acoustic systems and "deep" bass. - Radio hobby, 2000, No. 5, p. 59-65.

3. Korzinin M. Passive radiator in loudspeakers 6AC-2. - Radio, 1984, No. 2, p. 41.42.

4. Kunafin R. New sounding 6АС-2. - Radio, 2000, N ° 3, p. 14.

5. Calculation of the filter. - URL: http: // www. aie.sp.ru/Calculator_filter.html (19.02.15).

6. Smirnov I. "Melody" sounds better. - Model constructor, 1985, N9 10, p. 22.

7. Ephrussi M. Loudspeakers and their application. - M .: Energy, 1976.

With which the serially produced electrophones "Melodiya-103", radio "Melody-104" and magnetoradiols "Melody-105" are completed. Its disadvantages include a low sound pressure level at the bass and a significant unevenness of the frequency response in terms of sound pressure in the 4-6 kHz range, caused by the small volume of the box: only 5 liters instead of the optimal one - 9 liters.

To reduce the unevenness of the frequency response of the loudspeaker in the entire frequency range to about 12 dB and reduce the lower limit of this range to 50 Hz (instead of 63 Hz), a homemade PI is mounted in it, which is made from a failed dynamic head of the type either. To do this, the magnetic system is removed from the head and the voice coil is cut off, instead of which a metal disc is glued in with epoxy resin " 3 »Weighing about 4 g with a MZ threaded hole in the center. The PI design is shown in Fig. 1. It fits into a 110mm hole cut in the center of the back of the loudspeaker. To ensure the tightness of the closed box between the emitter and the wall, it is necessary to lay a rubber seal, and in order to increase the equivalent volume of the body box, partially fill it with cotton wool. In order to avoid the ingress of cotton wool on the diffuser and the PI metal disk, at a distance of 20 mm from it, partitions are installed (glued to the body with any glue) made of special foam rubber used in air filters of domestic motorcycles. The baffle has additional sound-absorbing properties that reduce the unevenness of the frequency response of the loudspeaker in the frequency range below 500 Hz. A cross-section of the converted loudspeaker is shown in Fig. 2.

To reduce the unevenness of the frequency response of the loudspeaker in the frequency range of 4-6 kHz, a third-order crossover filter is installed instead of the separating capacitor in it (Fig. 3), the crossover frequency of which is 4 kHz, the slope of the frequency response of the low-frequency link is 12 dB, the high-frequency one is 18 dB per octave. The filter coils are made by winding the wire on round frames (Fig. 4) with diameters 65 (L1) and 44 mm (L2). The first coil should contain 156 turns of PEL 1.3 wire, and the second - 107 turns of PEL 0.72 wire.

The converted loudspeaker is recommended to be adjusted using a resonant frequency generator. To change the resonant frequency of the PI, metal screws of various lengths are screwed into the threaded hole of the metal disk from the outside, having correspondingly different masses. If it is necessary to use a large mass, you can use a piece of bar solder of the corresponding diameter, cutting the thread MZ at its end. At the end of the adjustment, the screw is fixed with a drop of nitro enamel and closed with a plastic cap made of half a table tennis ball. The cap is glued to the PI diffuser from the outside. The improvement of the loudspeaker parameters can be judged from Fig. 5, which shows the frequency response and frequency characteristics of the input impedance module before (dashed lines) and after (solid lines) rework, measured by known methods. One way to significantly improve the sound of your loudspeakers

Radio 3 magazine number 2000. SOUND ENGINEERING

R. KUNAFIN, Moscow

These miniature loudspeakers, which were once equipped with electrophones of the first complexity group "Melody-101-stereo", are considered by many to be hopelessly outdated. The author of this article claims that they are still capable of providing quite high-quality sound reproduction.

The magazine "Radio" has more than once published articles about the alteration of the 6AS-2. Usually it boiled down to the manufacture of new loudspeakers based on the dynamic heads used in these speakers. I suggest that radio amateurs go for a more economical and, I think, more interesting bullet, trying not only to get rid of the flaws of the factory design, but also to make the most of its advantages.

The main defect of the 6AC-2 is a strong vibration of the walls of the case, which not only gives the sound an unpleasant coloration, but also makes it unbearably "dirty". To eliminate this drawback, you must first of all remove the decorative speaker grille, remove the back panel and the head unit (in order not to break the plastic base, it is recommended to cut the sealing layer from the inside and push the unit out from behind) and thoroughly clean the parts from foam rubber and putty residues. All four walls of the case and the plane of the rear panel should be pasted over from the inside with one layer of linoleum with a fabric base using Bustilat glue. You need to glue it with the smooth side.

To obtain higher parameters, it is advisable to mechanically decouple the heads and the body. To do this, the plastic base of the head unit is strengthened by gluing on it from the back side a cover made of hard plastic or plywood 6 ... 10 mm thick (Fig. 1). In the lateral sides of the cover, a round file was made for a taper with a diameter of 133 mm, as shown in the section. The final adjustment is made in place by sawing the grooves in the cover for the wires going to the HF head. The inner surface of the plastic base is coated with a thick layer of plasticine.

The diameter of the mounting holes must be such that the screws pass through them freely, without touching the cover plate and the base of the head block. A flange (Fig. 2) made of plywood must be firmly glued into the body. According to the same sketch, a gasket from a soft material is cut out (in the author's design, vacuum rubber with a thickness of 3 mm is used). Washers made of the same material are placed under the screw heads. When assembling, the head block is centered so that it does not touch the body. The tightening of the screws is finally determined by listening.

The head 25GDN-1-4 (1 OGD-34) is connected to the UMZCH directly, like 8 of the original design, and the 6GDV-1-16 (ZGD-2) is shunted with a 5.4 Ohm resistor, in the author's version it is composed of four connected in parallel resistors MLT-2 (two 20 and two more 24 ohms). Such a shunt not only equalizes the frequency characteristics of the sensitivity and electrical resistance of the head, but as a current divider serves as an effective damper of its resonances, including the main one. The 6GDV-1-16 head noticeably emphasizes the noise components of the signal, and with additional resistors it sounds much softer.

It may seem that the shunt will act as an unwanted low-pass filter, cutting off high frequencies as the head resistance increases. However, the inductance of the 6GDV-1-16 head is not high, and its frequency response has a noticeable rise in the high frequencies, and as a result, the sound turns out to be better balanced. Thus, it became possible to connect the head through the simplest first-order filter - an 8 μF capacitor. It is composed of two capacitors with a capacity of 4.7 and 3.3 μF (K78 or K73-16. In an extreme case, K73-11 is also suitable). Mounted mounting, with a strong twist and good soldering. It is also advisable to replace the connecting wires, at least with good copper network wires, but not in PVC or rubber insulation.

When assembling the speaker, the joints are coated with plasticine and the head block is attached. The body itself is completely filled with a sound absorber: pieces of batting are tightly stacked in layers. The rear panel of the speaker cabinet should be inserted with little force. Such a dense packing allows significantly weakening resonances and excluding sound bubbling.

At the time of listening, the speakers are installed in the middle of the room or at some distance from furniture and walls, which should not strongly reflect sound. They need to be placed directly on the floor, vertically, with their heads up; for this purpose, the back panel is equipped with soft legs or spikes. This arrangement, in fact, is the "highlight": the elongated shape of the body and the coaxial arrangement of the heads allow you to get a circular directivity pattern, with all the benefits that come with it.

It should be noted that the parameters of the speaker are optimized after 50 ... 60 hours of operation, required for running-in the heads; in the future, with regular use, the characteristics do not deteriorate and daily "warm-up" is not required.

Since the author was not able to remove the frequency response of the speaker. during its tests, in the presence of a group of experts, a gramophone record was listened to with a recording of sinusoidal signals with fixed frequencies (IZM ZZS 0201-02). An extremely flat frequency response was noted, as well as the imperceptibility of the rolls at the frequency of the mid-range and high-frequency bands (5 kHz). The falloff of the frequency response is noticed only at a frequency of 63 Hz, and the lowest effectively reproducible frequency can be estimated at 40 Hz. The lowest frequency reproduced without distortion is designated on the disc as 31.5 Hz, which allows even organ music to be reproduced quite naturally (!).

In general, the experts noted the surprisingly clean timbre accurate sound of the converted speakers. The sound of 6AC-2 was recognized as preferred by all experts when comparing it with the sound of such well-known "shelf" speakers. like "Rogers Studio 3". Rogers LS3 / 5a. "B&W DM 302". The main and decisive advantage of 6AC-2 is its ability to convey the unique individuality of singers 'voices and musicians' playing. By the way. 6АС-2 are quite capable of providing in a room with a volume of 50 ... 60 m3 sound pressure approaching a "disco" one.

Were tested the converted speakers with a power amplifier with feedback on the load current (see, for example, the article by M. Sapozhnikov "Non-standard inclusion of microcircuits in the UMZCH" in the magazine "Radio", 1998, No. 2. p. 23).

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These miniature loudspeakers, which were once equipped with electrophones of the first complexity group "Melody-106-stereo", are considered by many to be hopelessly outdated. The author of this article claims that they are still capable of providing quite high-quality sound reproduction.

The magazine "Radio" has more than once published articles about the alteration of the 6AS-2. Usually it boiled down to the manufacture of new loudspeakers based on the dynamic heads used in these speakers. I suggest that radio amateurs go for a more economical and, I think, more interesting bullet, trying not only to get rid of the flaws of the factory design, but also to make the most of its advantages.

The main defect of the 6AC-2 is a strong vibration of the walls of the case, which not only gives the sound an unpleasant coloration, but also makes it unbearably "dirty". To eliminate this drawback, you must first of all remove the decorative speaker grille, remove the back panel and the head unit (in order not to break the plastic base, it is recommended to cut the sealing layer from the inside and push the unit out from behind) and thoroughly clean the parts from foam rubber and putty residues. All four walls of the case and the plane of the rear panel should be pasted over from the inside with one layer of linoleum with a fabric base using Bustilat glue. You need to glue it with the smooth side.

To obtain higher parameters, it is advisable to mechanically decouple the heads and the body. To do this, the plastic base of the head unit is strengthened by gluing a hard plastic or plywood plate 6 ... 10 mm thick on the back side (Fig. 1). In the lateral sides of the cover, a round file was made for a taper with a diameter of 133 mm, as shown in the section. The final adjustment is made in place by sawing the grooves in the cover for the wires going to the HF head. The inner surface of the plastic base is coated with a thick layer of plasticine.

The diameter of the mounting holes must be such that the screws pass through them freely, without touching the cover plate and the base of the head block. A flange (Fig. 2) made of plywood must be firmly glued into the body. According to the same sketch, a gasket from a soft material is cut out (in the author's design, vacuum rubber with a thickness of 3 mm is used). Washers made of the same material are placed under the screw heads. During assembly, the head assembly is centered so that it does not touch the body. The tightening of the screws is finally determined by listening.

The head 25GDN-1-4 (1 OGD-34) is connected to the UMZCH directly, like 8 of the original design, and the 6GDV-1-16 (ZGD-2) is shunted with a 5.4 Ohm resistor, in the author's version it is composed of four connected in parallel resistors MLT-2 (two 20 and two more 24 ohms). Such a shunt not only equalizes the frequency characteristics of the sensitivity and electrical resistance of the head, but as a current divider serves as an effective damper of its resonances, including the main one. The 6GDV-1-16 head noticeably emphasizes the noise components of the signal, and with additional resistors it sounds much softer.

It may seem that the shunt will act as an unwanted low-pass filter, cutting off high frequencies as the head resistance increases. However, the inductance of the 6GDV-1-16 head is not high, and its frequency response has a noticeable rise in the high frequencies, and as a result, the sound turns out to be better balanced. Thus, it became possible to connect the head through the simplest first-order filter - an 8 μF capacitor. It is composed of two capacitors with a capacity of 4.7 and 3.3 μF (K78 or K73-16. In an extreme case, K73-11 is also suitable). Mounted mounting, with a strong twist and good soldering. It is also advisable to replace the connecting wires, at least with good copper network wires, but not in PVC or rubber insulation.

When assembling the speaker, the joints are coated with plasticine and the head block is attached. The body itself is completely filled with a sound absorber: pieces of batting are tightly laid in layers. The rear panel of the speaker cabinet should be inserted with little force. Such a dense packing allows significantly weakening resonances and excluding sound bubbling.

At the time of listening, the speakers are installed in the middle of the room or at some distance from furniture and walls, which should not strongly reflect sound. They need to be placed directly on the floor, vertically, with their heads up; for this purpose, the back panel is equipped with soft legs or spikes. This arrangement, in fact, is the "highlight": the elongated shape of the body and the coaxial arrangement of the heads allow you to get a circular directivity pattern, with all the benefits that come with it.

It should be noted that the parameters of the speaker are optimized after 50 ... 60 hours of operation, required for running-in the heads; in the future, with regular use, the characteristics do not deteriorate and daily "warm-up" is not required.

Since the author was not able to remove the frequency response of the speaker. during its tests, in the presence of a group of experts, a gramophone record was listened to with a recording of sinusoidal signals with fixed frequencies (IZM ZZS 0201-02). An extremely flat frequency response was noted, as well as the imperceptibility of the rolls at the frequency of the mid-range and high-frequency bands (5 kHz). The falloff of the frequency response is noticed only at a frequency of 63 Hz, and the lowest effectively reproducible frequency can be estimated at 40 Hz. The lowest frequency reproduced without distortion is designated on the disc as 31.5 Hz, which allows even organ music to be reproduced quite naturally (!).

In general, the experts noted the surprisingly clean timbre accurate sound of the converted speakers. The sound of the 6AC-2 was recognized as preferred by all experts when comparing it with the sound of such well-known "shelf" speakers. like "Rogers Studio 3". Rogers LS3 / 5a. "B&W DM 302". The main and decisive advantage of 6AC-2 is its ability to convey the unique individuality of singers 'voices and musicians' playing. By the way. 6АС-2 are quite capable of providing in a room with a volume of 50 ... 60 m3 sound pressure approaching a "disco" one.

Were tested the converted speakers with a power amplifier with feedback on the load current (see, for example, the article by M. Sapozhnikov "Non-standard inclusion of microcircuits in the UMZCH" in the magazine "Radio", 1998, No. 2. p. 23).

Each acoustic system (АС) 6АС-2 radio "Melodiya - 101, 104, 105 - stereo" and their modifications, electrophone Melodiya - 103, issued by the Riga Radio Plant named after Popov Software "Radiotekhnika", consists of two loudspeaker heads (round dynamic direct radiation) installed in a box made of glue plywood 10 mm thick. The loudspeaker heads are mounted in the housing coaxially (axis means axis) relative to each other on a plastic base (without a front baffle board). The advantages of such an arrangement of loudspeakers are flat, without dips, directional characteristics, which is impossible to achieve with spaced, even closely spaced, heads. But, on the other hand, all designs of such loudspeakers have one common drawback - due to the interference of sound waves emitted simultaneously by several heads, the total amplitude-frequency characteristic (AFC) of sound pressure, in the zone of their joint action, acquires a number of local peaks and dips on medium and high frequencies. To eliminate this phenomenon, it is necessary either to increase the slope of the frequency response of the crossover filters (which greatly complicates their design), or to bring the radiation centers of the heads closer together. The absence of a front reflective board entails significant losses due to the diffraction of sound waves (baffl-step effect) - one of the types of distortions that affects the sound quality of any speaker system. This type of distortion appears in the frequency range from 100 to 800 Hz and is a smooth decrease in the acoustic pressure generated by the speaker system below a certain frequency (for 6AC-2, this calculated frequency is 732 Hz). The actually measured loss is 3-4 dB.

To damp the moving system of the main head Gr 1 (Fig. 1) at frequencies close to the frequency of its mechanical resonance, the entire free volume of the box of the 6AC-2 acoustic system is filled with cotton wool. In addition, the Gr 1 head is small. All this determined the low sensitivity of the 6AC-2 speaker system and caused a drop in its frequency response in the low-frequency range (Fig. 2).

Figure: 1. Acoustic system 6AS-2: a) side view; b) front view (with the decorative grill removed): 1 - connecting hole; 2 - back wall; 3 - gasket; 4 - box; 5 - loudspeaker head Gr 1; 6 - wooden square to increase the rigidity of the box; 7 - capacitor C; 8 - decorative lattice fastening spring; 9 - the base of the loudspeaker heads; 10 - decorative lattice; 11 - loudspeaker head Gr 2; c - appearance; d - electrical schematic diagram

Figure: 2. Frequency response of sound pressure of acoustic system 6АС-2

The specified features of the speaker system are compensated by the characteristics of the low frequency path during the development of "Melodies". To equalize the frequency response of the sound pressure of acoustic systems, the LF path of the radio has a frequency response with a significant rise in the low frequency range (at a frequency of 60 Hz, about 14 dB). The required value of sound pressure during the operation of acoustic systems is provided by the increased output power of the LF path (the maximum output power of the Melody-101-stereo radio is about 15-20 W).

From the above, it follows that the 6AC-2 and the "Melodiya" amplifier are designed to work together. To connect the speaker to a non-standard signal source, you need to modify it.

Among the shortcomings, it should also be noted, vibration of the walls of the case, significant unevenness of the amplitude-frequency characteristics in the mid-frequency region. The latter is due to the fact that the 10GD-34 low-frequency head, which also performs the function of the mid-frequency link, has a sharp drop in the frequency response of the sound pressure from 4.5 kHz (Fig. 3, a). The high-frequency head 3GD-2 is switched on through a first-order filter with a cutoff band of 10 kHz. It is quite simple to equalize the frequency response of the sound pressure at mid frequencies - to lower the cutoff frequency of the filter. In a similar speaker M. Korzinin installed a filter with a crossover frequency of 4 kHz. However, in this case, the frequency of the main resonance of the HF head, which is 4.5 kHz, is higher than the cutoff frequency, which is not desirable. The 3GD-2 speaker operating at the frequency of the main resonance cannot provide full-fledged high-quality sound. In addition, the proposed filter is laborious to manufacture (winding two inductors). There is an easier option. For this, the isolation capacitor of the high-frequency head is installed with a capacity of 8.8 μF (film capacitors connected in parallel: two 3.3 μF each and one 2.2 μF with an operating voltage of 400 V). Since such a refinement will bring the cutoff frequency close to the frequency of the main resonance of the HF head, the latter is shunted with a 5.4 Ohm resistor with a power of 3-5 W. Such a shunt will equalize the frequency characteristics of the sensitivity of the heads, electrical resistances and, most importantly, damp the resonances of the HF head, including the main one. The latter, in this case, is connected in antiphase with respect to the woofer head (Fig. 4). In the described design, two series-connected 2.7 Ohm resistors with a power of 5 watts are used. The total capacitance of the filter capacitors is calculated using an online calculator. The calculation takes into account the total DC resistance of the 16 ohm head and the 5.4 ohm shunt resistor. Thus, the 3GD-2 head will work already at 4.5 kHz, providing full-fledged speaker sound in the entire range. It should be noted that one of the design features of the 3GD-2 is the presence of a silk domed diffuser, which provides a relatively flat frequency response of the sound pressure from 2 to 18 kHz (Fig. 3, b).

Figure: 3. Dynamic heads АС 6АС-2 and their amplitude-frequency characteristics of sound pressure: a - 10GD-34; b - 3GD-2

Figure: 4. Electrical diagram of the 6AC-2 acoustic system (modified)

To equalize the sound pressure of the acoustic system in the low-frequency region, following the example of I. Smirnov, a phase inverter (PHI) is added to the speaker. The author used a plastic pipe with an inner diameter of 50 mm and a length of 100 mm. This decision is correct, since heads with low (Q

The long-term practice of operating the 10GD-34 heads in acoustic design with a bass reflex has determined the best frequency of its tuning - 55 Hz. Calculate the dimensions of the port of the phase inverter for the specified frequency using the computer program BassPort. Based on the calculations obtained, a hole 35 mm in diameter is cut out in the rear wall of the speaker housing (Fig. 5.) and a piece of cardboard tube with an inner diameter of 32 mm and a length of 130 mm is glued into it. A board made of foil-clad fiberglass with dimensions of 50 X 50 mm with filter elements and surface-mounted conductors is also installed on the back wall. Felt 10 - 15 mm thick or foam rubber is glued to the remaining free space. The filler (cotton wool) is removed.

Figure: 5. Location of elements on the rear wall of the speaker cabinet: 1 - rear wall; 2 - sound absorbing material; 3 - bass reflex port; 4 - filter board

In conclusion, the seams of the walls are sealed, the walls themselves are covered from the inside with a self-adhesive vibroplast 1.5 mm thick or linoleum, the LF head is provided with a soft attachment (elimination of vibrations of the walls of the case). Four rubber feet are mounted on the back wall.

After such a simple and simple revision of 6АС-2, both the objective indicators of measurements of the frequency response of sound pressure (Fig. 6) and the subjective examination of listening to music programs note a significant improvement in its sound quality. A measuring microphone, a PC and the RightMark Audio Analyzer 6.2.4 program are used to test the heads and the acoustic system. ...

Figure: 6. AFC of the sound pressure of the 6АС-2 acoustic system after revision

Two loudspeakers have, following the example of R. Kunafin, speakers upward. It is allowed to operate the 6АС-2 with an amplifying device delivering a power of 15 ... 25 W per channel, which is quite enough to ensure high-quality sounding of a room up to 100 m 3.

For those who consider flipping the loudspeaker enclosures inappropriate, the loudspeakers are supplemented with a 1st order link that corrects high frequencies in the baffl-step band (Fig. 7). This circuit compensates for the rise in the loudspeaker frequency response associated with the transition from omnidirectional radiation to half-space radiation. The transition frequency Fd is 700 Hz, and the attenuation level N is 6 dB (from the frequency response graph in Fig. 6). In this case, the value of the compensating resistor Rk is taken equal to the load resistance Rн - 4 Ohm, the inductance of the compensating coil Lk is 1.3 mH. More precisely, the resistance of the resistor and the inductance of the coil are selected based on subjective impressions or measurement results.

Figure: 7. Link of the 1st order correcting high frequencies: a - schematic diagram; b) - frequency response

Literature.

1. Deryavin V. Transistor stereo radios of the first and higher classes. - M .: Communication ", 1979.

2. Alekseev I. About distortions of frequency characteristics of small-sized acoustic systems and "deep bass", Radiohobby No. 5, 2000.

3. Korzinin M. Passive emitter in loudspeakers 6AC-2, - Radio No. 2, 1984.

4. Kunafin R. New sounding 6АС-2. - Radio No. 3, 2000.

5.http: //www.aie.sp.ru/Calculator_filter.html.

6. Smirnov I. "Melody" sounds better. - Model constructor # 10, 1985.

7. Ephrussi M. Loudspeakers and their application. M. - Energy, 1976.

8. Afonin S. Creation of acoustic systems at home. - M .: "Eksmo", 2008.

9. Aldoshina I. High-quality acoustic systems and radiators. - M: Radio and communication, 1985.

Date of publication:17.06.2015

Readers' opinions

• trape / 08.06.2018 - 04:00
  8
• Author / 04/29/2018 - 15:00
  Symmetrical and coaxial arrangement of elements in loudspeakers leads to undesirable resonance phenomena. The FI cannot be centered! Select capacitors based on your capabilities. The main thing is that the total capacity corresponds to that on the diagram.
• Dmitry / 04.01.2018 - 21:20
  If you bring the filter out, is it better to center the FI? Is it possible to put capacitors one at 6.8 and two at 1 μF? Thank you, Dmitry
• Author / 07.01.2017 - 00:22
  Uv. Evgeniy. In this speaker system, the heads are located coaxially, that is, on one axis. There will be no effect from the low-frequency head supplemented by the high-frequency horn, since its central part is covered by the high-frequency head and the high-frequency radiation of the horn will be significantly attenuated, interfering and distorting the sound. For this design, it is not necessary to install a dust cap in the form of a cone in the woofer head.
• Eugene / 01/04/2017 - 22:52
  Vladimir, hello! Tell me why you did not use in this AO (with a phase inverter), a dynamic head 25GDN-1-4, modified according to your method (with an HF cone). Is it worth putting a modified and 25GDN-1-4 in these speakers?
• Pavel / 02/10/2016 - 18:59
  and the filter can be taken out