Modernization of the acoustic system 75AC-001. Part 1. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Modernization of the acoustic system 75AC-001. Part 1. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Speakers

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Experienced readers probably remember numerous articles on improving the sound quality of acoustic systems 10MAS-1, S-90 (35AC-212), etc., which were regularly published in the Radio magazine in the 70s and 80s . With the advent of imported audio equipment on the domestic market, interest in this topic, in theory, should fade away, but as editorial mail shows, this is far from the case. There are still a lot of domestic products left in operation, which are now quite easy to "bring to mind" - fortunately, any materials and audiophile materials are available to everyone. We will now tell about one such "upgrade". In two days (except for the time spent on thinking and acquiring the necessary details), our experimenter Konstantin Bystrushkin managed to "put the voice" on one of the best (if not the best) domestic acoustic system 75AC-001. We are sure that his advice will help improve the sound of any "soviet" speakers.

Acoustic systems 75AC-001 (aka 150AC-001), which under the names "Corvette" and "Cleaver" were produced by the plants "Okeanpribor" (Leningrad) and "Krasny Luch" (Krasny Luch), respectively. At the same time, significantly more Corvettes were produced than Cleavers. Both models have exactly the same sets of loudspeakers, circuits and design. However, in addition to the speakers, Krasny Luch also produced a set of speakers for both speaker options, and therefore some audiophiles still believe that Cleavers sound better on native speakers than Corvettes. A possible reason for this is the "rationalization proposals" implemented in the process of serial production of the latter. So, for example, over time, the formulation of the multicomponent compound for impregnating the diffuser of the 30GDS-1 midrange driver was simplified, and the internal vibration isolation of the housing was excluded.

75AC-001 are the latest development of VNIIRPA them. Popov (Leningrad), introduced into mass production, and became a kind of "swan song" of our column construction. When designing, for the first time in domestic practice, mathematical modeling and optimization of the parameters of heads and separation filters with the help of a computer were widely used. In addition, the speakers have a number of other advantages. Firstly, they have efficient loudspeakers (100GDN-3, 30GDS-1 and 10GDV-4) of a new generation, which made it possible to achieve a record (for the late 80s) sensitivity of 91 dB(W/m). The calculation of circuits on a computer provided a wide frequency range of 25-25000 Hz with a small frequency response unevenness and a low level of distortion. Secondly, the use of phase-correcting chains in the crossover filter circuit and the placement of the emission centers of the HF and MF heads in the same plane made it possible to significantly improve the uniformity of the PFC. Thirdly, the large volume of the case, low resonance frequency and the large diameter of the woofer cone provide a sufficiently deep and powerful bass.

Finally, air-core coils wound with high-quality large-section copper wire and metal-paper MBGO capacitors, which are characterized by low losses and distortions, are used in the design of the crossover filters. The circuit elements are installed on a cardboard base using surface mounting. Unfortunately, there are several hefty fly in the ointment in this big barrel of honey. The greatest contribution to sound damage is made by excessively long wires from an ordinary "puddle", as well as numerous elements of the protection circuit through which the sound signal goes to the loudspeakers. This was partly justified: the powerful 75AC-001 could be used not only in everyday life, but also for sounding houses of culture, small discos, etc. The node contains, in addition to 2 relays, 9 more transistors, 3 LEDs, 18 ordinary diodes, 40 resistors (!) And 8 capacitors. And all this goodness is somehow included in the sound chain! Numerous experiments have shown that due to the high sensitivity of the speakers, it is almost impossible to overload them in a living room, especially when you consider that, according to passport data, a short-term input power can reach 300W.

In short, in real conditions there is no benefit from ultrasound, while the sound suffers considerably because of it. With the woofer, despite the absence of relay contacts in its circuit, the situation is not much better. LF components also pass through the US board and a 5A fuse. All this leads to a significant increase in the length of small-section wires that are by no means of audiophile quality. The same cable was used to mount the filter board, which was also not made in the most optimal way: the elements are interconnected by additional jumpers soldered to the caps on the board. This further increases the overall length of the wires in the path of the audio signal. Now, finally, the reasons for unsuccessful attempts to make the sound of speakers more worthy with the help of audiophile speaker cables have become clear. Obviously, even the best of them is not able to compensate for the loss of quality in the internal wiring.

By the way, the 75AC-001 input screw terminals also leave much to be desired. The design of their contact pads is such that it is impossible to connect "forks" and "spatulas" that provide a convenient and reliable connection. More or less satisfactory results are achieved when clamping a stripped cable into them, however, when screwing the terminals, part of the cores is squeezed out. And, finally, the scanty amount of damping material inside the case for such a large internal volume was surprising. According to reference data, ATM-75 fibrous material made of super-thin glass fibers glued with phenolic resins should be used in 001AC-1. However, in appearance, the sound absorber was very reminiscent of ordinary viscose cotton wool. Because of this, parasitic sound radiation from the back of the woofer leads to the formation of standing waves. In addition, the wave reflected from the rear wall of the housing acts on the rear side of the diffuser, causing the signal front to be pulled. As a result - an indistinct and booming sound on the bottoms.

So, based on the foregoing, the following main areas of "upgrade" were identified:

exclusion of unnecessary elements from sound circuits;

replacement of internal conductors with high-quality ones made of oxygen-free copper;

optimization of the connection scheme to reduce the length of conductors;

replacement of input connectors with powerful screw terminals;

pasting the inner surface of the body with an effective sound-absorbing material.

Pic 1.

About 200-300 rubles and 2 man-days were spent on the proposed revision. You will need: 6-8 m speaker cable made of oxygen-free copper (OFC) with a cross section of 2.5-4 square meters. mm (approximately 10-20 rubles / m); two pairs of screw terminals, preferably gold-plated, of a universal design that allows you to connect "bananas", "spatulas", "forks" and bare wire (about 40 rubles); two air filters for kitchen air cleaners (sold in Svet stores, 20 rubles/piece); a little building sealant or plasticine.

After the necessary material base is created, you can proceed to the autopsy. The easiest way to do this is to place the patient on their back with the loudspeakers up. Now we recommend to adhere to the following sequence of operations.

First, the decorative trims of the loudspeakers are removed, and then they themselves are removed from the case. In this case, all connecting wires are carefully soldered. Be especially careful when dismantling the 100GDN-3 woofer - this process, due to the considerable weight of the speaker, is a very difficult task. Then unscrew the nameplate of the indication board and unsolder the connecting wires from the LEDs. After sealing the hole under the board, the LEDs and the nameplate are installed in their original place.

After that, through the hole in the woofer, you can begin to dismantle the separation filter and ultrasound boards. But first you need to unsolder the wires from the input terminals and the SG-5 socket on the rear wall of the case and unscrew 6 nuts around the perimeter of the filter board. The removed boards are separated from each other, after which the ultrasound is thrown out.

Pic 2.

At the next stage of work, it is necessary to remove all connecting conductors on the filter board by unsoldering them or biting them off with side cutters. Resistors R6, R7 and R9 are also removed from there (according to the manufacturer's scheme). Now the RF head is directly connected in parallel with the resistor R4 ("plus" to C13). Accordingly, the woofer must be connected in parallel to the C12-R3 chain ("plus" to C12). Thus, the electrical circuit of the crossovers is noticeably simplified.

Now we need to replace the internal wiring. In this case, when mounting the filter board, I used a Vampire Wire OFC Speaker Cable with a cross section of 2.5 sq. mm. They also connected midrange and HF loudspeakers. In the low-frequency circuit (from the crossover to the input terminals), a wire with an increased cross-section of conductors up to 4 sq. mm was used, it is desirable to solder it, taking into account the preferred direction of current flow (usually coincides with the direction of marking). Keep in mind that the sound character of most speaker cables can improve markedly after a few hours of listening. Therefore, it will not be superfluous to pre-warm up the new conductors by driving them before starting the installation.

Pic 3.

And some more practical advice. To reduce the number of connecting conductors, it is advisable to abandon the use of mounting clips on the filter board, and instead use the leads of resistors and capacitors, soldering the rest of the circuit elements directly to them. To reduce the influence of the return current of a powerful woofer on the circuits of the midrange and tweeters, the connection of the negative bus of the low-frequency, midrange, and high-frequency links to the "minus" of the input bus should be made only at one point. For this purpose, it is convenient to use the appropriate terminal of the capacitor C8 or C11.

After that, the crossover board is installed in its old place in the speaker cabinet.

To install new input terminals, you need to remove the round plastic cover on the rear wall of the speakers and remove the old ones, then bore the holes to the required diameter. After installing the universal clamps, a wire from the filter board is connected to them from the inside (if it cannot be soldered, you will have to use plug connectors that are tightly screwed with nuts from the inside). Then the cover with connectors is installed in its original place.

You are now ready to connect and install your speakers. We advise you to measure the lengths of the connecting cable with a small margin, as this will simplify the operation. First, the midrange is mounted. Since 30GDS-1 does not have a terminal polarity marking, it should be determined using a 1.5 volt battery, noting the diffuser displacement when its polarity is changed. Having installed the midrange head, it is desirable to seal the hole for entering its box of connecting wires. Then a "tweeter" is installed, after which the speakers are closed with a decorative nameplate. The second ends of the connecting cables of the loudspeakers are soldered to the terminals of the corresponding elements on the filter board.

Pic 4.

Before mounting the woofer, it is necessary to acoustically dampen the internal volume of the case. The best filler for this is a kitchen air cleaner filler, which is a fine-fiber synthetic material. One sheet will be enough. It is glued with the "Moment" to the side and rear walls of the case opposite the opening of the woofer. As a filler, you can use ordinary technical or viscose cotton wool, placing them inside the case. When determining the optimal amount of cotton wool, it is recommended to adhere to the norms for optimal damping: 8-11 kg per cubic meter. m. of internal volume.

The last operations are connecting the woofer to the filter board and installing it in the speaker housing, as well as installing a decorative ring-lining on the loudspeaker.

Well, now you can proceed to the most pleasant part - listening to the modified speakers and evaluating the results. A subjective examination of the sound of the "Corvette 75AC-001 SE" was carried out when they were connected to a stereo system as part of a Sony CDP-761E CD player and a Sony TA-F511 amplifier. A half-meter pair of Monster Cable Interlink 300 was used as an interconnect cable, and two-meter pieces of Jamo with a cross section of 4 square meters were used as an acoustic cable. mm.

To begin with, with the help of the disc "Ours in the City" (an appendix to the "Salon AV" 3/98), the correctness of the phasing of the loudspeaker connection in the speakers was checked. If everything was done correctly, then when playing in-phase sources, the virtual source of pink noise will be located exactly in the middle between the speakers, and when out of phase, it will “run up” to the speakers. Otherwise, before carrying out further auditions, it makes sense to once again carefully check the correctness of the installation.

The sound of the speakers was then evaluated using recordings of a wide variety of musical genres and directions, including classical and popular music: "The best of Mozart" (1996 Karussell International), "The best of Batch" (1996 Karussell International), "Famoust balet" vol. 1 and 2 ("The rose collection"/Elar music a/s 1992), Paul Mauriat - "Nostaljazz" (Philips 512 146-2 DDD 1992 Phonogram), The Beatles - "Please, Please Me" (Parlophone/EMI), The Beatles - "Rubber Soul" (Parlophone/EMI), Deep Purple - "Machine Head" (EMI), etc.

Compared to the original version, the sound of the speakers has changed significantly. Particularly noticeable changes occurred in the mid- and high-frequency ranges. The sound became truly transparent and crystal clear, while it was enriched with the finest nuances and musical details that were not heard before. For example, the rustle of metal brushes on the cymbals was reproduced with extraordinary authenticity, while in the original version of the speakers their sound had a somewhat noise-like character. The changes made have clearly benefited the low-frequency range: the bass has become noticeably clearer and more collected. Apparently, this is due not only to the introduction of a sound absorber inside the case, but also to the improvement in the damping of the woofer due to the use of a much lower-resistance cable with a large cross section. In a word, the effectiveness of modernization is obvious and more than paid off all the costs of its implementation.

Are there any reserves for further improvement of the sound quality of the 75AC-001? Significant improvement should not be expected, since the proposed set of measures makes it possible to eliminate the most obvious mistakes and flaws in their scheme and design. However, it is apparently possible to slightly "polish" and "lick" them. For example, by installing a second pair of screw terminals and implementing the option of connecting to an amplifier using the bi-wiring method. It is possible that by choosing other types of speaker cables for 75AC-001, you can achieve their more balanced and sophisticated sound.

Replacing the metal-paper capacitors in the audio signal paths on the filter board with polypropylene or other audiophile grade capacitors of equal capacity can give a noticeable result. Finally, today there is a huge selection of various vibration-absorbing pads and washers for installing loudspeakers, special coatings for pasting the inner surface of cases. But here we should not forget about the principle of reasonable sufficiency: each subsequent improvement will cost more and more. Especially when you consider that 75AC-001 have a number of fundamentally irreparable shortcomings.

Firstly, the large width of the front panel inevitably leads to the diffraction of sound waves, and as a result, to the coloration of the sound and uneven frequency response. Secondly, the large distance between the woofer and midrange speakers leads to the fact that a more or less uniform sound field in a wide frequency band is provided at a distance of at least 2-3 meters from the speakers, and in small rooms they sound very peculiar. Thirdly, powerful unshielded magnetic systems of woofers and midrange speakers do not allow their use in "home theater" systems. Fourth, despite its high sensitivity, the 75AC-001 does not work well with tube amplifiers, since a heavy woofer requires effective damping with a low output impedance. There are also "fifth" and "sixth" ... But in any case, if you know how and love to do something with your own hands, you will get real pleasure from any alteration.

Author: Konstantin Bystrushkin, Salon AV; Publication: hi-fi.ru

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Vladislav, satanist3513@gmail.com
Can you please throw off the diagram of the converted crossover, I’m also sorting out the Cleaver 150ac 009, I’m not a radio engineer myself and based on the photo I remade the filter and apparently something went wrong that the middle ones were slaughtered and the high ones weren’t enough, in the transmission the low frequency interrupts the mid and high frequencies.

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