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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Modernization of the acoustic system 75AC-001. Part 2. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Speakers In the previous article, we started talking about how you can independently refine domestic speaker systems in accordance with audiophile ideas about sound quality. The object for the experiment was the popular Soviet speakers 75AC-001, which still have a high reputation even among advanced listeners. As it turned out, after a relatively simple and not very burdensome modernization for the family budget, they can sound very good. Judging by the letters from readers who still come to the editorial office, the topic of finalizing the columns interested many of them, and therefore we decided to return to this issue again. But first, for those who did not fall into the hands of No. 11 of the magazine for 1998, we briefly recall the "content of the previous series." So, the 75AC speaker systems were conceived "by the mind" and, in principle, should have sounded very decent. However, all the charms of mass assembly, constructive simplifications, suboptimal mounting of the electric filter board, terrible internal wiring, plus the presence, as it turned out, of useless protection, completely "kill" the sound. Therefore, it is more correct to consider 75AC-001 (aka 150AC-001) as a good set of DIY parts. It includes fairly decent loudspeakers with high sensitivity, elements of an electric filter (coils, capacitors, wire resistors), as well as a body blank veneered with natural veneer with holes already cut for loudspeakers. Such sets are very popular abroad (we also sold them in some salons and on "Gorbushka"), as they allow handy audiophiles to realize their ideas and at the same time save a considerable amount. Therefore, at the first stage of the upgrade, or rather, tuning 75AC-001, they were completely dismantled, including the removal of all internal wiring, including connections on the filter board, and unnecessary components (protection devices and quenching resistors). Then, based on the remaining elements, the crossover filters were redesigned according to the optimized scheme. At the same time, all internal installation was carried out with a high-quality acoustic cable made of oxygen-free copper. To dampen standing and reflected sound waves inside the case, sheets of a fibrous sound absorber used as a filter in kitchen hoods were glued to the back and side walls. In conclusion, it was proposed to replace steel "soviet" terminals with high-quality gilded "posts". That, in fact, is all that was done at the first stage of work. Such simple and, in general, obvious improvements to the basic design had the most positive impact on the sound quality. The new version of the loudspeakers was called 75AC-SE, and for some time they fully met our requirements. However, after a series of tests, it became clear that although the Special Editions sound better than their original version, they still do not reach the level of decent Hi-Fi. It became obvious that the full potential inherent in these domestic systems is far from being exhausted. Gradually, the desire to once again take up the soldering iron matured and finally brought them to mind. Moreover, by this time the author managed to upgrade other components of the tract. In particular, transistor switches were removed, shunting the output in vinyl and CD players, and reed switches were used instead, in addition, all internal wiring was replaced in the turntable, including the connecting interconnect cable. Subtleties of articulation As before, before the start of tuning-II, a plan was developed for its implementation. As a first step, it was decided to try a two-wire connection to the amplifier using the bi-wiring method, since testing of foreign speakers convincingly demonstrated its effectiveness. Then it was supposed to increase the damping of the LF and MF loudspeakers by increasing the amount of sound absorber inside the case and in the midrange box. Finally, we wanted to achieve better articulation of the bass, which, although it became more collected after the first stage of the upgrade, still did not sound convincing enough. Actually, it all started with the bass, since it is the sound in the low-frequency range that causes the greatest complaints from the owners of the 75AC. The reason for the "mumbling" in this case is quite obvious: the huge 100GDN-3 cone is too heavy to be effectively damped even by the ultra-low output impedance of the power amplifier. Under these conditions, there is only one way out - to influence it with acoustic methods, i.e. place an additional sound absorber inside. The deterioration of the 75AC sound on the bottoms can also occur due to the departure of the woofer resonance from the bass reflex tuning frequency. After 8-10 years of operation, the corrugation inevitably dries up, and its flexibility decreases, which leads to an increase in the resonant frequency of the loudspeaker. Worse, in this case, the acoustic quality factors of 100GDN-3 also change. And what to do now? Irina Arkadyevna Aldoshina, who at one time led the development of 75AC-001, recommends experimentally measuring the parameters of an "aged" woofer (resonant frequency Fs in the open air and full quality factor Qts) and using well-known methods to calculate a new phase inverter tuning frequency. Then tune it to a higher frequency by reducing the length of the pipe. You will have to cut off some part of it with a hacksaw, but take your time, remember "measure seven times ...". Check again that your calculation is correct. And it's better to make sure experimentally that the bass will really improve after that. To do this, we recommend making two tubes up to 60 mm long (slightly more than half the length of the phase inverter - 104 mm) from thick cardboard or other material, one of which should fit tightly into the other. The outer diameter of the outer 80 mm, which corresponds to the diameter of the port. After that, a standard phase inverter is removed from the speaker housing. This is not difficult to do, since it is not fixed in any way and is held in a hole in the front panel due to a snug fit. Then a cardboard "telescope" is tightly inserted in its place, and by changing its length the most acceptable bass is achieved. The optimum is best caught not by ear, but by the speaker's input impedance curve. However, the experiments carried out in a real room with an area of 20 sq. m showed that the nature of the sound of the 75AC at low frequencies when setting up the phase inverter did not improve as noticeably as one might expect. Much simpler and more effective was simply damping the port with insert plugs. To do this, washers with a diameter of just over 80 mm are cut out of sheet foam rubber - it is best of all dark in color so that they do not contrast with the front panel for secure fixation in the pipe. The optimal thickness of the liner depends on the density of the material. It is best to take a not very thick sheet in order to be able to change the damping depth by installing several of these plugs. The speakers are fed with a sound signal with a large level of low-frequency components, and by changing the number of earbuds, the highest quality of bass is achieved. It is easiest to appreciate it on the big drum ("barrel") or double bass, the sound of which should become much clearer. But don't get too carried away, because if you tightly clog the phase inverter port, the latter will stop working altogether and the acoustic design will turn into a "closed box". Which is not good, since 100GDN-3 was not calculated for this. According to the author, the best results are obtained if, after installing the liners, the airflow intensity drops by about half. The total thickness of medium-density foam rubber in this case will be approximately 20-30 mm. This is the most successful compromise: the bass becomes more accurate and elastic, without losing too much in depth and intensity. It will be much easier to notice the effect if the high frequencies are "crushed" by the amplifier's tone controls. Better yet, listen to the low-frequency section separately by connecting the speakers in a two-wire circuit. Four wire game.
Now the midrange will live here.
Pasting with a sound absorber the pipe of the midrange loudspeaker and the rack.
Same as above, plus gauze mat #2 (see text).
Bi-wiring sockets outside...
...and from within.
"Minimalized" crossover. It is no longer possible to simplify it any more - everything superfluous has already been soldered.
Contact? There is a contact! Thus, we have smoothly approached the next stage of the upgrade - the organization of the bi-wiring scheme. For him, you will have to purchase three meters of speaker cable made of OFC oxygen-free copper. Taking into account the high power of the speakers, the recommended cross-section of conductors is at least 2.5 - 4 sq. mm. It is believed that pure copper is preferable for wiring the low-frequency section, and silver-plated copper is better suited for the mid/high frequencies. Now the market offers a huge number of all kinds of cables, and there is simply an endless field for experimentation. In the author's version, which gave good results, a Jamo OFC cable with a cross section of 4 mm (80-100 rubles / m) was used to connect the woofer, and Vampire Wire OFC Speaker Cable with a conductor cross section of 2,5 square meters was used for the top and middle. mm (40-50 rubles / m). They were also used for internal wiring in speakers. Then you should consider purchasing quality input terminals. In principle, of course, you can do without them by soldering the cables directly to the input of the crossover filters. This saves a decent amount and avoids several contact transitions. But on the other hand, you practically lose the possibility of further tweaking, i.e. selection of speaker cables (do not disassemble the same speaker each time to replace it). In addition, the seal is "tightly" inconvenient, because. it becomes more difficult to connect speakers due to the risk of getting tangled in cable coils. Therefore, if you decide to tune the 75AC to the maximum, then do not be stingy and take good gold-plated "screws". Unfortunately, they are quite expensive: not cheaper than 300-400 rubles. for one pair (there are products of well-known companies, not Chinese consumer goods). Since two of these pairs are required for each speaker when bi-wiring, bi-wiring them will cost a pretty penny. Plus, you need to splurge on connectors - "spatulas" or "bananas" - 4 pcs. for each cable. You will need 16 such connectors, and the price of each is at best about 30-40 rubles. This is how accounting works. However, if you have already managed to modify the speakers to the level of 75AC-SE, your costs will be halved, since you should already have half of the cables and connectors. Purchased connectors should be pressed onto the stripped ends of the cable of the desired length. This is best done with a special crimping tool, but with some accuracy, good results can be achieved with ordinary pliers. After the cables are ready, you can begin to "open" the speaker systems. We will assume that the starting point of tuning-II are the speakers of the 75AC-SE level. On the cover with connectors, we install an additional screw pair for midrange / HF loudspeakers. Then we remove the crossover board from the case. We disconnect the inputs of the midrange and high-frequency filters from the low-frequency loudspeaker circuit and connect them to our screw pair with separate cables. (Please note that for reliable soldering of copper conductors of a large diameter cable, an electric soldering iron with a power of at least 80-100 W is required, so stock up on it in advance. Keep in mind that cold soldering can negate all your efforts to improve sound quality.) Then we produce corresponding mounting changes in filter circuits, i.e. we separate the "earth" circuits of each of the loudspeakers. Actually, on this "electric" stage of tuning-II is completed. All further operations are associated with damping the internal volumes of the case to combat the reflections of sound waves inside it. To do this, you will have to spend a little more money and purchase several sheets of fibrous sound absorber (filter for kitchen hoods), one package of which today costs about 100 rubles. This material is necessary to absorb the energy of standing sound waves from the back of the diffuser. A porous polymer sheet is rolled into a tube and glued to the inner walls of a cylindrical box that isolates the midrange loudspeaker from the rest of the volume. At the same time, in order to leave enough space for the "basket" 30GDS-1, it is necessary to retreat from the upper cut of the box by about 50 mm. To increase the degree of midrange damping, it is a good idea to place a small mat of loose cotton wool on the "bottom" of the sound absorber pipe (cotton balls are also suitable). After that, the same sound absorber inside the speaker housing (through the hole for the woofer) should be pasted over the outer surface of the phase inverter pipe and the insulating box of the midrange speaker, as well as the wooden stand-strut between the rear and front walls. Then one more mat needs to be sewn from gauze, into which about 0,8-1 kg of cotton wool is loosely stuffed. Mat #2 is placed between the wooden brace and the midrange box at the top of the cabinet. At the same time, it should be fixed (with glue, using buttons or shoe studs) on the side walls and on the spacer so that the mat overlaps about half the height of the case. The clearance is necessary so that the sound wave from the back side of the 100GDN-3 can freely penetrate into the upper half of the body. At the same time, the presence of a sound absorber ensures efficient dissipation of the energy of sound waves reflected from the upper wall of the housing. Particular attention should be paid to the fact that mat No. 2 does not block the phase inverter hole. To avoid unwanted overtones due to sound waves from the woofer, try to hide the wires from the midrange and tweeters under a layer of sound absorber. In extreme cases, they can simply be fixed on the side walls. Feel the difference OK it's all over Now. Now you can assemble the speakers (which, by analogy with the tuning depth indexing system introduced by Ken Ishiwata at Marantz, we will assign the conditional index 75AC-SE mkII) and start testing them. If your amplifier has two pairs of speaker output terminals, it makes sense to connect the cables to different pairs. Moreover, many amplifier models provide the ability to switch the speakers connected to it, which makes it possible to listen separately to the sound of the speakers in both the low and mid/high ranges. Before you start listening, make sure that the polarity of connecting all the speakers in the speakers is correct. With correct speaker phasing, the sound source of a mono program or pink noise recording should be exactly between the speakers. It's also a good idea to "buzz" the speakers with a sliding sound tone signal in the frequency band 20-20000 Hz (tracks 16-18 on the CD "Our people in the city!", Supplement to "Salon AV" No. 3/98). The volume of the speakers should be more or less uniform throughout this range, excluding the lowest frequencies. If something seems suspicious to you, once again check the correct installation of the filters and the phasing of the speaker connections. If you don't have this or another test CD, then just listen to a recording you know well to see if there are any anomalies in the sound. After the "qualification tests" we launch the full program test: the most diverse sound material was listened to - from heavy metal to classical music and instrumental jazz. And what do we hear? According to the first (always the most correct) impression, the naturalness and sound resolution of the 75AC-SE mkII at medium frequencies have increased by almost an order of magnitude. On vocals, for example, not only the finest nuances of the manner of performance, but also the breath of the singers, the rustle of clothes and other quiet sounds that were previously ignored in most cases by the "old" 75AC (even the SE versions) were clearly audible. The tall ones, figuratively speaking, have become "thinner than a human hair" and louder than the Valdai bell. If earlier the sound of brushes on plates simply pleased, now it delights: you can even hear how individual wires slide along the surface of the plates. And what is the sound of a triangle of a symphony orchestra! The bass also picked up and almost got rid of the "mumbling". Although, for the sake of justice, we admit that it was still not possible to achieve from the 75AC the same clear sound on the bottoms as in high-end imported Hi-Fi stereo systems. However, perhaps a fair share of the blame for this lies with the room itself, since good acoustics of listening rooms in audio equipment salons is achieved only after special treatment of their walls and floors. The size of the listening room is also important: it is very difficult to achieve a clear bass in small ones (especially square ones) because of the standing sound waves that inevitably arise in them. Therefore, even from very good and expensive speakers, it is rarely possible to hear a truly correct bass in a real living space. Incidentally, the room where your stereo "lives" for preventive purposes is useful from time to time to "buzz" a sliding sound tone. At the same time, the resonances of the glasses of bookcases, the clinking of dishes and other similar parasitic sounds are perfectly revealed. Indeed, under normal conditions, they are masked by the sound of music, but subconsciously perceived by the brain as "dirt". If such flaws are found, the room must be "cured". For example, smear the ringing glass in the closet and in the windows around the perimeter with mastic or sealants, put rubber rings under the shelves with dishes, etc. If there are pronounced resonances in the frequency band of 100-500 Hz, try changing the location of the speakers in the room and "buzz" it again. Uneven sound at high frequencies and their excessive brightness can be reduced by hanging a carpet opposite the speakers on the wall and placing a carpet on the floor. If, on the contrary, the room is too muffled and the tops are sorely lacking, use the RF tone control in the amplifier and compensate for these losses. Well, now is the time to enjoy the sound of the 75AC SE mkII speakers, which you may not immediately recognize - it will change so much. Given the clear improvement in sound quality, the cost-effectiveness of speaker tuning is undeniable. Indeed, otherwise, in order to achieve an adequate level of sound quality, one would have to buy new imported acoustic systems worth no less than $450-600. PS. Once you get the hang of your speakers, you'll probably want to further improve the sound of your stereo system. This is the time to use the author's proposed method of searching for "bottlenecks" ("Catching fleas in a scientific way", "Salon AV" No. 3/99) and identify which of the components is now the most "weak link". With an increased class of speakers, it may turn out to be a signal source or amplifier. A quite reasonable question arises: why not modify them too? Moreover, our magazine has already published articles on the "cold", "warm" and "hot" upgrade of Hi-Fi components (for example, "The best is the enemy of the good", "AV Salon" No. 2/99). It is possible that in the near future the editors will return to this topic again. In the meantime, we wish you success in the noble cause of upgrading and tuning your Hi-Fi equipment. After all, nothing pleases the heart of a real audiophile so much as the sound of his own stereo system brought to mind. PPS Before starting the complete dismantling of the AU, we advise you to once again soberly evaluate your qualifications and decide whether such a labor feat is within your power. Author: Konstantin Bystrushkin, Salon AV; Publication: cxem.net
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