6АС-2 – new sound

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equipped with electrophones of the first complexity group “Melody-101-stereo”, are considered by many to be hopelessly outdated.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, this requires a little refinement of the Russian acoustics.

The magazine “Radio” has more than once published articles on the revision 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 shortcomings of the factory design, but also to make the most of its advantages.

The main defect of the Russian acoustics 6АС-2 – 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 (so as 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 the remains of foam rubber and putty.

All four walls of the case and the plane of the rear panel with the help of Bustilat glue should be pasted over from the inside with one layer of linoleum with a fabric base. 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 has been cut for a cone 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 assembly. 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 degree of 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 ohms and two more 24 ohms). Such a shunt not only equalizes the frequency characteristics of the sensitivity and electrical resistance of the head, but also, 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 extreme cases, K73-11 are 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 obtain a circular directivity pattern, with all the ensuing advantages.

It should be noted that the parameters of the speaker are optimized after 50 … 60 hours of operation, necessary 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 did not have the opportunity 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 decays 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 6АС-2 was recognized as preferred by all experts when comparing it with the sound of such well-known “shelf” speakers as “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 “disco”.

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).

Source: Magazine “Radio”, No. 3, 2000, ZVUKOTEKHNIKA