Second wind 75АС-001

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Created on 18.10.2008 14:59.

Updated 09.16.

Author: Audiomaster.

Refinement of the Russian speaker system 75AC-001, for example, the upgrade of the crossover and the improvement of the acoustics case.

Specifications 75АС-001

Parameter 75 AC-001 Cleaver 75 AC-001 Corvette
Speaker type: 3-way speaker with bass reflex 3-way speaker with bass reflex
Frequency Response Range: 25 (-19 dB) – 25000 Hz 25 (-19 dB) – 25000 Hz
Frequency response in the range 100 – 8000 Hz ± 3dB ± 3dB
Sensitivity 91 dBA 91 dBA
Characteristic sensitivity 0.73 Pa√W 0.73 Pa√W
Rated electrical resistance 8 ohm 8 ohm
Minimum impedance value 6.4 ohm 6.4 ohm
Limiting noise power 100 watts 100 watts
Ultimate long-term power 150 watts 150 watts
Maximum short-term power 300 watts 300 watts
Weight 30 Kg 30 Kg
Dimensions (WxHxD), cm: 38.6x71x34 38.6x71x34

History:

Acoustic systems 75AS-001 (aka 150AS-001), which under the names “Corvette” and “Cleaver” were produced by the factories “Okeanpribor” (Leningrad) and “Krasny Luch” (Krasny Luch), respectively. At the same time, “Corvettes” were produced significantly more than “Cleavers”.

Both models have completely identical sets of loudspeakers, circuits, design and characteristics of the 75AC-001. However, in addition to speakers, “Red Ray” also produced a set of speakers for both types of speakers, and therefore some audiophiles still believe that “Cleavers” sound better on their 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 medium-frequency diffuser 30GDS-1 was simplified, the internal vibration isolation of the housing was excluded.

75AS-001 is the latest development of VNIIRPA im. 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 crossover filters using a computer were widely used.

In addition, the speakers have a number of other advantages. First, they are equipped with efficient loudspeakers (100GDN-3, 30GDS-1 and 10GDV-4) of the new generation, which made it possible to achieve a record (for the end of the 80s) sensitivity of 91 dB (W / m).

Pic 1. Speaker 100GDN-3

The calculation of circuits on a computer provided a wide range of frequencies 25–25000 Hz with a small uneven frequency response 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 high-frequency and mid-frequency heads in the same plane made it possible to significantly improve the uniformity of the phase response. Thirdly, the large body volume, low resonance frequency and the large diameter of the woofer cone provide sufficiently deep and powerful bass.

Finally, in the design of the crossover filters, air-core coils wound with high-quality copper wire of large cross-section and MBGO metal-paper capacitors are used, which are characterized by low losses and distortion.

The circuit elements are mounted on a cardboard base using a hinged mounting. Unfortunately, this large barrel of honey contains a few hefty flys in the ointment. The largest contribution to the sound spoilage is made by overly long wires from an ordinary “tin”, 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 conventional 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 practically impossible to cause their overload in the living room, especially when you consider that, according to the passport data, the short-term input power can reach 300W.

In short, in real conditions, there is no benefit from ultrasound, while the sound suffers greatly from it. With the bass driver, despite the absence of relay contacts in its circuit, the situation is not much better. LF components also pass through the UZ board and a 5A fuse.

All this leads to a significant increase in the length of small-section wires that are by no means audiophile quality. The same cable was used when installing the filter board, which is also far from 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 audio path. The reasons for the failed attempts to make speakers sound more worthy with the help of audiophile speaker cables are now finally understood. Obviously, even the best of them is not able to compensate for the loss of quality in the internal wiring.

By the way, the input screw terminals of the 75AC-001 speaker also leave a lot to be desired.

The design of their contact pads is such that it is impossible to connect “forks” and “blades”, which provide a convenient and reliable connection. More or less satisfactory results are achieved when the stripped cable is clamped in them, however, when the terminals are screwed in, some of the cores are squeezed out. And, finally, the scanty amount of damping material inside the case was scanty for such a large internal volume.

According to the reference data, ATM-1 fibrous material made of super-thin glass fibers glued with phenolic resins should be used in 75AC-001. However, in appearance, the sound absorber was very similar to ordinary rayon wool. Because of this, spurious emission of sound from the back of the woofer cone 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 delayed. As a result – indistinct and boomy sounding at the bottom.

Refinement of acoustics 75АС-001

So, on the basis of the above, the following main directions for the refinement of the 75AC-001 speaker system were determined:

  • exclusion of unnecessary elements from sound circuits;
  • replacement of internal conductors with high quality oxygen-free copper conductors;
  • optimization of the connection diagram to reduce the length of the conductors;
  • replacement of input connectors with powerful screw terminals;
  • pasting the inner surface of the case with an effective sound-absorbing material.

About 200-300 rubles and 2 man-days were spent on the proposed revision. You will need: 6-8 m of Oxygen-Free Copper (OFC) speaker cable with a cross-section of 2.5-4 mm2 (about 10–20 rubles / m); two pairs of screw clamps, preferably gold-plated, of a universal design that allows you to connect “bananas”, “spatulas”, …

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