Created on 07.09.2007 16:04.
Updated 10.25.2019 19:03.
Author: Igor Alekseev.
After the creation of the speaker system of the Trumpet Vojta project and the purchase of the DVD + Receiver set by their owner, they needed a Subwoofer. Necessary ? it should be so. Moreover, one comrade had for a long time been gathering dust in the corner of the active acoustic systems S-70 of the Riga radio equipment plant, which it was a pity to throw away (after all, with EMOS), and there was no desire to listen. people have long been listening to vinyl through a tube amplifier on a homemade three-strip on speakers 2A-12, 4A-28 and 10GD-35RRR.
So it’s decided. We recycle the S-70 to make a pair of sabs (second to ourselves) – i.e. for a good cause. First, we disassemble the AC-ki. What we observe in the process: a set of numbered speakers was made at the factory on various 75GDN-5 speakers, one with a ferrite magnet, the other with alnico. Oddly enough, but their TS parameters were, to put it mildly, identical.
Again we have to state the fact that the heads are ON THE LEVEL! The calculation of the closed box for them (case S-70) was made IMPROPERLY! It is the volume in which both the maximum travel of the diffuser of the heads at 50 Hz and the maximum long-term input power to them coincide.
The design of the amplifying part, fixed on the rear removable wall of the speaker system, is a high-tech bundle of wires connecting the boards with electrolytic capacitors dried out from heat and time. Definitely a complete dismantling.
This is the question that haunts to this day: how could the printed circuit boards of the power amplifier be covered with such a layer of dust and dirt, if the boards were in a sealed casing, smeared with sealing mastic on all sides?
The boards were washed with water, the trimmer resistors and capacitors (K70 and K50-6) were replaced with new, tested ones. This is the question that haunts to this day: how could the printed circuit boards of the power amplifier be covered with such a layer of dust and dirt, if the boards were in a sealed casing, smeared with sealing mastic on all sides? The boards were washed with water, the trimmer resistors and capacitors (K70 and K50-6) were replaced with new, tested ones.
The iron plate, fixed outside the speaker system and acting as a radiator of the output transistors, was moved to a new case with minor modifications – the input jack for the tulip, the power jack for the standard power cord for the PC, the toggle switch for turning on the head re-phasing relay and the trimming resistor of the level control were fixed …
As a result of “trepanation” from a pair of S-70, tweeters and a semi-finished body remained. The rest: power electrolytes and internal waste went to the landfill.
The work began with measuring the TS parameters of the 75GDN-5 heads. After that, several types of acoustic design were modeled, taking into account the real possibility of placing the finished sub in the room. The choice was made in favor of the 4th order Bandpass design.
The maximum output power of the ULF from the S-70 was determined at the level of 45W of a sinusoidal signal in the 50-500Hz band, which is sufficient for both the swinging of the head and for the non-stressed mode of operation of the standard power supply.
The cases of the sub are made according to the proven technology of “puff sandwich” birch-fiberboard-pine, 35mm thick, glued under the press. The upper compartment of the housing is designed to accommodate the electronic part, the dynamic head is fixed on the lower partition. The partitions are installed in the body using “silicone pads” on glue and screws, which ensured complete tightness of the joints. Thanks to the installed partitions, the case turned out to be absolutely deaf.
The storage electrolytic capacitors of the power supply are new, all power wiring is made with high-quality wire. The layout (preliminary) of the electronic part is shown in the photo. The relay is used to rephase the connection of the dynamic head to the output of the power amplifier. using switches for these purposes is unreliable, and inverting the phase of the signal at the PA input leads to unnecessary complication of the circuit.
The rear wall of the case in the upper part has a cutout, in which the amplifier board is installed practically in the thickness of the material and is fixed on the standard radiator plate.
The frequency response of the subwoofer (real values of sound pressure) in a room with an amplifier output power of 45W is shown below:
As for the bass quality. The main drawback of purchased subwoofers, built as a WL or FI, is that there is no difficulty in matching, not with front speakers at the frequencies of their joint operation (although there are also many problems there, which is why an adjustable low-pass filter is included in the electronics of industrial subwoofers, etc.). The main disadvantage is the docking of the sub with the frequency response of the room in the lower part of the range. Excessive protrusion or lack of the lowest frequencies (30-60Hz) causes large values of the group delay time (GDT) of the reproduced signal at these frequencies.
If they say that the sub is “cinematic”, then most often it means that it does not fit the given room and no adjustments can correct this situation.
This sub sounds “loose and tight”. And nothing can be done about it. It can only be treated by replacing it with another model / manufacturer.
With regard to this subwoofer, we can say the following: the real group delay at a frequency of 50Hz does not exceed 10ms, and at frequencies of 120-150Hz – 5ms, which fully fits into the IEC recommendations. Its sound is unobtrusive, sometimes you get the feeling that the subwoofer is not working, but when you really turn it off, the sound of the main speakers immediately becomes “flat and not as expressive as it was”.