Refinement of acoustic systems of foreign production

Currently, on the Russian radio equipment market, you will be offered many inexpensive mini-systems, which include acoustics for a house of average quality. Usually these are two- and three-way bass-reflex systems with the simplest crossover filters on non-polar oxide capacitors. For example, the Technics SC-EH60 mini-system I bought from the Japanese company MATSUSHITA was equipped with SB-EH60 acoustics. The quality of its sound clearly did not correspond to the parameters of the amplifier 34 of the system itself, which was revealed when a higher-class speaker system was connected to it. This circumstance prompted me to make an attempt to modify the “SB-EH60”.

This acoustics has two cone-type tweeters with paper cones. Due to their design features, such heads have very low electro-acoustic parameters, but since their cost is low, they are widely used by manufacturers in relatively cheap audio equipment. The heads are mounted on the inner side of the plastic front panel, which makes the sound of high frequencies fuzzy, blurry.

LF-MF frequencies are reproduced by a head with a paper cone with a diameter of 14 cm. The sound of the MF range is tolerable, except for the well-perceived spurious emissions on the frequency response of the speaker system. As for the reproduction of low frequencies, it is accompanied by booming and resonant sounds introduced by the insufficiently strong housing of the acoustics when operating at high power.

The fact is that the body is made of 12 mm chipboard, there is no sound-absorbing material in it. The crossover filter of the speaker system is made on only two non-polar oxide capacitors. The filter section frequencies are 3 and 8 kHz.

When modifying the speaker system, I tried to replace the crossover, but this did not give positive results, mainly due to the low quality of the tweeters. Therefore, I decided to replace the high-frequency heads of the acoustic system of the mini-system with one domestic 10GDV-2-16, which has better characteristics. Naturally, the crossover filter had to be replaced as well. Its circuit is shown in the figure.

Since the low-midrange head effectively reproduces the frequency range up to 5 kHz, the crossover frequency of the filter was chosen equal to 5 kHz. The filter parts are mounted on a fiberglass board. Coil L1 is frameless, its winding is wound on a mandrel with a diameter of 21 mm and contains 153 turns of PEL-1 0.57 wire. Resistor R1 – PEV-3. Capacitors C1. C2 – MGBO for an operating voltage of 160 V.

Since it was not possible to install the 10GDV-2-16 head inside the speaker case, a separate HF box was made for it, placed on top of the mini-system speaker. The dimensions of its body are not critical, it is only important that the head is fixed from the front side of the box through a rubber or foam rubber vibration-absorbing pad. It is advisable to lay out the inner surface of the HF box with cotton mats.

The HF speaker heads of the mini-system are disconnected from the UMZCH and left in their places.

Now for the further refinement of the speaker system. Considering that the “SB-EH60” AC case, like most other mini-systems, is non-separable, we had to remove a part of the back wall to strengthen it with a hacksaw. Previously, a panel with sockets for connecting an acoustic system to the UMZCH and a phase inverter pipe were removed from it. Then, through any of the holes formed, you need to carefully cut out a part of the back wall around the perimeter so that 15 mm on each side remain to its edges. Instead of the removed part of the back wall, a new one made of 12 mm plywood is installed. It is necessary to cut out holes for the bass reflex pipe, panels with sockets for connecting the UMZCH and wires connecting the speaker to the HF box.

The new back wall is fastened with screws overlapping the remaining edges of the old one. But before installing it, you should damp the rear and external surfaces of the LF-MF diffuser by soaking them with a Guerlain solution in gasoline. To do this, you need to prepare a solution of the consistency of liquid sour cream and grease the rear surfaces of the diffuser and corrugation through the windows in the diffuser holder. Then do the same with the front surfaces of these head parts. When the solution is dry, this operation is repeated one more time.

To strengthen the enclosures of the loudspeakers of the existing acoustics, I recommend installing spacers made of wooden blocks with a section of 20×20 mm inside them. The length of the spacers should be chosen so that they fit snugly and without distortions between the side walls of the loudspeakers. They should be attached to the walls themselves using metal corners. All seams of the housings should be coated with plasticine, the old ones should be removed and new separating filters should be installed. After that, evenly fill the entire internal volume of the loudspeaker enclosures with cotton wool (about 800 g), leaving a free space opposite the phase inverter pipe. To prevent cotton wool from getting inside the woofer-midrange heads, the windows of their diffuser holders should be sealed with gauze or any other acoustically transparent material.

Upon completion of the above works, it is necessary to make holes in the newly manufactured rear walls of the loudspeakers for installing the bass reflex pipes and strips with sockets connecting the speaker system to the UMZCH. Then, through another specially made hole in the back wall, pass the connecting wires of the HF loudspeaker and connect them to the outputs of the LF-MF head.

After refinement, the sound of the acoustics has improved significantly. The bubbling and unpleasant side-tones in the low-frequency range have disappeared, and the reproduction of the highest sound frequencies has also improved.

According to the given methodology, home acoustics such mini-systems as “Panasonic CH40”, “Panasonic CH32”, “Daewoo AMI-940”, “Sharp CD-C451H” and others. Their owners were very pleased with the improved sound quality.