Improvement of the Microlab B-72 speaker system. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Speakers

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It just so happened that I used this set of speakers as a computer one. They were bought on the principle of the best that was enough for that moment. In 2008 it was 600 wooden rubles. At the moment, their cost is 1200-1300 wooden ones.

It is hard to argue that as a background and just a sound source, they fully win back their price, but over time, the incredibly disgusting sound began to ask for something to be changed. It is difficult to describe the sound of such a class of acoustics, but in a nutshell, there was no lower bass at all, some significant sound pressure started from 60Hz, grew sharply to 80Hz (where the whole body buzzed terribly), the absence of at least some articulation in reproduction , not just details, at least some. When watching movies, I had to turn the bass to a minimum, otherwise the whole sound, including dialogues, was reduced to a continuous resonant boom. I won’t even talk about the stage, it simply didn’t exist, all the sounds were clearly stuck to the speakers. The whole sound is like through some kind of cotton veil. In general, a typical picture for active speakers for 1200r. However, for some reason there was a clear understanding that these speakers can still do more. On the Internet, there was not a single example of a tweak of this acoustics, but there were pink snots of mp3 fans from the built-in speaker of a mobile phone, about the excellent sound of this model. There were also characteristics from Microlab, where they shamelessly lied about magnetic shielding and frequency response from 30Hz ...

So at some point in time, all this stuff was collected in a bag and taken to work. The situation was complicated by the fact that at work we are dealing with digital technology, therefore, from the measuring equipment - a mediocre oscilloscope and Chinese multimeters. In many ways, I had to rely on my own ears and the audio frequency generator on the computer, since the ears are trained and relatively experienced. All alterations were made from improvised materials, sometimes not quite those that I would like to use, but maybe this is for the better, the tweak should be commensurate in terms of labor and costs with the original donor, otherwise it becomes meaningless.

First of all, I dismantled the active speaker and gutted the transformer and amplifier from it and studied it. The guesses were immediately confirmed. TDA2030 in a typical inclusion with maximum savings on parts. But for starters, the speakers themselves - the case, although made of MDF, but due to the small thickness of the walls, rang like a glass when tapped. I did not glue the spacers, I went a simpler and no less effective way for such cases - vibroplast M2 over the entire inner surface.

After rolling, the cases became deaf as if they were made of concrete, I added a little holofiber, they usually pack a lot more, but I was afraid to stifle the speaker in a compact case and create inoperable conditions for FI, you can also lose punch if you get carried away with stuffing, which I didn’t want. I didn’t touch the FI, oddly enough they didn’t make aerodynamic noise, I didn’t want to thoughtlessly lengthen it, and there’s nowhere to go, the wall is close, but I don’t think it’s possible and right to make completely new and bent without calculations and measurements. The entire filter in these speakers consists of one capacitor per tweeter at 2,2 microfarads, which is good in its own way, if it were not a polar electrolyte of unknown origin. Replaced by K73-17 2,2uF 63V Playing with more complex filters without measuring equipment would be stupid. The wires inside were already soldered at the factory and their quality was quite consistent with the quality of everything else, so they remained unchanged. The passive speaker was all, the conversation continued with the former active one.

Initially, the plans were to modify the cases, change the electrolytes to non-electrolytes and change the TDA2030 to TDA2050, however, after the case was finalized, it became clear that the trance and amplifier in the column were guaranteed nothing to do, bass appeared, and with them a 50Hz background, actively induced by the transformer to the woofer. This was not heard before. Yes, and the difference in the sound of an empty and stuffed column was clearly audible, so it was decided to take the amplifier with power into a separate case, however, leave the tone block in the speaker case. in the case, it occupies little and is generally passive, so it was also decided to leave the line-in connectors in place so as not to drag interconnects back and forth, however, I decisively got rid of connectors of dubious quality in the internal wiring using wire cutters and a soldering iron.

The connector to the amplifier along with the cord was used from a dead Pioneer sound processor :) Through it, directly from the column from the tone block, the signal is supplied to the input of the amplifier, and the LED on the tone block is fed from the amplifier, and the signal and power of the LED are in shielded pairs, there are a lot of them inside this cable, decided to play it safe. The connector is cool, with a latch. I glued the power switch in place, filled all the holes from the fasteners with hot glue, glued it on the outside with black arokal, and on the inside with vibroplast. The containers in the tone block are made from lavsan from birth, so I didn’t touch anything else there. On this, the former active column was collected and all conversations with it also ended.

The amplifier began to present surprises and surprises almost immediately. The board itself is divorced well: the earth is a star, the diodes in the bridge are shunted by filmmakers, there are places for soldering ceramics for power supply in front of the amplifiers, though nothing is soldered there. After the diode bridge, there are 2 electrolytes of 4700 microfarads and 2 of 100 microfarads each, although they are shifted specifically to the amplifier of the right channel, and a jumper of a humorous section goes to the left. I strengthened the jumper, soldered 100 microfarads of electrolytes and soldered them to the paws of large electrolytes on the reverse side of the board lying down, at the same time shunting them with ceramics of 0,1 microfarads and filling it all with hot glue. The amplifiers themselves were changed from TDA2030 to TDA2050, why they don’t do this at the factory is still a big secret for me (during the tweak process, the opinion was born that the TDA2050 is more demanding on piping, so with the TDA2030 you can save not only on the chip itself, but also on strapping, but I don’t care about the sound).

Electrolytes of 1 μF were used as input separators. A search in the bins gave birth to a dead Kenwood audio processor, 2 680nF and 2 540nF films were extracted from it, brown, there is no more data on them =) I soldered in parallel, one in place of the electrolyte, the second on the reverse side of the board. Filled with glue. The result of the test inclusion was surprising - a strong background of 50 Hz in the right channel, an instant warm-up of the amplifier and transformer, and hard clipping even at low volume. I went to look for short circuit or ground. Have not found. He began to change the capacitors in the channels in some places, the culprit was found quickly.

According to the datasheet, in parallel with the speaker, there should be an anti-ringing RC chain of 0,1 μF non-electrolyte and a 1 Ohm resistor for the TDA2030 and 2,2 Ohm for the TDA2050. The Chinese, on the other hand, had a jumper instead of a resistor, and with the TDA2030 it worked fine, but from 2050 it stopped and broke through the capacity. I reasonably decided that with a wire length of less than half a meter, this circuit is not needed and dropped out the capacitances. The background went away at first, but when warming up, the amplifier became unstable and oscillated at random frequencies. I soldered k73-17 at 0,22uF 45V in their place, it didn’t help, the capacitance didn’t die, but there was no stability. As a result, I soldered 1,2 ohms each (which I found) instead of jumpers and removed everything as if by hand, generation is not audible either with ears or with an oscilloscope, the radiator is barely warm, the trance is cold. I didn’t find out why I consistently generated only the right channel, replacing the amplifier with another new one did not give any result, apparently somehow connected with the wiring of the left channel.

In the feedback circuit there is an electrolyte to the ground at 22 microfarads, as much is required according to the datasheet. For good, I would also change it to at least a unipolar one, but this was not found, so I replaced it with a 47 microfarad electrolyte, subjectively the bass became slightly fuller, literally on the verge of audibility. It remains to compete with HF noise with the speakers turned on. Datashit advises to put power ceramics, 0,1 uF, as close as possible to the microcircuit. I decided to take the datasheet literally (after fiddling with the resistor) and soldered these capacitors directly onto the paws. The tone has changed...

He began to try in turn everything in a row, capacitors falling under the arm. The timbre, amplitude, frequency of the noise changed. As a result, complete silence was achieved with capacitors marked M15 - I don’t know how much it is, probably 0,15 microfarads. Now the amplifier is absolutely stable, it does not heat up more than 40 degrees and it is possible to distinguish between on and off without a signal at the input only by the LED, even if you bring your ear close to the tweeter with the volume completely turned up to maximum. On this, the creativity with the amplifying part was completed, all capacitors and protruding elements were mercilessly filled with hot melt adhesive.

Now nutrition

It is known that Microlab stings on transformers in life with all the ensuing consequences - insufficient maximum power, clipping, penetration of channels into each other by power, trance ringing, etc. On the native trance, 9 Volt (bipolar) 0,8A was written, but these characteristics also raised strong doubts. I did not rewind, the iron is frankly weak.

Chamber of Commerce and Industry 250-220-51 was extracted from the bowels of the cabinet. The bipolar 11V and the declared current of 0,9A were already somewhat more interesting, the iron was clearly larger and the build quality was many times. It was a pity to lose precious volts on the diode bridge, so almost all the stash was brought to light, but no Schottky diodes were found, so they were stupidly chosen with the lowest transition resistance and soldered instead of the native ones. All this happiness was collected in a case from a motorcycle charger, it barely fit. I glued the transformer to the zippers, excellent vibration and electrical isolation, the amplifier was bolted, I sealed the unnecessary holes with an oracal, I didn’t put the case on the ground, I also applied cosmetics. I'm fine, but time was running out. Later, the body was rated as "so disgusting that I like it", at the same time it provokes listening to the Sex Pistols;)

The results

The rumble and mumbling at low frequencies have completely disappeared, while the speakers begin to press quite audibly already at 30 Hz, which is really surprising. In general, the articulation and dynamics of the bass were very pleasing, although subjectively it became less, but the point here is the lack of body resonance, so now the bass control is at its maximum, you still need to kick the 4-inch driver. Clipping has moved aside, the detail has increased monstrously, as if a cotton veil has been pulled off the speakers, the stage rises like children go to school, rare echoes stick to the speakers, the sound has become unobtrusive and pleasant. In general, the columns jumped over their class and the one next to it from above. At the moment, I’m sure that they can easily outplay regular consumer goods systems up to 5000r confidently, they would still have fabric tweeters and not plastic ones, but buying specifically for them is too cool, so if only they’ll drop it for free;) HF at 14-00, more - plastic cuts the ear, less deaf already. As a bonus, I got the opportunity not to turn off the speakers at night and put a cell phone at least on the amplifier itself, before a call or SMS in a zone of 2 meters from the active speaker caused a guaranteed obscene reaction from all households. Especially at night.

In general, I am 100% satisfied with the rework, the result paid off both the initial investment and the investment and work on fine-tuning, the result exceeded my expectations.

Microlab knows how to make good constructors for further refinement, which was proved once again.

Author: Eduard Witlif; Publication: cxem.net

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