Cyber Acoustics MMS-20 amplifier blows speakers

[lti]

I have had these computer speakers for over 12 years, and recently the amplifier in them has been giving me trouble.
First, the sound was very quiet with a humming sound coming from the right speaker. I partially recapped and resoldered the amplifier and it sounded great... for a few days. Then, one speaker suddenly blew. It worked perfectly the day before, but the next day the right speaker had a well-done voice coil.
I have had headphones connected to the speakers ever since because the amplifier is able to produce tolerable sound from my computer's terrible onboard audio. However, the right driver in my headphones is now making buzzing and rattling noises.
I checked the voltage across the headphones and there is no DC voltage across either channel.

I also have another problem with the amplifier. When I turn the power off, there is a huge voltage spike across the input jack. I discovered this accidentally by connecting the speakers to the audio input on my computer.

What is going on? I have pictures of the voice coil and of the amplifier board.

[Th3_uN1Qu3]

Re: Cyber Acoustics MMS-20 amplifier blows speakers

I have had these computer speakers for over 12 years

Then maybe it's time for a new set of speakers. Not a lame attempt at speakers.

That under the little heatsink is a TEA2025. They are known to fail intermittently. My dad had an old 2.1 set which used TDA2822 for the sats and TEA2025 in bridge for the sub. Apart from the absolutely tiny "sub box" the speakers were actually engineered properly. With crossovers and everything. Anyway, after many years the sub failed. The TEA2025 IC was bad, but it would work for ~20 minutes after being heated with the soldering iron. Then it would die again.

I bought a new one, only to have the same thing happen within an hour. I blew three new ICs till i found one that worked. Later, the transformer in the speakers was found to operate the ICs at their absolute maximum ratings. It was a proper transformer unlike what i've seen on more recent sets of the same type. The new ICs simply didn't make the cut - either they custom ordered them when the set was made, or the ones i bought were fake.

Anyway, you can find better use for your time. Throw 'em in the bin.

[lti]

Re: Cyber Acoustics MMS-20 amplifier blows speakers

Later, the transformer in the speakers was found to operate the ICs at their absolute maximum ratings.

That's the problem. The chip is only rated for 9V in bridged mode. Most of the subwoofers I've seen are rated for 4 ohms, and the chip is not designed to power 4 ohm speakers with a supply voltage higher than 6V in bridged mode. If the chip was running at its absolute maximum voltage, it would be receiving 15V.
This amplifier was also poorly designed. The SVR cap was only rated for 10V, but the included AC adapter produced 12V. The entire set of speakers was designed to be powered by a 9V power supply, and that is why the original drivers fried.

Anyway, I would like to keep these speakers and possibly even find a replacement driver for the one that smoked. Modern computer speakers that I can afford don't sound very good. Also, I don't believe in the "it's junk if it doesn't have a subwoofer" bullshit.
The big mess on the input of the TEA2025 is somehow able to reduce treble and boost bass, placing both within the levels that a properly working audio source would produce instead of what my old computer produces.

Why doesn't the quote show up properly?

[Th3_uN1Qu3]

Re: Cyber Acoustics MMS-20 amplifier blows speakers

You forget to put the ending /, that's why. [/QUOTE]. And don't give me that BS, yiu can buy a decent set of speakers for under $50. Of course you don't need a sub... if your main speakers have 8 inch or larger drivers.

Drivers that will fit your set are available at Parts Express but they will cost more than your plastic boxes were ever worth.

[lti]

Re: Cyber Acoustics MMS-20 amplifier blows speakers

What if I don't have $50?

Also, I bought a pair of drivers from Parts Express, but they are not shielded (they were advertised as shielded speakers), have the worst sound quality of any speaker I've ever heard, and cause the amplifier to run extremely warm, even though they are the same impedance as the original drivers. These were great speakers when they worked.

Finally, I would agree that you don't need a sub if you have 8 inch or larger speakers, but it is hard to find subs that give decent sound for what I listen to.

Anyway, I want to at least have an amplifier that doesn't blow speakers or some circuit that will slightly boost bass and reduce treble so I can use headphones on my computer.

[Th3_uN1Qu3]

Re: Cyber Acoustics MMS-20 amplifier blows speakers

Anyway, I want to at least have an amplifier that doesn't blow speakers or some circuit that will slightly boost bass and reduce treble so I can use headphones on my computer.

Change out the TEA2025, lower the supply voltage, and only use it with headphones.

I haven't bought anything from PE due to what shipping would cost, but they generally have a good rep. Maybe you got the wrong drivers. I know what kind of drivers you are talking about. Around here they are still available in electronics stores (only the 4" not the 3" version though), but they are very inconsistent in parameters, and they have a tendency to buzz on certain notes. I wouldn't bother buying new drivers of this type. I bought 4 and 98% of the time they're sitting around doing nothing.

[smason]

Re: Cyber Acoustics MMS-20 amplifier blows speakers

yiu can buy a decent set of speakers for under $50. Of course you don't need a sub... if your main speakers have 8 inch or larger drivers.

.

Each person's "decent" is different, but I'm more than happy with the Logitech X-140 speakers at about $30.00

I'm not a gamer, and I'm over 30 so I don't need pounding Boom Boom bass.
I find they sound very good for $30.00 and don't take up much desk space.

[lti]

Re: Cyber Acoustics MMS-20 amplifier blows speakers

Why do I need to replace the TEA2025? The outputs are coupled and the coupling caps are still good. There is no voltage across the drivers or headphones.

If anybody is wondering, these are the replacement drivers:
http://www.parts-express.com/pe/show...number=299-066

[Scenic]

Re: Cyber Acoustics MMS-20 amplifier blows speakers

There is no voltage across the drivers or headphones.

right now there isn't, but when it heats up there probably will be.
the TEA2025 was ran above its max. specs all the time.. no wonder it goes tits up someday..
didn't kill itself completely yet, but already has intermittent heat related problems..

[lti]

Re: Cyber Acoustics MMS-20 amplifier blows speakers

Again, the TEA2025 is being run in stereo mode, so there is a coupling cap on both outputs. No DC voltage should get through the caps.
Also, this amplifier was never run above its maximum specs. That was another person's amp.

[nfiniti9]

Re: Cyber Acoustics MMS-20 amplifier blows speakers

You replace some speakers that were likely 3 watt RMS with some that are 25 watts RMS. The amp TEA2025 is 2.5 watts. Theres no way in hell those drivers are getting enough power to move them correctly.

[lti]

Re: Cyber Acoustics MMS-20 amplifier blows speakers

The old speakers were 3 watts RMS. If a 2.5W amplifier was able to blow a 3W speaker, it should have no problem powering a 25W speaker. Those speakers suck no matter how much power they are receiving. A 1.5" 0.5W speaker powered by an LM386 has more bass and clearer, more natural sound than those 25W speakers powered by a 25W RMS amp.
Also, those speakers sound equally shitty when they are powered by a 25W amp as they do when powered by a 0.5W amp.

[nfiniti9]

Re: Cyber Acoustics MMS-20 amplifier blows speakers

The old speakers were 3 watts RMS. If a 2.5W amplifier was able to blow a 3W speaker, it should have no problem powering a 25W speaker. Those speakers suck no matter how much power they are receiving. A 1.5" 0.5W speaker powered by an LM386 has more bass and clearer, more natural sound than those 25W speakers powered by a 25W RMS amp.
Also, those speakers sound equally shitty when they are powered by a 25W amp as they do when powered by a 0.5W amp.

Sorry that is just not correct. Underpowering will ALWAYS blow a speaker faster than overpowering. I'm not arguing that the speakers suck, I'm sure they do, but you cannot take a much larger mass and put it on a weak IC amp like that. I guess you will find out why if you don't believe me.

[Th3_uN1Qu3]

Re: Cyber Acoustics MMS-20 amplifier blows speakers

Again, the TEA2025 is being run in stereo mode, so there is a coupling cap on both outputs. No DC voltage should get through the caps.

I missed the coupling capacitor part - What makes you think the caps are good? Drop in new ones.

Sorry that is just not correct. Underpowering will ALWAYS blow a speaker faster than overpowering.

Say, when there's a 200W speaker on a 50W amp, yes. With this power difference... no it won't blow them. But it won't sound good either.

I think the blown voice coil on the speaker driver is simply due to age. As for the headphones, most are way too sensitive to be plugged into a 2025 or similar. I blew plenty of headphones myself in the same way.

[lti]

Re: Cyber Acoustics MMS-20 amplifier blows speakers

The caps are only a year old.

When headphones are plugged in, the output of the amp goes through a pair of 390 ohm resistors before going into the headphones. If these resistors were not there, my headphones wouldn't last very long.

How could age cause a voice coil to burn up? The coil is dark brown and you can see melted enamel from the wire. The material the coil is wound around is burned, as you can see in the second picture. The coil is shorted as well. Its resistance reads 0.04 ohms.

Finally, what is the best way to reduce the supply voltage? Should I buy a new AC adapter or just connect a voltage regulator between the current power adapter and the amp?

[lti]

Re: Cyber Acoustics MMS-20 amplifier blows speakers

How does the bass/treble boost circuit work? The only things with power going to them are the TEA2025 IC and the power LED.

[lti]

Re: Cyber Acoustics MMS-20 amplifier blows speakers

I have traced the circuit to a 22uF cap that has the positive lead connected to pin 15 of the TEA2025 and its negative lead connected to a resistor and a diode in series, whch are then connected to the base drive resistors of each transistor. The voltage on the negative side of the cap starts at around 3V and slowly drops to 0.23-0.24V.

I have attached a picture of this mess.

[lti]

Re: Cyber Acoustics MMS-20 amplifier blows speakers

This amp killed another set of headphones. What is going on? It is always the right channel driver that blows.

Also, the output coupling caps and the filter cap are warm to the touch, even though they aren't near any heat sources. The new caps are rated for higher ripple current than the originals (unless the original filter cap was low ESR - the series isn't marked). The caps around the heatsink are also warm, but they are being heated by the TEA2025B IC.
The original output coupling caps were 470uF 10V Rubysuns and were replaced with 470uF 35V Xicons. Both the original caps and the replacements had a temperature rating of 85°C. The filter cap was a 1000uF 16V Jackcon rated for 105°C and was replaced by a 1000uF 35V Xicon rated for 85°C.

[Th3_uN1Qu3]

Re: Cyber Acoustics MMS-20 amplifier blows speakers

This amp killed another set of headphones. What is going on? It is always the right channel driver that blows.

Throw it away already. Unless your headphones were the $2 variety that is, in that case you simply buy another pair.

Also, the output coupling caps and the filter cap are warm to the touch, even though they aren't near any heat sources.

That's because you changed bad caps with bad caps.

[lti]

Re: Cyber Acoustics MMS-20 amplifier blows speakers

The new caps are better than the originals unless the filter cap was low ESR.

I won't throw it away yet. I am using $10 headphones, so I don't think the amp will damage better headphones.

I also noticed that there is less bass from the left channel when using speakers, but I can't hear this when using headphones. The coupling caps are almost perfectly matched, so there must be mismatched components in the bass/treble boost circuit.