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Old 10th February 2006, 09:30 AM   #1
Mordor is offline Mordor  Italy
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Default Help me to fix POWER ACOUSTIK please!

I've an hold Power acoustik U.s.a. amplifier. I don't know what model, on th chassis, and pcb isn't write. Some information:
Is two channel with 4 transistor for channel ( 2negative & 2positve), is about 30 x 30 cm. The chassis are black and is a monoblock (alluminium fusion) similar to hold rockford fosgate with a silver signature "power acoustik usa".
The amp work but not good. The problem is a distortion.
The distortion are big when there are only bass frequency, and are a high harmonic distortion. The bass frequency are good, but there is this high harmonic distortion. The problem are in 1 channel. Now i try to invert power mosfet ( left ch to right ch and right ch to left ch), i've tryed to change all opamp, now all are BurrBrown , i've changed supply capacitors with ELNA double sized. All the capacitors are good (not exploded). There are 9 transistor for channel that i think drive the power mosfet. how can i do? Anyone know old power acoustik amplifier? Any idea?
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Old 10th February 2006, 01:51 PM   #2
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Default Re: Help me to fix POWER ACOUSTIK please!

Quote:
Originally posted by Mordor
I've an hold Power acoustik U.s.a. amplifier. I don't know what model, on th chassis, and pcb isn't write. Some information:
Is two channel with 4 transistor for channel ( 2negative & 2positve), is about 30 x 30 cm. The chassis are black and is a monoblock (alluminium fusion) similar to hold rockford fosgate with a silver signature "power acoustik usa".
The amp work but not good. The problem is a distortion.
The distortion are big when there are only bass frequency, and are a high harmonic distortion. The bass frequency are good, but there is this high harmonic distortion. The problem are in 1 channel. Now i try to invert power mosfet ( left ch to right ch and right ch to left ch), i've tryed to change all opamp, now all are BurrBrown , i've changed supply capacitors with ELNA double sized. All the capacitors are good (not exploded). There are 9 transistor for channel that i think drive the power mosfet. how can i do? Anyone know old power acoustik amplifier? Any idea?
One thing that can happen when you switch out the stock opamps to a better part is non-symettrical voltage rails. Because the bias is usually higher, your +/-15V rails may sag. If this is the case, add more beef.

Was the one channel distorted before you changed the ops?
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Old 10th February 2006, 04:45 PM   #3
Mordor is offline Mordor  Italy
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There are 3 double op amp, one for buffer, one for lowpass and one for highpass. This evening i try to measure the voltage at opamp, but i dont think that this is the problem, beacause the source is common to every 2 channels, and only left is bad. Is not a distortion caused by saturation, is an high frequency distortion caused by bass freq, i think is a problem of the 0 point of the symmetrical transistors....
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Old 10th February 2006, 07:13 PM   #4
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Quote:
Originally posted by Mordor
There are 3 double op amp, one for buffer, one for lowpass and one for highpass. This evening i try to measure the voltage at opamp, but i dont think that this is the problem, beacause the source is common to every 2 channels, and only left is bad. Is not a distortion caused by saturation, is an high frequency distortion caused by bass freq, i think is a problem of the 0 point of the symmetrical transistors....

It is possible that one of the parallel outputs is open. I have seen that before and as a result you may have x number of good outputs on either the positive or negative rail and x-1 transitors on the other. It is rare, but it can happen. It would be wierd for the bias to be that different on one channel. But also check the device used for bias.
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Old 11th February 2006, 08:02 AM   #5
Mordor is offline Mordor  Italy
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i've measured base of all transistor and the voltage are symmetricallly on all channell, negative are equal to all negative transistors and positive are equal to all positive transistor. Power stage are ok, i've inverted righ transistors with left transistor.
At the ouput i've 0.25 volt dc on bad channel and 0.10 on the good channell. now i try to invert some transistor of bias.... from left to right....
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Old 11th February 2006, 08:45 AM   #6
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M,
Ive had issues with some car amps over the years.

Couple of tips.

Car amps are power pigs. They draw a lot of DC and if your bench supply can't deliver the current, no matter how many 'caps' are in the line, you will get low frequency distortion.

So if the amp works at low volume and screws up at high volume that is usually the simple answer. Worst case bring a car battery to the bench and use enough heavy wire to make the connections.

Another issues with auto equipment is mechanical. Very often a solder joint will fail and you won't be able to see it by eye.

Use a large magnifying lense and if there is any doubt at all, re flow all the heavy solder joints. Im going to assume this amp has a DC to DC converter section, heavy parts get jerked around when the car hits a bump, and enough of this movement will weaken solder joints. So at high current, the sound get's distorted. Same problem for marine radios, two way, and cb sets.

Run the amp and use a plastic stick to poke and prod the various parts, see if the outputs change on the scope.

One last thing that made me go blind one day.

Had a Craig auto amp that had a high frequency osc problem. The last thing I looked at was a by pass cap that failed. You might have a small part some where that has drifted out of spec, and causing you pain.

Much luck
Jack Crow
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Old 11th February 2006, 09:41 AM   #7
Mordor is offline Mordor  Italy
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Now:
i've inverted ALL transistor ( power and drive ). Nothing.
I see some small caps ( electrolitic ) serial to signal. If one of this with years is fu**ed? now i try to bypass or changhe.... This ampli is my death......
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Old 11th February 2006, 09:47 AM   #8
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Default The truth of the matter....

M,
Sooner or later you are going to solve this problem. It's just a matter of time and effort.

Somewhere in there is a defect, and it will be exposed.

On the truly mind numbing ones, I leave for a night or two, and the best ideas come about when your well rested. So take care of your self, and the electronics will be easy.

One of the tools that's made me a lot of money over the years is an ESR meter for caps. I have one here that works like magic. Other tech's see it and want to know what it is. It works on about .2 uF and up. It's saved me hours of pain.

Be well
Jack Crow
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Old 12th February 2006, 12:29 AM   #9
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If you have a scope and can see the distortion when you drive a low frequency sine wave into the amp, post a photo of the display and it may help with the diagnosis.

It's not a good idea to bypass coupling caps. Although it's rare in car audio, sometimes bypassing them will disturb the biasing of the next stage and cause distortion. Often, caps are used to bypass part of the signal to ground (as in the feedback from the output back to the differential amplifier). If this cap is bypassed, the amplifier will have higher DC offset than it should.
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Old 12th February 2006, 01:58 PM   #10
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Quote:
Originally posted by Mordor
Now:
i've inverted ALL transistor ( power and drive ). Nothing.
I see some small caps ( electrolitic ) serial to signal. If one of this with years is fu**ed? now i try to bypass or changhe.... This ampli is my death......
What do you mean when you say you inverted the transistors left to right right to left? You just swapped the channels PNP to PNP and NPN to NPN?

If you have a scope, you can just remove all of the output transistors and look at the signals to find where the distortion is(WITH NO LOAD!). With the help of a signal generator you can clip the signals and look to se if one channel is clipping before the other or just about any other problem for that matter.
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