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Perreaux 6000B Problems, Advice needed
Perreaux 6000B Problems, Advice needed
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Old 5th August 2018, 01:57 AM   #1
Quakeos is offline Quakeos
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Default Perreaux 6000B Problems, Advice needed

Hello, Gents hoping I can receive some 'desperate hope' in how to solve this issue with a beautiful Perreaux 6000b Amplifier. I'll also add this amplifier is from the 1980s and is MOSFET fed.

To keep up with the jones,

First of all, I pulled this 25kg block of joy out of my garage for the first time in around 20 years before it got retired as my dads DJ Amplifier. Intense dust and corrosion as usual and I did my best to clean it all and did so.

First impressions - sounds jaw-dropping, however, there is an issue.

Crusty volume knobs and dust was the first issue, contact cleaner solved this and I cleaned the entire PCB with PCB cleaner.

Channel b is perfect, distortion-free and a relatively low DC offset.

Channel a sounds just a wee bit different, and as you increase the volume, a distortion comes into play and at a medium-ish volume the distortion completely takes over the speaker. The DC offset in this channel is 100-120mv

I have a post on audiokarma trying to fix this issue, I've tested all caps and they all showed a healthy reading on my digital multimeter.

I cannot for the life of me find what is causing this issue, I am thinking of replacing all the transistors (not the FETs though, as they are unobtainable)
with new parts.

I will attach photos below of the amp internals and schematics and any feedback on where to go from here is highly appreciated. With this said, I am no electronics engineer but I have a basic idea of what I'm doing

Thank you DIYaudio, I appreciate everyone in advance who has taken the time to read this post and offer solutions. I just want to be a happy man with a great sounding amp again.
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Old 24th August 2018, 12:28 PM   #2
Mooly is offline Mooly  United Kingdom
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Perreaux 6000B Problems, Advice needed
This looks to be based on the old Hitachi application note for Lateral FET's.

Start with the basics and confirm the supplies are correct as measured at their destinations. The front end of each channel seems to have its own auxiliary supply so check they are correct. Do they fall when the distortion sets in ?

Is the bias setting voltage similar between channels ? That is the voltage across the preset.

Is the voltage across the emitter resistor of the constant current source similar between channels. It looks like Q5 but the diagram is very blurry.

It is possible the distortion is actually caused by AC instability (oscillation) and for that a scope check is needed to confirm or otherwise.

Don't replace any parts in hope and particularly transistors... that's a sure fire way to disaster.

Also check the supplies to the upper left opamp and again, a scope check of the opamp output would be useful.
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