All; I'm trying to fix this Audire Crescendo power amp I acquired. Both channels have output, but there's a lot of DC offset. One channel is 250mv and the other is 2.5v. Both channels drift to higher values if the load is taken off the speaker terminals. I do not have a schem and can't find one to buy.

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I haven't tried the phone number for Audire yet, as I understand the designer is retired. The circuit has NO DC offset control. It does have a bias control. Changing this setting does not affect the DC offset at all. What I've done so far: replace all electrolytics, as some were showing signs of age replace one bad 10 Ohm resistor and 1 4148 diode swap the outputs from channel to channel.

The amount of DC error does NOT follow the output transistors, so the outputs are not causing it. I'm kinda at a loss. I'm not expecting anyone to solve this for me, but it would be helpful to have suggestions on what to test and how.

Thanks for any help. BTW, I got this with an Audire Legato preamp which I got running and it sounds really nice. Click to expand.I should have guessed you would be the first to reply!

Thanks for the reference to that thread. I had never read it because I usually adjust offset by the manual instructions. This may be a case where I need to sit down with a pile of paper and a big eraser and just grind out a schematic for this thing. There doesn't seem to be one available, so that would be a service to people who try to repair one in the future. Looking at the schem of the input diff pair, the thing that immediately jumps out at me is that if the base of the 'left' transistor, rather than being referenced through the resistor to ground was referenced to a pot on a voltage divider between the + and - supplies, the DC offset would be adjustable.

But it's 1:00 AM and I shouldn't be trying to think right now. Well, I couldn't leave well enough alone, so I've been fiddling with this piece all evening. The only small signal transistors I had a bunch of were 2n5551 and 2n5401. I tested 30 of each in the simple Hfe tester in my DMM. The NPNs clustered around 168 +/- 5 and the PNPs around 108 +/- 5.

I picked a pair of each that matched each other. Using the pinout of the MPQ6600 as a guide, I installed the 4 single transistors in its place.

To keep them all the same temp, I cut a very small rectangular piece of 1/16' alu about 1/4 X 1/2 as a shared heat sink. I bent all 4 transistor bodies up against it and fixed them in place with silicone sealer as I have seen done. Upon starting the amp, the DC offset was 300mv, which was better than the previous 2500mv, but not good enough.