Help with DC offset and Bias on amp. Soundcraftsmen circuit?

[Spendor]

I have a power amp made in Australia that i believe was used as an install amp in Cinemas. It's made by Auditec or AVE. It sounds kind of good, but on checking DC its showing around 350mA. There are 2 1k adjustment pots which i assumed were for DC and Bias but they're wound fully anticlockwise and turning them clockwise only increases DC. Is this amp or style of amp familiar to anyone and how do i go about getting it sounding optimal - preferably with less DC on the outputs! There are no large emitter resistors and I'm thinking it might be similar to a sound craftsmen design? I haven't been able to source a circuit diagram, but have attached a pic.I have a scope, multimeter, audio tone generator and dummy load but not much else. Any help much appreciated!

 

[Robisme]

No emitter resistors on MOSFETs.

Rob

 

[Spendor]

Thanks - I know nothing about amplifier design. Any thoughts on how to optimise this amp?

 

[petehall347]

is there a model number on the amp modules ?

 

[Spendor]

The amp model is AVE 2074 (audio visual equipment ltd) and only has Auditec 074 written on the pcb. There is some info on the 2084 version, but it is a different amp topology with different output devices.

 

[petehall347]

i cant find any info on Auditec mosfet amps .

 

[Spendor]

Yes i have searched and found nothing much. I believe it could be a variation on this circuit. It shares similar output devices - any thoughts?

 

[Spendor]

Sorry this circuit attached. Similar MPSA transistors, 1n4742 diodes and output fets.

 

[petehall347]

the pots are more likely for bias . saying that there are 2 of them for some reason

 

[Schmidlapper]

For a Soundcraftsmen amp using the schematic you posted, the bias is set with the pot, .8V typically.

If the DC offset is to high, it is corrected by replacing Q1, Q2, Q3 with well matched ones. DC offset is nominally 50mV or less. For a Soundcraftsmen the ZTX694B is a suggested suitable sub. I physically bonded Q1 and Q2 together with some heat paste between them to help keep them thermally synced. In rebuilding my A2801 the final offset I got was 35mV both channels, well within spec.

Just be aware I’m speaking of a Soundcraftsmen, yours may well be different.

 

[Spendor]

That's good advice and perhaps a starting point for me - I have adjusted to 800mV. Unfortunately one side has DC offset around 20mV and the other about 350mV. I will check transistors and see if I can match up with some others I have lying around. Thanks.

 

[Schmidlapper]

In some cases it has been reported that Q4 was the issue, in my case the three mentioned corrected the high offset problem. Also Q4 is a different style transistor than the other three.