stumped by distortion at moderate-high volume

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My amp is an old Rega Mira II. It sounds fine at low volume, but at moderate to high volume there is significant distortion in both channels, which is especially noticeable on speech (sounds like a mic being overloaded).

Pre-amp output is clean; so it's a definitely power amp. issue.

However, I cannot find anything obviously wrong. Voltages around the power amp circuit all seem quite normal as far as I can tell; there is no DC offset; QC is fine.

I've attached the circuit (power amp section is same in 2000 and Mk II).

Can anyone give me a clue as to where the problem might be?

Thanks!:confused:
 

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  • Mira 2000 main pcb.pdf
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Is this an amp that used to work OK, and now is playing up; or a newly acquired unit?

Sounds to me like there may be a power supply issue, the voltage rails are sagging/modulating badly when called upon to supply significant current. Try monitoring these rails while the amp is working under significant load ...
 
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You'll need a 'scope to look at power supply modulation properly. DMMs typically give average readings within the refresh cycle, so may not tell you much.

You could simplify things by just temporarily adding extra main electrolytics of say, half to equal the present value to verify whether there is something amiss with the supply.
 
It has Impedance-detecting VI limiting (freely translating from Spanish) built around TR15/17 and surrounding components (and equivalent ones in the other channel).
Will limit current for resistive loads below 2r2 or real world (reactive) speakers below, say, 3 ohms or 3A into shorts.

So it's easy to trigger on complex loads; did you switch speakers or something?
 
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Have you measured the distortion at the higher levels into a dummy load? fft?

Just thinking laterally have you ruled out the speakers? Is the distortion across the board or at higher frequencies? Getting an fft showing the spectrum would be interesting. Reason I asked about the speakers is because I had this problem and it was because I had too low a slope on the crossover. Increasing it to 4th order fixed it. Of course in your case if it used to be fine, then it is not the design of the crossover, but it could be degraded components...

Tony.
 
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