Op amp distortion

First time post .. please treat me gently :-) .. but help would be
appreciated.

As part of a "powered monitor" (not for the PC but wedge-shaped speaker cab
with built-in amp for use on stage) I assembled a commercial power amp
(already tried and tested for several years in another application) and a
'front end' comprising two dual op-amps (LM358 or TL082) to provide two
balanced input preamps and a mixer. In my experimenting I found the sound
was distorting - a kind of "roughness" occurring at any volume level - which
a bit of "web research" suggested might be crossover distortion.

Long story short - well, shortER anyway ;-) - by a process of injecting the
source audio at various points in the chain, I established that the power
amp was (predictably enough!) working fine, but both "new front end" stages
suffered with the same distortion problem. As recommended by one bit of
research, I tied the output pin of the mixer stage to the negative supply
rail and this made a substantial improvement. Unfortunately, doing the same
for the 'balanced input' stage made no difference that I could see (er,
...hear!). Changing the op amp from the original LM358 (as is now working
successfully in the mixer stage) to TL082 made no difference (Thinks .. I
haven't tried a TL082 in the mixer, maybe I should, just to see ...)

Anyway, bit of a tall order, perhaps, (especially without seeing a
schematic, but there's nothing much different than you will find in a book
of "standard applications for analog operational amplifiers") but can anyone
suggest what might be adrift and what else I can do to fix it? (All the DC
levels seem to be what one might expect - like, everything pretty close to
zero volts AFAICR except for the + and - 15v supplies).

I've puzzled over this for hours and got nowhere, so all suggestions are
welcome.

Thanks,
Mike.


.

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