quad 520 help - noise when turning off

[aardvarky]

Indeed, you already wrote so in post #2.

no problem.
i'm pretty sure i did swap the speakers around just to rule it out and it was the amp channel.

 

[Charles Darwin]

Could be.
I wonder if the 520f is centre tapped and he's looking at the wrong schematic - or the 510, or whatever the 520c is?
you don't see many of the straight 520's from my looking around.

The 510 is a mono version of the 520 and some 520s had a ground lift switch and input transformers fitted. Without those transformers input is single ended despite the xlr sockets.