So, I've put the lid on and hum has appeared. It's the same on every input so there's a problem somewhere.
Any ideas?
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How odd. Is the case and lid made from aluminium or steel?
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Have you got the case earthed?
Is the case earth commoned with the in/out socket negs? They may need separating if so.
What I'm suggesting is that it may (or may not ) be best if the signal negs are not earth to the case.