Oh, OK these are not "active" input preamps.
I can see how your first fix, moving the op amp inverting input reference to the transformer secondary worked.
If it had been an active preamp, e.g. a THAT1510, then I would have suggested referencing the differential amp to the input for the same reason. (pin 5)
Low noise grounding tips in mic preamp
- mediatechnology
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Re: Low noise grounding tips in mic preamp
Well, gents. I was wrong. The problem was not the reference gnd. What eliminated the hum was a temporary wire that went from the op-amp gnd pin to transformer gnd (op-amp "-" pin doesn't have direct connection to GND, instead it goes through the RC). When looking carefully I noticed that gnd pin on the op-amp (DOA) didn't have any internal connections except bypass caps. So I started digging further and found a filtering cap that was a part of a relay driver circuit. This cap was fed unregulated DC. And it was dumping this ac ripple into the agnd. What the temporary wire did was sort of bypassing this part of circuit.
So I connected the cap to cgnd instead and bingo - everything is quiet! Now I need to find time to tweak the second preamp....
So I connected the cap to cgnd instead and bingo - everything is quiet! Now I need to find time to tweak the second preamp....