C84 "buzz" on output.

[tourtelot]

On my first-build C84, I have a "buzz" on the output that I discovered because I plugged in a ribbon mic and cranked the gain way up. This isn't present on the other three C84s that I have built. It sounds like EMI noise but it isn't related to proximity to the PSU in the rack. It is not 60HZ AC induction. It gets louder when a mic is plugged in but I can hear it just listening to the output alone. The tone characteristic remains the same whether a mic is plugged in or not. Just the level changes. And it changes with an increase or decrease in front-panel gain.

This is not the faint OPAMP "hiss" that I can hear at high gain settings on the other three units. Otherwise, the noisy unit sounds identical to the other three and tracks levels the same as well.

Anyone have a good place to start looking for this "buzz"?

D.