I had a HUM in my rig that I could not isolate. I finally realized it was gone when I had nothing plugged into the McIntosh. That made me think it had to be the SMS preamp.
There was nothing else in the signal chain for this test. I tried several different cables to connect the pre to McIntosh to eliminate that possibility. No difference. I sent the preamp to Brad.
So, then I was a position where I had no choice but to try a different preamp, which I should have done before sending it to Brad.
I tried 2 different preamps and guess what? The HUM was there with either one.
The only thing left was the input jacks. I started wiggling them while a preamp was connected and I could immediately hear the HUM going away and returning as I wiggled them.
They were not making a good ground contact to the housing of the McIntosh causing the HUM.
I replaced them with switchcraft 1/4" TS inputs. Connected everything. HUM was completely gone.
First show: 1973
Last show: 1994