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Grateful Dead Music Forum

A place to talk about the music of the Grateful Dead 

 #138255  by hippieguy1954
 Sun Apr 13, 2014 5:57 am
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.
 #138258  by Searing75
 Sun Apr 13, 2014 8:44 am
Now you know, and knowing is half the battle!
 #138268  by hippieguy1954
 Sun Apr 13, 2014 3:58 pm
Yep! You guys are both correct! That's why I thought I'd pass it on!

Peace!