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Classic ground loop

Posted by Chris O on September 15, 2012 at 11:14:37:
Your left and right RCA input jack grounds should be tied together right at the jack. Then then that single ground should be tied to the signal ground of the circuit like it was prior to the modifications. When you use your meter you should measure less than 1 ohm resistance to ground. The hum is limited to the input section, not the power supply or anything a cheater plug will help you with because it stops when you short the inputs.

I will bet that if you disconnect the ground from the RCA jacks, one might still be grounded through the chassis. You might also have a bad solder joint on the RCA jack ground. You can't create a loop if there is only one path, somehow you have more than one. Keep looking, you will find it.