11-06-08: Zapper
It seems the 2 outlets were on different phases of you house electric, which induces a hum. With them on the same outlet/phase, hum is gone, makes sense.
You house is probably fine. You can use a 3-light tester to see if the wires are reversed on either outlet.
Rockvirgo,
You can check if your problem is as Zapper mentioned in his post.
If you have a volt meter set the meter on a scale of at least 250Vac or higher.
Insert one test probe in the small slot of one of the duplex receptacles and the other probe in the small slot of the other duplex receptacle. {Small slot is the hot)
If you do not get any reading that indicates each hot is fed from the same Line of the electrical panel.
If the meter reads around 240Vac then the two circuits are fed from opposite Lines in the panel.
Note, if the receptacles are too far apart for the leads of the meter use a drop cord. Just plug one test probe in the small slot of the female end.
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Here is another test you might try......
Take a piece of wire and connect one end to the center trim screw that holds the the duplex receptacle cover on and connect the other end of the wire to the other trim screw of the other receptacle.
Plug your amp into one of the recepts and the preamp into the other.
Post back your results.
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By the way you can pick one of the circuit checkers at
Home Depot.
