House Wiring Defect?

Cford has it right. Be certain that the black wire(hot) is connected to the copper screw of the outlet, and the white wire(neutral) to the nickel screw. You might even check the breaker box connections, while turning off the power, as Dave suggested. You should only see black wires connected to the breakers, and whites to the neutral buss(could be reversed there instead of at the outlet).

Rather than pulling apart outlet and breaker boxes, follow Zapper's advice and get a 3 light tester. It looks like a yellow 3 prong adapter, with 3 lights on the back. You plug it into the outlet, and it will tell you if anything is wired wrong.

Thanks all for your responses. Those three light socket checkers are great but haven't been able to snag one yet. Visually, both outlets are wired identically and the breaker box looked ok, except for some red wires inside. My last guess is one of the two outlets is set up near the window aircon so maybe it has a separate ground.

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.