House Wiring Defect?

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.

Do not forget that house panels are 208-230 volt in the USA anyway and you should take your outlets from the same side of the panel. Typically break 1 is one side of the panel and breaker 2 is the other side so when you power your room you should use breakers that are all even or all odd. This can also increase the differace in ground protentials. Try stay away from the side your refrigerator or any moter is connected to. Also ask your elecrican if you panel is balanced, meaning even power distribution from side to side, this is not the same as balanced power so do not confuse the two. Some audio equipment is more sensitive than others to this problem.

Single phase
.

Please be aware that most "microphone" cables (i.e. from a "music store") are incorrectly wired -- they often have pin 1 tied to pin 0, which is the shell. As clever as this is for electrocuting musicians, we still have musicians so the premise isn't working well...

Seriously, you'll have to open both ends and determine if the wiring is 1==1, 2==2 and 3==3, with nothing else.

You also might try disconnecting EVERYTHING, then start from the sub and speakers BACKWARD, listening to each step.

And of course IF you have TV cable connected to anything then that's very probably your culprit.

Regards,
Barry