House Wiring Defect?

Not a problem. Only audio equipment is sensitive to slightly different ground potentials.

It might mean that the hot and neutral may be flipped one 1 of the outlets, so I would check it.

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.

Remember to turn off (at the breaker) the house power before you take the outlet cover off to inspect the ground and +,-.

I know it is semi insulting to write the above, but having grabbed 220 once myself (and 110 more than once), it is meant in the kindest way.

Best regards,

Dave

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.

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

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 good 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 wirinfg is 1==1, 2==2 and 3==3 , with nothing else.

You also might try disconnecting EVERYTHING, then start fro thje 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

Thanks Barry, yep I took apart my Kimber PBJ's overcomplicated Switchcraft XLRs to check their wiring before I figured out everything needed to be on the same outlet.

For the record, although the outlets are wired with three core wire, the electrician here at this circa 1930's rental says it's two core down to the box. One of the two outlets in this thread is a new one for the window A/C unit. The electrician says he used half the 240. From what I've read here all the 120 circuits are developed that way. My guess is these two outlets have different ground schemes.