[Steve Vol]

Help!  Any electricans out there? - I had a new dedicated 20 amp electric line run in my house for my audio equipment from the breaker box, and now I am getting a humming noise through my speakers that I did not have before.  I am running amps that I leave turned on all the time, and with my preamp turned off, I have a humming sound in the speakers.  When I plug the amps back into my old circuit, the humming sound stops.  Other than going back to the old house line, anyone have any suggestion/ideas on what would cause this?  Bad grounding on the new line??

Thanks for any ideas.

Steve

[Wayner]

Maybe you had a bad ground (or none at all) on your old lines and the new lines that are grounded are not friendly with your amps because you now may have a ground loop. Do your amps have a 3-prong grounded plug? If so, does the pre-amp have a 3-prong plug as well? are they all tied to the same circuit?

W

[mjosef]

Check to see if the old circuit is on a different phase than the new circuit.
If so, then its probable that the side on which the new line was installed has some noise generating appliances...solution then would be to have the electrician switch the new line to the opposite side.

[Steve Vol]

Wayner, thanks, both the amps and preamp have 3 pronged plugs.   What exactly is a ground loop? 

All lines come basically from the same circuit box, although there are 2 different boxes and the electrician ran a dedicated line from one of the boxes to my audio equipment.

mjosef, how would I know if I am on a different phase?

Thanks

[mjosef]

You will need to look at your breaker panel...your panel has 2 positives(each 120v) and one neutral feed from the main utility...each vertical row of breakers(the full size ones, not the half size) are divided equally between the two positives(phases), first breaker being on one side(phase) the next below it being on the opposite(other) side(phase)...
Why don't you just get the electrician back and have him/her do it, surely he would oblige if you are not happy. If you are not familiar with phases, you probably shouldn't be messing around in the open panel to switch the breaker over anyway.
Safety first!

Edit: correction in order for the (mis)use of the word phase(s)... residence are normally single phase. And there are two poles to that phase. 

[Wayner]

You have a potential ground loop because your pre-amp is grounded, your power amps are grounded and you have interconnects between them as well. Now the ground has a completed circuit. There can be a slight voltage to ground thru the equipment finding its way from the pre-amp ground to the commonly connected power amp ground and then back to the pre-amp via shielded interconnect.

You have several options. First I would lift the ground off of the pre-amp. See if the hum doesn't stop. Or you keep the ground on the pre-amp and lift the ground on the power-amps.

Or you could take the shielding off of the interconnect cables at the connector terminal, keeping them grounded at only one point. Some audio cables have direction arrows on them, pointing away from the grounded end. Some folks have played with isolated audio transformers with a 1 to 1 ratio, but I believe the sound quality may suffer.

Good Luck!   

Many audio manufacturers avoid grounding their equipment for this very reason.