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See if you can "predict" a future ground loop hum?

I was hoping someone could perhaps shed some light on whether or not I may be in trouble regarding ground loop hum....here's the situation:

I'm about to move into a newly built custom home that we designed with a music/library. I had the electrician wire 2 20 amp dedicated lines: 1 at the front of the room for a McIntosh MC 402 and 1 on the side wall for the other components, Classe preamp/Rega & NAD M5 CD players. No video will be on this system. The entire system will be balanced except the Rega Apollo and the REL storm subwoofer.

We're still currently in my "old" home with no special wiring. Last evening, I decided to hook up the MC 402 like I'll have it in the new home to test it out (I just recently purch. it and hadn't tested it yet). When I powered it on plugged into a socket in the front of the room (apart from the other components), there was an audible hum coming from the speakers. I then grabbed an extension and tried different outlets, and of course the one which had no noise was the outlet where the rest of the equipment is plugged into.

Now I'm scared to death that we'll move in to this nice room that I designed and could have the same problem. My question is will the dedicated lines (assuming they're wired correctly) solve this problem or could the same thing happen as in the old home that has no special wiring consideration?
audioguy3107

Showing 3 responses by almarg

almarg's avatar
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almarg
9,670 posts
 
I'm surprised by this, because I would expect that a balanced preamp-to-power amp interconnection would be pretty much immune to ground loop hum. Are you sure that the hum is coming from the main speakers and not from the sub?

I'm thinking that perhaps when you plug the MC402 into the different outlet, which may have a different ground potential as Zephyr suggested, it is affecting the chassis potential of the preamp (since the two chassis are connected through the shield of the xlr cable), which in turn could result in an offset between the grounds of the preamp and sub.

One thing I would try, regardless of the answer to that question, is to float the sub ground by using a cheater plug (a 3-prong to 2-prong adaper) on its ac power plug.

Hope that helps,
-- Al
almarg's avatar
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almarg
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Follow-up to my previous post: My reference to ground offsets between the preamp and sub assumed that the sub is connected at line-level; let us know whether that is correct, or if it is connected to the MC402 outputs at speaker-level instead.

Regards,
-- Al
almarg's avatar
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almarg
9,670 posts
 
I think that all that can be said about lifting an ac safety ground on a piece of audio equipment is that the risk of a problem is small but is not zero.

But if you are saying that the sub is not on the same ac outlet as the preamp and the rest of the stack, and if the hum goes away when the sub is disconnected, I would certainly try connecting it to that same outlet.

Also, let us know whether the sub is connected via line-level or speaker-level.

Regards,
-- Al

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