2. tech-talk
  3. 1261721779

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
xti16
1,495 posts
 
Did you try disconnecting any and all sources? First place I would start at is with the TV and cable/satellite boxes if any. Then the Rega and Nad. Last would be the sub. Once you find it's either a source or just the pre and amp combo a little more troubleshooting can be done.

That being I do have a ground loop too. I had dedicated lines installed witch didn't help at all (for the ground loop but sure dropped the noise floor). Then I added a balanced isolation transformer which helped a lot but didn't totally eliminate it but cut the hum way down and changed it to more of a slight buzz. My source is a meridian G08 to a Bryston B100 using rca's. If I lift the ground in the power cord on the Meridian it's totally gone. I have taken the meridian to 2 places and connected it with balanced ic's and hear no hum or buzz.
zephyr24069's avatar
Ag insider logo xs@2x
zephyr24069
1,183 posts
 
The same thing could happen (and has to me) in an old setup with non-specialized but separate circuits; it happens due to the equipment plugged into the 2 separate circuits seeing different ground potential (at least as I understand what I've heard from electricians and audiophile friends who are good at this sort of thing). Have your electrician fix the situation so that both circuits have the same ground potential. In addition, you'll want to look into something from Granite Audio called the "Ground Zero" floating ground station. Check out my system as posted on Audiogon. I have the same setup and went through the problem you are referencing both in an older home, and with a newer home with custom-installed 20-amp circuits...
almarg's avatar
Ag insider logo xs@2x
almarg
8,673 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
More to discover