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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
11 responses Add your response
tfkaudio
591 posts
 
As I understand it, the safety issue is real in theory, but not in practice. If, for instance, you have a gound loop and use a cheater plug to lift the ground on the amplifier power cord, this is ok because the amp is still grounded via the interconnects to the preamp (assuming the pre-amp is grounded via a 3-prong power cable).

Now, let's say you make the foolish mistake of removing the interconnects to the amp with the amp turned off but still plugged in. You now have a REAL safety issue because you've just removed the component's path to ground.

So, as I said, it's not a safety issue in pratice because you generally wouldn't leave an amp in the plugged-in-but-not-connected state for very long (usually only while making component changes).

However, as any component manual will tell you, unplug ALL affected components before removing interconnects. Follow this rule 100% of the time and you will be fine.

Cheers.
bundy
127 posts
 
Finished my basement.
Put in 3 dedicated 20amp outlets.
Had hum as preamp and amp into different sockets.
Removed ground pin on amps cord.
Hum gone.

10 years of happy operation.

I think that as long as there is a gound somewhere in the circuit, you are fine. Mine is through the preamp.
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stonedeaf
101 posts
 
Wouldn't get too exercised about this potential problem just yet.Your new home was wired by a electrician who understood what you were looking to do with this room? He should have been careful to get the grounding right.
Don't know how old the existing home is - but in general - older wiring ( especially if you are not the only tennat during the home's history) is always a crap shoot - hidden hot boxes or my favourite - three wire receptacles that aren't hooked up to anything on the third wire are always fun.
And for reasons I really don't understand - the number of wire nuts I've had just plain fall off of 3rd. wire (safety grounds) when i pull receptacles out of wall boxes is just plain crazy.There's a reason they're called crafts unions - problem is like any other profession - there's a percentage of don't give a damm slobs wearin the same hats.
How WELL your AC wiring is done really does matter.
audioful
1 posts
 
Have used 3 to 2 prong plugs for several years. No problems. Have them plugged into conditioner/surge protecter which has 3 prong into wall plug. Good luck
xti16
1,525 posts
 
Lifting the ground would be like using a 2 to 3 prong ac adapter on the various power cords.
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

audioguy3107 OP
40 posts
 
Yes, as soon as I get a chance I'll disconnect the sub....I didn't even think about that, I just assumed it was amp/preamp since the hum disappeared when I plugged it into the same outlet as the rest of the stack (NOT inc. the sub). Thanks for all the info.....I'm no electrician, but would somebody chime in about the safety of lifting the ground? I've read all kinds of information regarding safety/electrocution/fire hazard. What is the true risk of lifting the ground on 1 offending component? Thanks!
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almarg
9,670 posts
 
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
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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
xti16
1,525 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
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zephyr24069
1,239 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...
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