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Discussion Starter #1
Hi,

I had a nice, quite system until I added several external Mackie power amps. Now I have a persistent, and fairly loud, hum that stays at a constant volume. The hum seems to be coming from the my SC-05 AVR that I'm now using mostly as a preamp/processor. As I spin the dial to change inputs, the hum will stop for a moment while the system is buffering. And if I turn the AVR off, the hum stops completely. Is that likely to be a ground loop? If so, what's the best way to go about finding the source? I've already tried moving things between outlets, rerouting cables, changing how things are stacked (that actually made it a bit worse), disconnecting various things from the inputs, changing what bits are plugged into specific outlets, and everything else I could think of. So is there anything else I'm missing?

Thanks
 

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It is definitely power bleeding through somewhere. Do you have a power filter on your system. I know they are costly but they are well worth their investment.
 

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Discussion Starter #3
Yeah, sorry, should have mentioned that I'm using a furman and an APC. I've got a couple tripplites that I tried, even though they are just "surge protectors". But they at least let me plug into different outlets to see if that was the cause.
 

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This is a 60Hz hum usually caused by having equipment on more than one circuit that may be sharing things like dimmers for lighting or a freezer/fridge. Make sure that all your equipment is plugged into the same lag off of the electrical panel.
 

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Discussion Starter #5
Everything is, so far as I can tell. One breaker shuts down this entire room.

But my question remains; is this likely to be a ground loop? The problem didn't start until I added the extra amps. When it was just the AVR, the system was dead silent, other than a bit of hiss, up to +12db.
 

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Yes, a ground loop is very likely the issue. How are you connecting the amps do they have balanced or unbalanced inputs?
 

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Discussion Starter #7
The amps have balanced and unbalanced inputs. My AVR only has unbalanced outputs (RCA), so I'm using the unbalanced inputs. I have an adapter and tried that on one channel to see if it made a difference, but no joy.
 

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Elite Shackster , HTS Moderator Emeritus
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Doesn't matter what signal wire you use, if you add the grounding plug and have another source to ground (usually cable TV or satellite signal wire) it will add the hum (even if you use unbalanced). Many of us had this problem when adding a BFD parametric EQ for the subwoofers.

Your choices are:

Cheater plug (lifts the third prong and just leaves the one path to ground, eliminating the hum)
One main power block that accepts the cable TV signal as well to reference everythign to the same ground at the power conditioner. This has worked for me once and failed me once (two different power strips), so your mileage may vary.
Lastly, they sell isolation transformers for the cable signal that lift the ground, removing it as a path and usually eliminating the hum. I don't think this works for satellite, though, since the shield carries a voltage -- but I may be wrong there.

A quick way to verify is to just unplug the cable tv/satellite wire and see if the hum goes away.

Good luck.
 

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Discussion Starter #9
Yes, I had a hum after adding my BFD, but moving the plug from the Furman where the AVR is plugged in to the APC stopped that hum.

However, yes, this is being caused mostly by the cable. I unplugged that and the hum is greatly reduced, but there's still a small bit there. What's strange is that I did *not* have that hum before I added the Mackies. What's even stranger is that I'm running it through the APC which is supposed to filter the input from that.

And there's no way around that hum? It's a cable box.
 

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Try lifting the grounds on the Mackies, pro grade amps are notorious for introducing a ground loop when used on consumer end systems.
 

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Discussion Starter #11
Thanks. Hopefully guitar center will have something in stock. I'l give Fry's a shot too.
 

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Discussion Starter #12
I ordered a VRD-1FF. It's supposed to be a ground lift for the cable. We'll see if it works. Thanks for all the help.
 

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We have a whole thread dedicated to solving ground loop problems, you might want to see what remedy has helped others.

BFD Ground Loop Thread

Since the noise is going away when you disconnect the cable TV feed, that’s the thing to look into. Check to make sure the service is properly grounded to the electrical service ground stake, not a water pipe or something like that. While you‘re there, give the stake a once-over, as it’s very likely your problem begins and ends right there. See my post from the above mentioned thread for an example of all the problems that can be found at a ground stake that can contribute to noise.

Once you get that lined out, if you still have noise, then we can look into other potential problems. But the ground stake should be the first thing you look at, not the last - which is what most people seem to do (and I’ll include myself there).

Regards,
Wayne
 

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Discussion Starter #14
Yeah, as soon as I got home last night, I checked to verify that the cable system was properly grounded. It's grounded with the rest of the electrical system, so it looks like the ground should be good. I can call the cable company and complain, but it's probably faster to just get that ground isolator for the cable. I may at least try contacting the cable company to see what they say.
 
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