Entertainment Center: TVs, Stereos, VCRs and DVDs - Correcting a ground loop

Hello all,

I have what I can only assume is a ground loop hum in a newly installed subwoofer amp. All of my equipment to date (mostly Sony stuff) has had 2-pronged plugs (no grounds). I haven't had any trouble with it at all. I took an old 15" DBX subwoofer I had laying around and installed a new 270 watt plate-type amplifier in it. It has a 3-pronged plug.

When I first plugged the thing in the hum was so loud that you couldn't even hear any of the signal (music, tv, whatever) coming out of it. I narrowed that down to the cable TV antenna. If I unhooked the CATV the hum went away. So I put a ground blocker on the antenna cable and it cured it. No more hum - but this was at my 'test' location where I could plug RCA cables from the sub-out on the amp into the RCA jacks on the amplifier.

Happy to have solved the problem, so I thought, I pulled speaker wire 30 feet across the room and put the sub in the corner (opposite the sub at the other end of the room). This time I'm using line-level inputs. I hooked them up and there's a hum. Not nearly as bad as before I 'fixed' it but it's noticable. I unhooked the cable TV. That's not the problem.

I have a 'cheater' plug on there right now which I know is not a good thing. The 'cheater' plug fixes it so it's got to be a ground loop. The sub amp is plugged into a different circuit than the receiver. So I ran an extension cord over to the same circuit. Still hums. Then I plugged it into a 3rd GFCI circuit. Still hums. So I'm not sure what to do with it at this point. Can/should I ground the chassis on the receiver (I think there might be an optional grounding screw on the back of it) to the house wiring ground (which is my water pipe)? Theoretically then both the receiver and the sub amp would be going to the same ground.

Any advice would be appreciated.

Thanks,
Jim

You have some good ideas that would work in theory. Its just very interesting when you disconnect the ground pin which has no functional affect other than for safety this problem goes away.

If i were you i would leave the cheater plug on. And if your all safety crazy attach a wire from the case to the box in the wall so the case is grounded. Its just weird that neutral line is not causing a ground loop hum and the 3rd pin is. With the cheater on it just make sure no one touches it or spills beer on it when you have a party. Basically with out a case ground you will never know if it is energized or not because of a hot wire in the device touching the case. This is the reason for that 3rd pin, and why you shouldn't disconnect it, safety issues which could be fatal could result. I am just interested if you run a wire from the case to the wall outlet and ground it to the box if this hum will comeback. Also another safety device that is in the amp is a transformer, yep in this case it is a safety device to. Because if someshorts in the amp current will be limited by the core of the transformer going into saturation.

Yeah, I'm kind of baffled by it. In theory, the 60 hz hum is caused by the amp (with the 3-conductor plug) finding a different ground than the receiver. Since the receiver doesn't have a 3-conductor plug, it's anyone's guess where it's getting its ground from. So I grounded the case of the receiver to the water pipe which is ultimately where my electric service is grounded. Still hums.

So I'm going to leave the cheater plug on it. Most of the amp in question is buried inside the sub. The sub is jammed in the far corner of the room. The part of the amp that does stick out is behind the sub at shin-level. No one's going to touch it. Though if we get drunk enough I may dare someone to put their tongue to it. Or not.

The only other thing to do is to hammer a ground rod into the ground and tie my audio equipment grounds there. If I get more serious about the setup I may do that. In the meantime the cheater plug works fine.

Thanks for the reply,
Jim

If you are running line level signal through 30 feet of speaker wire, you are probably picking up hum through the wire. Line level signals should be run with shielded cable, not speaker wire. Try a shielded wire and see if it goes away. I played in a band for 25 years and have heard more hum from amps than you can imagine.

Nashcat

If you are running line level signal through 30 feet of speaker wire, you are probably picking up hum through the wire. Line level signals should be run with shielded cable, not speaker wire. Try a shielded wire and see if it goes away. I played in a band for 25 years and have heard more hum from amps than you can imagine.

Nashcat

That would explain why I don't get a hum with the RCA jacks hooked up. But it doesn't explain why the hum goes away when I take out the ground.

I'm using Monster in-wall cable (which wasn't cheap) but I don't think it's shielded. When I say 'line level' I'm talking the speaker-out jacks on the receiver. Is this the correct terminology? (I used it because the lugs I'm using on the sub amp say "LINE IN"). I guess more appropriately I'm using the high level outputs on the receiver to run 30 feet (actually closer to 40 feet) into the high level input on the sub. When I use low-level inputs (with only a 12-ft. cable) I don't get noise.

Thanks for the info.

Jim

Jim,
I'm not sure what kind of amp that you are running, but there is a chance that you are overloading the front end of the amp with the speaker level input. Unless the amp is designed for speaker level inputs, there will be a impedence mis-match between the two. On most amps that I have seen, the line level input refers to an input from the pre-amp section of the receiver, instead of from the speaker outputs. If you'll post the brand and model of your amp, I'll see if I can find some more info on your problem.

I can't tell you how many times I've had to "lift" the ground, to get rid of hum. A "cheater" plug, or two, was always in the equipment bag whenever our band played on the road.

Nashcat

Thanks Nashcat. It's some off-brand plate amp I got from Parts Express. It will take either high level or low level inputs. When I use the low level inputs it's fine. Maybe it's just because it's a cheap amp.

Is there a distance limitation on low impedance runs? I can swap out the speaker wire for a 50' run of shielded RCA patch cord. I'd feel a lot better if this thing was grounded properly.

Jim

Jim,
There really isn't a distance limit on low impedence runs, but there are limits to UNSHIELDED runs of any impedence. Try the shielded run and see if your problem goes away. It will probably be cheaper than the monster speaker wire, if you shop around, or make your own.

Nashcat