Need Help With An Aggravating Ground Loop Problem

[Paul Marossy]

I am playing for a rock opera Easter production at church and so I thought it would be cool to have my amp in stereo, especially for the U2 song we will be doing. So I thought I'd break out my old ART SGX2000 and use it with my '74 Fender Twin Reverb. I was getting some really nice tones with that and a couple of my DIY pedals. Everything sounded great at home, no problems.

So, when I get there, I get everything all set up and I have a HUMMMMMM from hell when I turn on the amp. It's very loud, and any distortion at all sends it WAY over the top and it's totally not usable. The ground switch on the back of the amp doesn't do anything to help. But if I pull one of the channels from the effects unit, 90% of the hum goes away.

I am really baffled by this. Since the Twin Reverb is a true two channel amp, it seems like it's perfect for doing stuff in stereo from a stereo effects unit. There's not two different amps involved, and so I thought I shouldn't have any problems with ground loops as there is only the ground of one amp to deal with. The DIY pedals are being powered from a 9V outlet on the SGX2000.

So my question how the heck can this be happening and do you think it's due to "dirty power"? Both the effects unit and the amp have grounds on the power cords. But what if the grounds are defeated in the power system, would that cause problems like this? I did notice that they cut off the ground connection on some of their power cords on the stage. I believe that it's a dirty power issue because there is no problem at all when I am at home. I need to find a solution to this problem, and quick. Or I will have to rethink everything.  

[blooze_man]

The only thing I can think of is maybe the sound system at your church isn't grounded well. The one at mine isn't...

[Paul Marossy]

Maybe. But I play there all the time and never have a problem, though. I usually use a mono setup thru various tube amps I own which all have grounded power cords, and my normal pedal board is powered by a wall wart that does not have a ground on it.

[John Lyons]

Sideways question:
Using a mono amp with a stereo processor isn't giving you stereo.
Dual mono inputs into a mono amp...Maybe with a JC120 as it is dual mono amps
side by side into separate speakers (two separate mono amps in the same cabinet)

Maybe the fx unit has some kind of out of phase thing going that is fooling with the
Twins mono inputs..Hmmm still seems odd.

[Paul Marossy]

Sideways question:
Using a mono amp with a stereo processor isn't giving you stereo.
Dual mono inputs into a mono amp...Maybe with a JC120 as it is dual mono amps
side by side into separate speakers (two separate mono amps in the same cabinet)

Maybe the fx unit has some kind of out of phase thing going that is fooling with the
Twins mono inputs..Hmmm still seems odd.

The Twin reverb has two independent channels, one for left and right channels of the effects unit. Yeah, not a true stereo amp, but it sounds more or less the same.

I don't know about the out of phase thing, could be. It think it must be a power supply issue, and probably because of how the amp is grounded. It's an old school hand wired stripboard arrangement with everything grounded to the chassis in quite a few locations.

There may not be a realistic solution, but I did think of using a DI box in series with one connection to see if that would help at all. I guess I could try it and see if it helps at all.

[Tubebass]

Do you have everything in your setup plugged into the same outlet?
Have you tried lifting the ground on one of the signal cables to the amp?
Though this probably has no bearing on your hum issue, don't forget that the Twin's channels are out of phase with each other.

[Paul Marossy]

Do you have everything in your setup plugged into the same outlet?
Have you tried lifting the ground on one of the signal cables to the amp?
Though this probably has no bearing on your hum issue, don't forget that the Twin's channels are out of phase with each other.

Yeah, everything was plugged into the same outlet, and on a single power strip. I could try lifting the ground on one of the cables, that might work well enough to make it an acceptable situation.

I have one more rehearsal where I could try this out, and then I have two dress rehearsals and then seven shows to do where there will be up to 2,500 or more people in attendance, so I want to get this right!

[wavley]

First I would get an outlet checker and check the power to see if it's grounded. 

I always thought that the channels of a Fender were out of phase with each other and that's why it sounds thin when you jump them together Marshall style, but that doesn't really matter because 1. you said it sounded good at home 2. I don't really think that a phase problem would cause your hum.

Personally I've been battling a ground loop myself and I built one of R.G.'s humfree things and got rid of it, I still haven't found the cause of the loop though, all of a sudden something in my rig decided that it didn't like my Space Echo (which I've gone over and over with the service manual and doesn't hum by itself)

I've been known to lift a ground on a signal cable or a di box just to get through a gig.

I also have an air conditioner that makes dirty power in my house so I have to turn it off when I record.

Maybe if you have the parts and time it's time to build a power conditioner, they're never a bad thing to have, or maybe try an Isobar

[David]

Does the practice area have fluorescent lights, perchance?  I've had some difficulty with those over the years.
After having gotten a few healthy pokes from mics that were grounded improperly during my gigging days, I went out and got one of those outlet checker thingies and used it religiously at every place we played at.  Most of the time the power was in good shape with correct wiring and good grounds.  There was the one time when some moron had...  well, never mind...

[Paul Marossy]

No flourescent lighting. The sanctuary has a lot of big metal halide type lights in it, and they can be dimmed. I'd say it's at least 15,000 sq ft with about a 20 ceiling, exposed structure & ductwork painted black, etc.

I haven't asked, but I have a feeling that they have some floating grounds going on there.

[wavley]

No flourescent lighting. The sanctuary has a lot of big metal halide type lights in it, and they can be dimmed. I'd say it's at least 15,000 sq ft with about a 20 ceiling, exposed structure & ductwork painted black, etc.

I haven't asked, but I have a feeling that they have some floating grounds going on there.

Could it be the dimmer making the noise?

[Paul Marossy]

Anything is possible, but the dimmer wasn't in use at the time...

[TELEFUNKON]

Dimmers "sizzle" - they don`t "hum".

Is it possible to turn the power plug of either the amp or the effect`s power supply around by 180 degrees, and if so does, it make difference?

Or: did the transport of your equipment damege the connection of one of the big caps in amp or FX? 

[Paul Marossy]

Can't reverse the polarity on the power supply to the amp. The ground switch on the amp has absolutely no effect on the hum, either.

[Paul Marossy]

OK, problem solved. I feel really dumb now, but it was the 9V outlet on the effects unit. When I tested it this afternoon, I got the same problem as I had the other night. When I tried a wall wart instead, 95% of the hum went away. So... I think there must be a filter cap on that 9V outlet in the effects unit that needs to be replaced.

I'm just happy that I figured this out!