Eliminating Ground Loop -- Transformer or just lift the shield?

[hourglass]

Since I only have enought electronics know-how to make me dangerous, I'd appreciate any insight that you guys might have into solutions for this situation...


My situation is this:

I have a preamp which takes a mono signal, adds some FX and splits it to left & right line-level outputs.

I'm plugging these outputs into two separate power amps. Power amp 1 causes no problems.

However, power amp 2 seems to have a ground loop and I get 60hz hum whenever it is connected to the preamp (on either the left or the right output.)

Last night I eliminated the hum in power amp 2 by plugging the output into a direct box (whirlwind IMP 2) which was in turn plugged directly into a LowZ to HiZ transformer. (so I'm stepping an unbalanced line level signal down to a balanced line-level signal and then immediately stepping it back up to an unbalanced line-level signal...) The 2nd transformer was plugged into Power Amp 2's input.

With the IMP 2 in normal mode, I had 60hz hum. When I lifted the ground on the IMP 2 the hum disappeared.

So my question boils down to -- is there a simpler way to do this?

Can I use a single transformer to accomplish this? Maybe something with a 1:1 input/output ratio? I see that people make & sell 1:1 transformers but I have no idea which one I would need. If anyone is aware of the specs, the pre-amp involved here is a Pod XT, so I guess I need to know the max output level to select the right transformer. I have no idea what it's putting out...

If I can't use a single transformer to accomplish this, does anyone know what sort of transformer is in the IMP 2 direct box? I know that I could use two of them to accomplish the same thing I'm doing now.

For either solution, I only need raw transformers because I will be running this signal through a passive attenuator box that I built and I have room in the box for a transformer or two. (This would also add a bit of heft to the box and help to sit still on top of my amps.) For what it's worth, the amp hums with or without the attenuator in the circuit.

Finally -- is the transformer solution overkill? I have about 18 inches of cable between the pre-amp and power amp 2. Could I get away with simply lifting the shield @ the pre-amp output side of that cable? Or would that risk further problems down the road with RF leakage?

Actually, that last one was not finally, cuz there's one more thought. Somehow, somewhere (in some loud, dark, dirty club) the ground prong broke off of the power cable for power amp 2 (which is the one with the hum.) I haven't had time to wire up a new cord for it, but I have been wondering if this is the root of the whole issue. I'm thinking that the pre-amp has always given me ground issues and I've always attributed that to the fact that it uses a wall-wart and doesn't have a direct connection to earth. But as I said in the beginning, I really don't know enough about this shit to know if that matters...

Thanks so much for any advice.

Ryan

[nosebleedaudio]

This depends on a couple of things:
1; is the output (source)balanced?
2: is the input of both amps balanced?
If the answer to both is yes then dropping the shield should work.
A 1:1 trans such as a Jensen JT-11-FLCF will solve the problem.
Overkill? depends is nothing else works...
The worst thing is if you have an electronic balanced out and one amp is unbalanced the other balanced, I would never lift the AC ground on a power amp to solve a ground loop.

[hourglass]

Originally Posted by nosebleedaudio

This depends on a couple of things:
1; is the output (source)balanced?
2: is the input of both amps balanced?
If the answer to both is yes then dropping the shield should work.
A 1:1 trans such as a Jensen JT-11-FLCF will solve the problem.
Overkill? depends is nothing else works...
The worst thing is if you have an electronic balanced out and one amp is unbalanced the other balanced, I would never lift the AC ground on a power amp to solve a ground loop.

Nothing in the system is balanced. Signal originates at a guitar, ends up at unbalanced inputs of power amps.

I'm currently using a TS cable out of the Pod so it's unbalanced. Technically, the Pod does have the option for balanced outputs, so I suppose I could use a TRS cable and then just use the 2nd transformer I have now and not connect the shield on its input.

As for the ground prong on the 2nd head - it truly did break off at a gig. I do at least know better not to hack them off on purpose.

When I find some time (like in the next 10 years or so) I'll slap a new power cable into my amp. If that doesn't eliminate the hum I'll look into that Jensen tranny.

Thanks for your advice.

Ryan

[dkatz42]

Yikes. Replace that power cord. Now.

Aside from being potentially lethal (I was nearly killed playing a gig 25 years ago on shared equipment because some fool cut off the ground prong) the lack of said ground prong means that the chassis of that power amp is not at the same potential as the rest of your kit, and there's 60 Hz leakage current going through the shield of your signal cable.

If everything else is the same but you're getting hum from the ungrounded power amp, well, it seems like the answer is staring you in the face.

Not to mention that you can fix your problem for $1.39 at the hardware store without mangling your sound by going through multiple transformers. Even if this doesn't fix the problem, you've improved your life expectancy.

[nosebleedaudio]

Originally Posted by dkatz42

Yikes. Replace that power cord. Now.

Aside from being potentially lethal (I was nearly killed playing a gig 25 years ago on shared equipment because some fool cut off the ground prong) the lack of said ground prong means that the chassis of that power amp is not at the same potential as the rest of your kit, and there's 60 Hz leakage current going through the shield of your signal cable.

If everything else is the same but you're getting hum from the ungrounded power amp, well, it seems like the answer is staring you in the face.

Not to mention that you can fix your problem for $1.39 at the hardware store without mangling your sound by going through multiple transformers. Even if this doesn't fix the problem, you've improved your life expectancy.

I agree 100%...
Get one of those $9.75 Hubbell # hbl5266c plugs, far better and 10 times easier to wire, regardless what size wire it is...

[hourglass]

Thanks guys.

I suspected the cord could be the issue (and it's given me a few jolts to the lip off of a vocal mic in the past) so I'll replace it first thing tonight when I get home.

Ryan

[hourglass]

Spent $3 at Lowe's and got a nice fat plug end.

Got home at 6 PM and had exactly enough time to sit down in front of the amp before the super wind/rain/hail storm of the decade swept through and knocked out the power.

Power came back at 6 AM so I got up and put the plug end on the cord to my amp.

The good news: The hum is gone from that amp.

The bad news: The hum is now in the OTHER amp and it's ten times louder than it was in the first amp.

If I do the same as before and lift the ground to one of the amp inputs then the hum goes away. Doesn't matter which amp gets lifted.

Ryan

[dkatz42]

You might want to check the continuity between the ground prong on the power plug and the chassis (particularly the unbalanced input jack) on the two power amps. See if the resistance is different. (Are the two power amps identical?)

Also, try measuring the resistance on the audio cable shields, or try swapping the cables and see if maybe you've got a dodgy cable.

This stuff *should* just work if you've got good, low-impedance paths to ground.

[hourglass]

About an hour ago, while standing in line at McDonald's awating my large Cholesterol Value Pack I think I realized what I screwed up...

Being that it was the early AM and I'm _NOT_ a morning person --

I'm sure I connected the BLACK wire to the ground prong.

Might make sense for a DC circuit, but not so much when we're talking about AC.

I'll check it when I get home. If that's what I did the two chassis are certainly not at the same potential.

As for the amps - no, they're not the same model. Same manufacturer, tho, for whatever that might be worth.

And the cables have been swapped six ways from Sunday. Plus other cables. And more other extra cables...

Ryan

Originally Posted by dkatz42

You might want to check the continuity between the ground prong on the power plug and the chassis (particularly the unbalanced input jack) on the two power amps. See if the resistance is different. (Are the two power amps identical?)

Also, try measuring the resistance on the audio cable shields, or try swapping the cables and see if maybe you've got a dodgy cable.

This stuff *should* just work if you've got good, low-impedance paths to ground.

[dkatz42]

Originally Posted by hourglass

About an hour ago, while standing in line at McDonald's awating my large Cholesterol Value Pack I think I realized what I screwed up...

Being that it was the early AM and I'm _NOT_ a morning person --

I'm sure I connected the BLACK wire to the ground prong.

Might make sense for a DC circuit, but not so much when we're talking about AC.

I'll check it when I get home. If that's what I did the two chassis are certainly not at the same potential.

As for the amps - no, they're not the same model. Same manufacturer, tho, for whatever that might be worth.

And the cables have been swapped six ways from Sunday. Plus other cables. And more other extra cables...

Ryan

Hmm, that's not good at all. You've created a new path of lethality. ;-) Also it means that your ground is carrying current, which is also not good, and that your chassis is at 120V if your outlet happens to be wired backwards, though it might pop a breaker or blow up your gear.

Remember--white goes to the sliver prong, black goes to the brass prong, bare goes to the ground prong.

First rule--no electrical work while impaired! Impaired carpenters end up missing fingers, but impaired electricians end up missing life... (This from a guy who plugged his sister's barrette into an electrical outlet at the age of three, and stuck his fingers into the back of a light switch to try to figure out the magic at the age of five...)

Wouldn't hurt to spend $10 on an outlet tester, which is always a handy thing to have around. They're not fabulous, but will at least tell you if your outlet is wired the right way around and there's at least some ground path. You'd be amazed to find out how many miswired outlets there are (or how many clueless folks put in three prong outlets without wiring the ground so that there'd be a hole for the ground prong to go into, which is probably worse than clipping the pin...)

[hourglass]

For what it's worth (which I understand isn't much based on the current situation) I wired the outlet that all this is hooked up to when we first moved into the house. I ran the cable from the new breaker in the panel into the wall I built for the studio and I'm certain the ground wire is connected on both ends. Pretty sure the circuit isn't backwards, but I'll double-check it.

Ryan

Originally Posted by dkatz42

Wouldn't hurt to spend $10 on an outlet tester, which is always a handy thing to have around. They're not fabulous, but will at least tell you if your outlet is wired the right way around and there's at least some ground path. You'd be amazed to find out how many miswired outlets there are (or how many clueless folks put in three prong outlets without wiring the ground so that there'd be a hole for the ground prong to go into, which is probably worse than clipping the pin...)

[hourglass]

OK...

I'm still alive. :) Didn't have time to post yesterday.

When I took the plug apart, I found that I'd done exactly as I suspected -- the black was wired to the ground prong (I also tested the original plug to be sure it was wired as you described.) I put the wiring in the proper order, plugged in just that amp and powered it up to make sure I didn't kill anything or anyone.

Powered up fine, so I connected my setup as previously described.

This time we have a winner. No hum in either amp.

Thanks for the advice.

Also, this is clearly something I should have done a lone time ago because the amp sounds a whole lot better, too. Sounds like it did when it was new.

Ryan