well, first, I've had ground loops before and fixed them by simply buying and using the proper equipment. but that didn't work in this case.

I'll try and make this as short but complete as possible.

Signal Path:
Guitar → Pedal Board → Amp → AmpFX Loop Send → MultiEffects Rack → AmpFx Loop Return.

I then line out to my Audio Interface on my PC from the back of my MultiEffects Rack and get a ground loop hum... BUT it's my Amp that's producing the hum.

I've put an Eliminator in between the Rack and the PC but it did nothing. Kinda makes sense right? since the signal is traveling from Amp to PC and there's no noise on the PC... not sure how it's coming back to my amp or how I could fix it.

So, thanks!! I will really appreciate any ideas here :)

Any major flourescent lighting near your amp? (long shot..but is an old prob sometimes) ....also...try running all power thru same outlet source if haven't yet. curious what model amp....

Hey, thanks for mentioning....  but no, no fluorescent lighting at all in here on on this rooms circuit.
 
I do have a lot of stuff in here tho, I thought about trying to run it all on one power source but there's too much for that... I can't imagine all the outlets I'm using... let's see, I'll take a count. ok, so 29.  That's all my gear, and my PC (with 5 monitors) and a few DC adapters too.  That seems like a lot to me, but also when it comes to a real studio maybe not very many, right?  Anyhow, everything that plugs in is plugged into good Belkin surge suppressors.
 
Amp is an Orange TH30h with a 4x12 cab, my baby! :) 
 
(edit: updated gear in my sig)
 
(2nd edit: wait, I just realized, I do have one LCD monitor, those are lit with florescent lights, I'll try unplugging it first thing tomorrow and report back)

Another culprit is circuits with variable voltage switches (incandescent light switches) which may not be isolated from the receptacles properly.
 
I interesting method for finding EMI is to use a single coil guitar (only one coil online) and run it around the area your gear is in. If you have that available, can leave your recording setup powered, but use a single coil (either just through the amp, or all the way to the audio interface). Just be careful of gains when doing it.

First, try running everything from a single outlet.
If that fails to eliminate the noise, run the line-out (from the Multi-EFX) into a DI box and lift the ground.

thanks for the help you guys...
 
mettelus, unfortunately no luck with the single coil experiment.
 
Jim, I assume you mean from a single duplex more so than an single outlet?  sorry if that seems like a stupid question...  the DI box I purchased has already been sent back but it did not have a ground lift circuit... but after a bit of reading and shopping I see that some of them do.  I'm guessing this would be a good next piece of equipment to try.
 
thanks everyone. the guy at sweetwater gave up on my problem and no one at gearslutz even commented.  so I truly appreciate you all helping me!  not sure where else I'd turn...

Michelle911
Jim, I assume you mean from a single duplex more so than an single outlet?  sorry if that seems like a stupid question...  the DI box I purchased has already been sent back but it did not have a ground lift circuit... but after a bit of reading and shopping I see that some of them do.  I'm guessing this would be a good next piece of equipment to try.

 
Yes, I meant a single duplex (pair) of electrical plugs.
They share a single ground...
If the issue is a ground loop (almost certainly the case), using a direct box with a ground-lift feature will solve the problem.  This is a common issue... and a common solution.
 
 

it might be a ground loop, it might be a pin-1 problem, it might be any number of things, and it is going to be difficult to diagnose on a forum, but let's give it a shot eh?
 
First - is everything properly grounded. If all the gear has a three prong power cord is the ground conductor on the outlet connected to ground? Don't laugh, I've run into that many times, especially with low cost outlet strips and power "conditioners". If you don't know how to test this, or don't have a friend that knows how to do this then go to a hardware store and pick up  one of those outlet testers (whatayaknow - even Amazon sells them - LINK) It won't identify every possible problem, but it will let you know if all three conductors are connected correctly.
 
Second - start taking things apart. It sounds like you've already started down the path. If you disconnect the multi-effects rack from the amplifier does the hum go away? In theory (and I love theories<G>) the output from the multi-effects rack ought to be isolated already. You didn't mention what this rack is, can you provide more detail?
 
Third - should have been first I suppose - you also didn't mention details about this isolator you inserted . What is it? Many such products are pure snake oil, the only way to get good sound and proper galvanic isolation is a transformer, and a good transformer is expensive.
 
Fourth - powering everything from a single outlet can work, but not if the outlet strips are poorly constructed. I've measured 10s of Ohms of impedance at 60 Hz on some cheap rack mount power strips. UGH!
 
If you can provide some more details we can make the hum go away. I've yet to see a power line noise problem that could not be solved. Some take longer than others to figure out, that's all!

thanks for that Jim, I didn't know about a ground lift.  seems like a lot of eliminators have that, except for the one I bought lol my luck, I bought a good one too...  anyhow I have a new one on the way, hoping it will fix things.
 
Bill, fortunately I know my room is all grounded properly, all new outlets and new wiring in the last ten years. Me and my dad did it ourselves (he's a contractor). I'm into home theater too, so yeah, have had ground loops pop up before, but always managed to fix them on my own with the proper equipment. This is the first I've run into one that's left me all ???  I have ground testers on each power strip/suppressor too and all read positive.  I'll have my new eliminator in a couple days, if that doesn't solve things I'm glad to know people here are willing to help me solve my issues - much appreciated!

Man, I got a lot of good stuff from this thread. Thanks everyone.