stereo looper use - ground loop?

[Tim Toberer]

I am trying to put together a small pedalboard with a stereo looper. My guitar is an acoustic with a sound hole pickup and a triple disc piezo (on bridge plate) both wired to a stereo endpin jack. The magnetic pickup volume and tone is on the guitar, but the piezo is wired straight to the jack. Volume is controlled on the amp. having it this way allows me to blend the sounds by turning the volume down on the guitar, which brings in more piezo. Works like a charm. I want to setup my acoustic lap steel with this same setup.

My question is, can I split the signal so I can route the magnetic pickup through a couple effects pedals to go into one input of the looper while carrying the piezo signal separately to the other input? Here's the potential problem, I want to rejoin the signal after the looper to carry it to my stereo amp in a TRS cable. I see a ground loop issue because they share a common ground. Seems like I would have to wire them with separate mono jacks on the guitar in order to avoid this. (which I don't want to do!) Hope that makes sense. I want to wire it like this so I can make acoustic and electric loops with the same guitar at the same time. I also want to use the percussive sounds of thumping on the top to make percussive loops.

[George Biner]

I see a problem in that you are taking two outputs, one from the magnetic effect boxes and one from the piezo guitar pickup, and connecting them together -- one usually doesn't connect one output to another, the stronger one will win and you won't hear (and may damage) the other one -- I think you're going to have to keep them separate or send them through a mixer (with gain elements like op amps or resistors buffering one output from the other). Then you also won't have any ground loop issues.

[Mike Auman]

The ground in your setup comes from your amp. It's plugged into AC power, and that earth ground (3rd lug) is tied to the chassis and signal grounds in your amp. That reference ground travels back toward your looper via the shield lead of your TS cable from looper to amp. The looper passes that ground reference backward thru both input cable shield leads, and so on, ending up at your guitar. Since the ultimate source of your reference ground is the amp, there's no problem splitting and merging your signal between guitar and amp. All the signal paths are referenced to the same ground source (your amp).

The problem comes if your setup is feeding two different amps. These can sometimes have two different AC grounds that are an ohm or two different. That's a "ground loop" and can cause noise due to current flowing between the two different grounds. In that case, you have to isolate the two amp grounds, and use only one of them as a reference for your signal chain. There are boxes that do that.

You didn't ask, but all of the power supplies in your setup are isolated from earth (AC) ground, and get their power reference grounds from the same place: your amp.

[Lee Baucum]

Ebtech Hum X (now Morley Hum Exterminator, I believe) is what I used to use when I was playing through two amps.

Plug one of the amps into the device and then the device is plugged in to the electrical outlet.

They claim is doesn't lift the ground....it just stops the ground loop and the hum.

~Lee