Ground loop buzz with stereo rig?

[Inazone]

Last fall, I finished putting together a rack rig for recording. Took it to the studio where we were tracking drums to lay down some scratch guitars. Everything seemed to be working great until I tried using my tuner (see below) to switch between my two preamps. There was a very audible buzz, and one of my preamps would intermittently cut out. Here is the rig:

 

- Sabine RT-7100 two-channel rack tuner (dedicated left and right ins/outs) with A/B switching

- ART DST-4 solid state preamp > ART Tube Channel EQ > left channel of Carvin Tube 100 power amp

- Peavey Rock Master tube preamp > Rockton Multi-Valve > right channel of Carvin Tube 100 power amp

 

At first I thought there might be an issue with the Sabine tuner feeding a single guitar signal to both preamps, so I unplugged everything from the tuner and ran through a simple A/B footswitch. Same problem. I unplugged my EQ and effects processors and tried again. Same problem. The only cure seemed to be unplugging one of the two preamps, at which point the buzz went away, and the preamp that was cutting out no longer did so.

 

I would be fine with unplugging whichever preamp I'm not using on a particular track, but it sure would have been slick being able to switch "sides" at the push of a button. Only the Peavey preamp has a ground lift switch, and I have since added some other units (Rocktron Hush IICX and BBE Sonic Maximizer) that have independent left and right channels to allow the preamps to share them, meaning that every device in the rack is connected to every other device to some degree. If it was a ground loop, would unplugging the cable between the A/B switch and one of the two preamps be enough to eliminate the buzz (as seems to be the case) even though they are still in the same rack and plugged into the same power amp and effects? As long as my guitar signal is only going to one preamp and the other is unplugged, everything works fine. I know that there are a couple of isolation products that supposedly will help prevent ground loops in racks, but I might be way off base about the nature of the problem.

 

Am I making things too complicated by trying to "share" so many pieces of equipment in my effort to have simple A/B switching? It's not my live rig, so if I can't switch on the fly, it's not the end of the world, but it would have been really convenient.

[LeVice]

Hey!

I'm not a specialist, but please consider this:

Wathever is your switching device(tuner ,a/b box,...) you have two cables going to a separate preamp. I think this could solve the problem: the jack going from the switching device should be unshielded, while the other end( in the preamp) can be shielded. Planet waves makes cables that are shielded on only on side.

Tell me if helps!

Cheers

[Inazone]

I wasn't able to find anything on the MF or AMS sites, and none of the cables on the Planet Waves cable page look like what you mean:

 

http://www.planetwaves.com/Pcablesdetails.aspx?ID=1

 

Any idea where you saw them? I would like to try some out if I can find them!

[LeVice]

Strange...

I don't remember if I saw them on the website. I found them in a shop, here in Belgium. I don't see why they would discontinued them.They were designed to isolte the signal from the guitar ground

3 options:

-ask for these cables in a shop

-send Planet waves an e-mail

-ask in the diy forum how to unshield one end of a cable. It's a pretty basic operation(one desoldering)

 

The reason I think (am sure,actaually)it will work is because the ground is a reference voltage considered by the circuit as 0V. It's rarely 0V, but that doesn't matter since this value doesn't change.

If you have 2 different grounds, you have 2 different reference value. The result is induced hum.

Such cable will isolate your signal from one ground and only one reference point will be considered. I think the ideal is plug the unshielded ends in your switching device but once you got the cables, you can also try to plug the unshielded ends in your preamps( just to experiment)

Hope you will find them very soon