Ground loops & whine on my guitar's output signal!

[HankMcSpank]

(Firstly, I know there's a musical equipt forum on here - but this post relates more to a solid state/electronics line of questioning)

Hello

I've come to a bit of a dead end & need a top tip or two!

I have a guitar with a little opamp circuit that I made built into the rear of the guitar- on batteries the guitar output is whisper quiet & to all intents & purposes free of noise.

First a diagram of my setup....



(apologies for the bad copy - the only bit that's totally illegible is the block at the bottom in the middle - it should read isolation transformer)

The guitar also connects back to a 'guitar unit' that also happens to provide +7V/Ground/-7V fed down a specialised 13 pin cable to the guitar - it's when I connect up that cable to my guitar that my problems start! In short, I'm getting a lot of noise & whine on my guitar's output- even when my onboard guitar cct is still running off batteries.

The 'guitar unit' manafacturer has acknowledged there's a possible noisy USB ground problem (they recommend cutting the USB ground wire - I did this it helped with the level of noise, but some faint whine remained - but it seems in this condition other issues kick in, so I reverted back).

I bought an isolation transformer - with this in situ, the noise significanlty drops - almost gone in fact, but what remains is a faint high pitched whine - even though it's low in volume, it's at a frequency that is very intrusive during quiet passages playing guitar. (I'd say about 4-5khz)

Now bearing in mind, the guitar's output signal is 'whine free' on batteries when there's NO 13 way cable from the guitar unit connected - it's only when the 13 way cable is connected to the guitar I get the whine (I have a power switch that flicks between the +9/0v/-9V battery power & the +7V/0V/-7V from the guitar unit)...when on batteries the only physical connection back to the guitar unit is the ground - it must therefore mean there's noise getting into my guitar signal from the guitar unit's ground feed somehow.

The guitar unit gets it's ground from the PC (which gets its ground from the AC mains) - I guess it's the PC feeding noise into the ground & ultimately the guitar unit that's the culprit....how can I clean the PC ground up that's feeding the guitar unit?

One other possibility is the guitar unit itself is adding the high pitched whine down the 13 way cable?

Do you think there'd be any mileage in snipping the USB ground wire & running a direct (non PC) AC ground to the guitar unit? At least this should eliminate the PC as the source of the 'ground whine' (or is this a dodgy procedure?!)

Any advice greatfully received!

[darkfenriz]

Try cutting '0' form the unit.