Recording: Sporadic ground noise with DI acoustic guitar

Question

Getting loud sporadic buzz/hum/noise when recording my Ovation Celebrity acoustic guitar. Using Logic Pro 9, running direct from the guitar's 1/4" output into a PreSonus Tube Pre-amp --> Focusrite Saffire Pro14 FireWire interface. Guitar has an under-the-saddle pickup. Recording direct since my room isn't conducive to mic'ing.

The noise comes and goes and is not affected by the instrument being played (it’s there just as it’s plugged in) and sounds similar to what would indicate a grounding issue (just louder), and if I touch the metal part of the instrument cable, it goes away. However through the headphones the noise almost sounds digital, if that makes sense. I’ve tried another electric guitar and a bass guitar through this set-up and there were no issues.

First thought is that it’s the guitar, but when plugged directly into an amplifier it was fine; no noise issue at all. I also plugged directly into the computer via a line-in port (and 1/8” to 1/4” adapter) and while the signal was low (due to lack of pre-amp), there was also no noise.

I’ve tried opening the pre-amp and blasting with compressed air (also compressed air on the Saffire interface’s knobs), dusting/the guitar and its input, replacing the 9-volt battery from the guitar’s EQ, using different cables, plugging the guitar into the pre-amp directly via the 1/4” cable AND with a balanced XLR (via a DI box), and I’ve also tried plugging the pre-amp into a different wall outlet (albeit the same wall outlet the power strip it was previously plugged into is using). I haven't tried changing the tube in the pre-amp because it's not been heavily used, and again, doesn't do this with any other instrument.

I’ve even unplugged the air conditioner (from an outlet on another wall), turned off lamps and fans, etc. And yes, I rebooted the computer/re-powered the interface, first thing.

I last recorded this acoustic guitar with this same set-up about 2 weeks ago with no issues, but I’m thinking this must be specific to the combination of this guitar and this recording setup.

Can anyone shed some light on what might be going on here?

Answer 1

If it looks like a duck, walks like a duck & quacks like a duck, then it's fairly safe to assume it's a duck.

If it stops when you ground yourself to the cable shield, then why not just tie a wire from there to your wrist. You could even use a computer grounding wrist band. I used to use solder wire, as it doesn't have a tendency to rattle against the instrument, you can bend it out of the way & it tends to stay where it's put.

For safety, make sure anything you subsequently touch isn't going to fire you backwards across the room. Licking the back of your fingers & touching your potential 'enemy' is the on-the-fly way to test for that - not, for instance, grabbing the mic with both hands.

If the recording device is a non-earthed, double insulated device that will provide ample opportunity for a floating voltage. (Read a million ill-conceived reports &/or 'fixes' for 'my computer tingles' on the web.)