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I'm sure this is a "ground" issue, but I'd really like to better understand what's going on.

I have a bass guitar plugged into the "instrument input" on my interface (ART TubeFire 8). When I monitor that channel, I hear a buzzing sound that goes away whenever I touch either the metal plate where it's plugged into the bass, or the little spring on the cable shielding.

The thing is, it only works when my hands are touching these items. Of course, I can't play whilst acting as a human ground, so I am trying to find some alternative.

I've tried balanced and unbalanced cables, and also added a direct box. The direct box helps in that it gives me more places to touch, but it doesn't affect the buzzing per se.

I imagine that the bass itself could be the problem, as it's about 20 years old. But it sounds perfectly fine when I "manually" ground it.

What's going on?

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Does your bass have one or two single coil or humbucker pickups? –  filzilla Nov 14 '12 at 0:17
    
@filzilla: that doesn't matter much here, humbuckers are only less responsive to magnetic coupling (which is never affected by touching ground with your hands). –  leftaroundabout Nov 14 '12 at 10:18
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1 Answer

This is a capacitive coupling issue. It can be fixed by properly shielding all the electronic parts, i.e. surrounding them with some conductor connected to ground. Hackish though it sounds, the "human ground" is actually quite a common "solution" to this problem: on most electric instruments, the strings are connected to ground, so whenever you actually play something your body is turned into such a shield. In the case of your bass, it's probably this connection that's broken, so the easiest fix would be to search for the wire that connects the electronics to the bridge and reattach it.

Of course, this whole idea of using the body as shielding is horrible engineering, it's not only ineffective but sometimes dangerous, one of the reasons why there were so many electrocution accidents in the early days of live sound technology (when somewhere a ground pin touched a live wire – which normally doesn't ever happen today, but it's still not very nice).

The proper solution is to internally shield the electronics, with e.g. grounded tin foil. I have done this on a couple of instruments, it's not that difficult: just cover the surface of the internal cavities with the foil and make sure it's connected to ground in some way. You need to be a bit careful that none of the signal wires or potentiometer pins connect to this ground, but it's not such a big deal.

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+1 There is also an alternative to tin foil: spray on conductive paint. I have used this on one of my guitars that had complicated internal routing. –  Rory Alsop Nov 14 '12 at 10:29
    
Good point, I've never tried this but I reckon it's in fact easier to handle than tin foil and should work just as well. –  leftaroundabout Nov 14 '12 at 10:35
    
How do you typically electrically tie your sheets of aluminum foil together and to the ground wire? Aluminum has a low melting point, and probably doesn't bond well with conventional solder (though I haven't tried to solder it myself). –  ObscureRobot Nov 14 '12 at 21:31
    
@ObscureRobot: I don't specially connect the foil (yes, I do mean aluminium) to ground, I just make sure that 1. all the foil parts are in contact with each other and 2. some washer (normally of the output jack and also of the potis) that's connected to ground anyway is pressed against the foil. –  leftaroundabout Nov 14 '12 at 22:38
    
a while back, I was contemplating using spray adhesive with aluminum foil to shield an audio project, but I'm fairly sure the adhesive is nonconductive. The mechanical pressure of a washer is a good idea, though. –  ObscureRobot Nov 14 '12 at 22:41
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