Through various PAs this guitar has a background buzz which disappears when I touch the exterior of the plate around the three potentiometers (Volume Bass Treble). Its a metal plate curved to the side-shape of the guitar and gold-plated. There is no problem when it's played at home through a Marshall 100wASD (acoustic amp). Any ideas? Rob
Next, just where are these various PA's that you're having trouble with? The first thing you should do is take your Marshall amp into the same environment as those PA's, and see what you get.
No buzzing? It's the PA equipment itself.
Buzzes like crazy? It's the wiring of the environment. If it's a bar, perhaps there are neon signs nearby?
The fact that the buzzing disappears when you touch a metal plate is a good sign. But sadly, being a nylon string instrument, there's no way to ground the strings, like there is with a standard electric guitar. That means that your only* viable recourse is going to be some internal shielding.
Of course, with a 4Kilobuck instrument, this may not be a comfortable thing for you to contemplate. If the buzzing problem cannot be cured in the environment, then your guitar is gonna have to "go under the knife", so to speak. You might consider contacting some reputable guitar technicians, and getting estimates on how much is involved in a job like this, and what it might cost.
Sorry I can't be of more help. The reviews around the net don't specifically mention humming or buzzing to be a problem, so I think that you may be able to clear things up without having to resort to drastic measures, but it's always best to know your options.
* Please, don't let anyone try to tell you that you could simply wrap a strip of small-gauge wire around your wrist a couple of times, and run the other end to the shell of the cable going to the amp. I shudder to even mention it, because it's just about the biggest safety no-no I can imagine. Simply resist the temptation, please. 'Nuff said.
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