Hello,
I've been working on an acoustic guitar with a built-in fishburn pre-
amp. The pre-amp has one input cable coming from the strings of the
guitar, and I believe these are probably piezo-electric devices.
There is another cable which goes to the 1/4" output jack of the
guitar. The problem is that when the guitar is plugged into an
amplifier (this is not the pre-amp), there is a loud and consistent
humming. If I put my hand on the strings or on the output jack, the
humming stops. I was told that this was a problem with the pre-amp.
But I did some checking and found that I could disconnect the cable
(which connects the pre-amp to the output jack) and the hum is
present.

So, now my circuit consists of the guitar amplifier, the cord (20'),
the output jack of the guitar, and a three wire cable (20 gauge, I
think) which is soldered to the output jack on one end and has a 3 pin
plastic socket on the other. That's the whole circuit and I still get
the hum. I unplugged the guitar cord from the output jack to see if
the hum was in the amplifier, and it wasn't. The hum is only present
when I have the guitar cord plugged into the output jack of the
guitar. So it appears that the cable is acting like an antenna.

Can anyone explain how this 18" cable is causing the hum? And how can
I fix it?