Ground loop/humming in electric guitar circuit

[Andy]

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?

 

[Norman Webb]

Andy wrote in message

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?

If the signal wire of the cable is partly shorted to the earth shield of the
cable you will get hum. It may not be a dead short but enough to cause the
interference. You get the same effect if you put your finger on the end of a
cable plugged into an amp. So the cable may be faulty. Borrow another to
try.
If the cable is ok then
If we are talking electromagnetic pickups then additional earth shielding
is required.
Shielding cavities with copper foil and earthing or spraying a metalic
paint can help.

I am not so sure about piezo pickups though.