Why is my guitar making a humming sound?

Factual Questions
1

Musical 'dopers, can you diagnose the source of this humming sound, given the symptons:

The guitar is an electroacoustic, plugged directly into the guitar-in socket of a 4-track recorder. I’m monitoring it through a pair of headphones, and there’s a slight hiss, but also a low humming sound. The most noticable thing is that the sound stops completely when I touch the metal jacket of the jack where it plugs into the guitar.

I guess that something, somewhere is not properly grounded…can anyone tell me what it is?

Thanks

pd

2

It’s humming because it doesn’t know the words!
Sounds like mains hum; not uncommon at all, but if this has only just started happening, then it could be indicative of a problem somewhere.

Is there anything about the setup (cables, headphones, power supplies etc) that you’ve changed recently?

3

Usually there is a ground wire that connects between the jack (where you are touching and making the hum go away) and the bridge of the guitar. As long as you are touching the strings, you’ll eliminate the hum the same as if you were touching the ground on the jack. Sounds to me like this ground connection is broken somewhere.

PS - (IMHO) Get yourself a decent sound card for your computer and download one of the many free track machine programs available. You’ll never want to go back to that old 4 track.

4

mangetout , I only noticed it when I started listening through headphones; maybe I’m just hearing stuff with extra sensitivity compared to just playing it through my amp. The headphones are kind of old (like, 20-30 years!); I’m going to borrow a new pair this weekend and see if that makes any difference.

engineer_comp_geek , I’ll check tonight and see if touching the strings has the same effect as touching the jack. The bridge is wooden, though, so I can’t really picture how grounding the jack to it would work. Do you mean that’s what happens on an electric guitar with a metal bridge?

I’ve tried using my computer for multi-tracking before; I just could never get it to work smoothly for me. Probably didn’t help that I’m using linux, and a old sound card, and trying to record multiple tracks at once. This is my first experiment with recording; because the technology’s fairly old I was able to pick up a mint 4-track on ebay for £50. Plus, I’m in awe of the slightly hack-ish aspect of recording tracks onto a medium that was designed for one! (OK, one in each direction.)

5

It really sounds like an open ground somewhere in the circuit. I would check (from my own experience) these things:

  1. Swap Cables. If the buzz goes away, screw off the base of the jack on the cable and see if the soldered connections are crapping out.
  2. If you have that internal preamp in the guitar that runs on the 9-volt battery, check the connections on that. Maybe swap the battery out.
  3. Check the actual jack on the amp and guitar. One time I got a buzz from loose connections from the inside of the jack on the amp; the positive side of the jack was bent just a little bit.
  4. Try changing the place you plug in your amp. Sometimes something on the circuit in the wall can give you a buzz. Kind of like playing too close to a neon sign transformer in a bar…

Good luck buddy!

6

There are some guitar technicians on the board who probably know better than I do, but every acoustic/electric that I’ve seen has some way for the ground connection to be made to the strings. On my guitar there are two metal posts that stick up on either side of the strings, which in addition to holding the bridge in place (the rest of the bridge is made out of wood) also connects to a metal strip that runs underneath the strings, where the bridge piece sits in. A friend of mine had an ovation acoustic/electric. The only metal connection on it was a metal post that stuck through the bridge, that on the surface looked like a metal dot used just for decoration, but if you rested your hand on that dot then the hum went away.

Hum is definately going to be more noticable through headphones than through an amplifier. You might try going in through a low impedence input (use an external mixing board if you have to) and see if that helps.

7

There’s always a small amount of mains hum from any guitar pickup, unless you go to the extreme of playing in a cave deep underground, powered by nice, clean batteries.

primer_dimer, I reckon your hum is caused by your guitar ground not being earthed to the Earth. If you’re playing through a guitar amp, this will always be grounded to the earth connection on the mains, and you shouldn’t notice a difference in hum if you’re touching the strings or not. However, your 4-track recorder will be double-insulated, which means that it’s not earthed at all. Consequently the strings and metalwork on your guitar will be “floating” with respect to earth, and will pick up any mains hum in the area. The hum diminshes when you touch the strings as you are partially grounding the guitar with your body when you do this.

Another source of mains hum is a ground loop, formed when 2 or more items in the signal chain are earthed separately. Normally the only thing that is earthed in a guitar setup is the amp, so it’s not usually a problem for guitarists these days. This definitely isn’t the problem in the O.P.