Hum When Not Touching Strings

[jasonn]

I have nearly finished my first build. It has two humbuckers, a 3 way toggle switch, one volume and one tone pot.

Everything works OK except for a small crackle and bigger hum that both go away when I touch the strings, bridge or jack. I have searched this forum and others and this seems to be caused by not running a wire from the bridge to the back of the volume pot - but I have dones this and double checked that there is a connection between the two.

I am comparing my new guitar with an old les paul copy using the same amp and cable. The old guitar has virtually no hum whereas my new build is bad.

To try and narrow down the problem I disconnected everything and wired a single pickups directly to the jack (+ the bridge ground to the jack). This causes exactly the same problem with either pickup.

The control cavity is not shielded but could this cause such a bad hum?

Any ideas what I am doing wrong?

[Prostheta]

Do you have a grounding wire connected to the bridge?

[jasonn]

> Do you have a grounding wire connected to the bridge?

Yes - athough it is attached to the tailpiece. I checked this with a multimeter and the connection seems OK.

[joshvegas]

If you don't have sheilding are you in a room with A CRT monitor/striplights or dimmer switches?

[jasonn]

If you don't have sheilding are you in a room with A CRT monitor/striplights or dimmer switches?

Thanks - I did not realise they could cause hum unfortunately there is no CRT monitor/striplights or dimmer switches in the room.

Do you think I should try shielding anyway?

[psw]

I have nearly finished my first build. It has two humbuckers, a 3 way toggle switch, one volume and one tone pot.

Everything works OK except for a small crackle and bigger hum that both go away when I touch the strings, bridge or jack. I have searched this forum and others and this seems to be caused by not running a wire from the bridge to the back of the volume pot - but I have dones this and double checked that there is a connection between the two.

I am comparing my new guitar with an old les paul copy using the same amp and cable. The old guitar has virtually no hum whereas my new build is bad.

To try and narrow down the problem I disconnected everything and wired a single pickups directly to the jack (+ the bridge ground to the jack). This causes exactly the same problem with either pickup.

The control cavity is not shielded but could this cause such a bad hum?

Any ideas what I am doing wrong?

Sorry to hear it not working properly.

A great post that has anticipated everything that i would suggest...right down to eliminating all the wiring and connecting the pickup directly with the jack! Obviously, as touching the strings kills the hum, the strings must be grounded adequately.

You have not mentioned the pickup type, but it is not unknown with HB's with 4 conductor wiring to be hooked up accidentally so that only one coil is operating (effectively coil split) and so as with most single coils, susceptible to hum and noise (like a strat) and the effect of the sting grounding more obvious. To test this, tap a screwdriver to the pickup poles when pugged in to be sure both coils are operating. If not you will need to try variations of wire combinations so that the two coils are wired in series for the full humbucking effect and power.

Do you think I should try shielding anyway?

Not at this stage, it just adds more complexity. Generally HB guitars are pretty quiet but if your wiring is messy, this would not help things. Wiring every ground back to a central point is always a good practice (such as the back of the pots or the selector) as ground loops (effectively creating a mini coil inside the guitar) can attract hum like a single coil.

Once everything is working right, good shielding can add even more noise reduction but I would do this only after the wiring is checked and working properly. Guitars like my old gibson les paul were not shielded and very quiet regardless.

Hope that helps...but it is the only thing that you don't seem to have tried yet...keep at it...

pete

[bluesy]

Let me throw this in.

On very simple setups - like one pickup (tried both single coil and humbucker) wired direct to the output socket no control), or also, one pickup through a volume pot to the output socket, I have found it is always necessary to touch the ground somewhere to make the hum go silent. The hum is not loud, but it completely goes away when you touch the ground wire. Connecting the ground to the strings via the bridge, or tailpiece, does nothing of itself, but it means that while you are playing, because you are touching the strings (which are grounded), the guitar does not hum.

[psw]

This is generally true...however....

I am comparing my new guitar with an old les paul copy using the same amp and cable. The old guitar has virtually no hum whereas my new build is bad.

You may get a little hum, but with an HB guitar you'd expect it to be only slight and completely go away when touching the strings. This symptoms sounds like a single coil...inherently noisy unless the strings are touched...so all I coul think of since the pots have been bypassed and the cable and amp tested with another guitar is that this could be it...however, it is something of a mystery!

pete

PS...an alternative to shielding is to use all shielded cables...but a bit tricky to work with and not necessarily the problem here.

[Prostheta]

Are your output jack wires reversed? This would mean your entire instrument isn't in fact grounded no matter how well it's shielded :-D

[psw]

Are your output jack wires reversed? This would mean your entire instrument isn't in fact grounded no matter how well it's shielded :-D

Ahhh....very true...and some pickup makers make it easy to do by colour coding their hot as green...down here the traditional colour for ground...worth a shot. Nice call!

[jasonn]

Are your output jack wires reversed? This would mean your entire instrument isn't in fact grounded no matter how well it's shielded :-D

Ahhh....very true...and some pickup makers make it easy to do by colour coding their hot as green...down here the traditional colour for ground...worth a shot. Nice call!

Thanks for the suggestions - I am learning a lot. I checked the output wires - they were OK. Just to make sure I then tried a different output jack with no improvement.

I tried tapping the coils and they are both working for each humbucker.

I tried removing the black metal humbucker rings but that made no difference either.

Then I took the bridge pickup from my old (no hum) les paul and put it in the new guitar wired directly to the output jack. It was just as noisy as the other pickups. Does this suggest there is something about my guitar that is causing the hum? The guitar is an unusual shape, i.e. it has a small body with heavily recessed tunamatic bridge and tailpiece - this means there is not a lot of wood between components.

The weird thing is when I put everything back, i.e. with just the new humbucker wired direclty to the output jack the hum was not quite as bad.

Could the unusual shape cause the hum? Here is my guitar with the toggle switch and pots removed:

I have run out of things to try so my next step might be to add copper shielding to the control cavity and pickup cavities.

The bridge humbucker is made by Schaller and the neck pickup is made by Artec.

Any more ideas greatly appreciated!

Edited August 17, 2008 by jasonn

[psw]

Nice looking guitar...no the shape wont make a difference...

When checking the pickup direct to the jack, are you wiring the string/bridge ground to the ground of the jack? Also, when testing them, put the jack in the guitar...don't just have the wires hanging out as this can cause some noise.

It is a mystery...still thinking you are doing all the obvious things I would have suggested and more...

Then I took the bridge pickup from my old (no hum) les paul and put it in the new guitar wired directly to the output jack. It was just as noisy as the other pickups. Does this suggest there is something about my guitar that is causing the hum?

See...this is interesting and would suggest that the guitar is at fault but electronically, the only connection to the actual guitar itself is the string ground. Check this again with a multimeter that the strings are connected to the cable ground.

Shielding does help, but if this is really bad, not just a faint hum, with HB's you would expect it to be pretty low and disappear when touching the strings...that is the only difference I can see between the two guitars perhaps. The guitar nutz forum has a bit about shielding and you can get even strats to be pretty quiet.

pete

[Prostheta]

Surprising. It really does sound like a case of the output jack wires being reversed. Sorry if you've already crossed this bridge, but I would try reversing them from where they are right now. It's such a common error, that i've been known to do it....sometimes....

[psw]

I checked the output wires - they were OK.

Yes, but did you reverse them?

Surprising. It really does sound like a case of the output jack wires being reversed.

On the other hand, as long as the strings are grounded to the ground on the jack, all that reversing the wires will do is cause the pickup to be out of phase resulting in the exact same sound as the other way...unless you wire two pickups out of phase with each other.

Although you have tested that both coils are working...they could still be wired in parallel, not series. This results in a low output almost single coil sound and although theoretically will be humbucking, you may be turning up the amp to compensate resulting in more background hum than usual.

Did the pickups come with a wiring scheme and colour codes for the wires? If four conductor, did two wires come already connected together...perhaps a picture or diagram might help...this is a tricky one. Does the hum disappear completely when the guitar is touched?

pete

[~davie]

check for grounding loops, and try star-grounding