Ground Hum

[topper]

Ok, in a passive bass, that's properly grounded, will you still hear a ground hum when you are not touching the strings? Is that normal?

 

[Jim Carr]

Ok, in a passive bass, that's properly grounded, will you still hear a ground hum when you are not touching the strings? Is that normal?

Is it a Jazz bass? How is your power?

I have a "properly grounded" Jazz with noiseless pickups that buzzes
at home thanks to poor power quality and wiring. There is a HUGE
amount of Radio Freq. noise in the area due to poor transformers and
pole top insulation issues. Plus, my house is 80 years old, and the wiring
is not well grounded.

When I go to one of my regular gigs that has very clean power, no hum!

 

[Jim Carr]

Tell us about your amp....and tone settings.

If the treble is really cranked, it will hum even more.

 

[topper]

Tell us about your amp....and tone settings.

If the treble is really cranked, it will hum even more.

I'm playing through a Hartke HA3000 amp.

It definitely gets worse with the bass's tone control in treble. With the treble cranked it sounds like frying eggs. And it completely goes away when the tone is in the full bass position. It also completely goes away as long as I'm touching the strings.

I'm not sure about the power in my house. I've only played this bass here at home and at my band's practice space and it does it in both places.

I can play with the tone in the full bass position, or keep my hands on the strings when I'm not playing. I just want to make sure this is normal. I've owned a number of basses over the years and it seems like some have hummed when I wasn't touching the strings and some haven't.

This bass is a custom build (my second) and is full of custom wiring. I don't expect to be able to make this sound as quiet as my Zon Sonus, but I do want to make sure I haven't done something really really wrong in the wiring.

I thought it was really weird that the hum went away when I turned down the tone control, but it sounds like that's not all that unusual.

 

[pkr2]

Look in the control cavity and see if it has a wire rout and ground wire for a bridge ground . Since it has a custom wiring scheme, it's possible that the bridge wasn't grounded which could cause the prob that you describe. You can also check it quickly by temporarily grounding it by holding the bare ends of a piece of wire on the bridge and the other end on the exposed metal of the input jack. Be sure to hold the wire on the isulation. If the hum goes away, either repair the connection at the bridge or the ground point in the cavity.

The connection at the bridge is made by pinching the wire between the body and the bridge when the mounting screws are tightened.

Are the pups humbuckers? If they are single coil, some hum is to be expected.

The hum should be loudest if the bass is oriented toward a source of interference such as a PC monitor or a TV set. If the hum doesn't diminish as you turn the bass 360 degrees, powerline interference is a good possibility. If the hum goes away when the bass is facing a particular direction, check for proper shielding.

If the ground wires are not star grounded in the cavity, you can experience hum caused by a ground loop even with humbuckers.

 

[Son of Magni]

If the bridge wasn't grounded the hum wouldn't go away when you touch the strings.

Most likely the control compartment isn't shielded. This is something you could help with conductive paint.

Also florescient (sp?) lighting is a major EMI source, as well as the things pkr2 mentioned.

 

[topper]

The bridge is definitely grounded. I've read a little on star grounding but found it difficult to incorporate in a complex wiring plan. So I took the easy way out and routed all the grounds to the back of the volume pot, then one to the bridge and one to the ground on the output jack.

The bass has some humbucker settings and some single coil settings. The ground hum happens no matter which setting I'm on.

I'm wondering if it's possible that I have some ground loops going on. Would those go away when I touch the strings?

You can see the wiring diagram here http://www.trey-mitchell.com/images/wiring5.gif (It's a beast).

I put a piece of shielding on the back of the control cavity cover plate and ran a ground wire to it, but didn't sheild the rest of the cavity. That might help.

If anyone spots any ground loops in the wiring diagram, let me know.

 

[pkr2]

Shielding the cavity may very well be the answer. It's something that should always be done, antway. You can't actually spot a ground loop on the schematic. Just moving the wires around in the cavity may stop the hum. Make sure that wires are seperated as much as possible.

If your amp has a ground lift switch, try all the positions.

I would bet that the shielding will help.

 

[topper]

Hmm... The wires are jammed in there like too much spaggetti. I could probably shorten a few of them and make some more space, but the bottom line is there's just going to be a lot of wires in there.

For now I'm going to turn my tone control more towards the bass position and see if I can still get the sound I want.

When I've got more time I'll consider taking the whole thing apart, cleaning up the wiring and sheilding the rest of the cavity.

Thanks!