Guitar hum issue

[Jack.Straw]

Hello. I'm building an Ashbory-type bass guitar. I'm using the ashbory preamp with the addition of an on/off switch. I also detached the LED from the circuit board. On testing the installation i'm getting a hum when the volume is not on full. On full volume everything is fine. Anything below that creates a hum. The hum is removed when touching any of the controls. I think I have everything grounded well, but i'm new at this. Anyone understand such things that can point me in the right direction?

Here is the schematic and close-up pictures of the preamp front & back:
Ashbory Bass Schematic

Here is a picture of my installation. I used shielded wire everywhere.
**broken link removed**

Thanks for any advice you can offer!
-Scott

 

[mneary]

You probably need to put shield between the sensitive electronics and you. Try a piece of aluminum foil lining the cavity where the amplifier sits. You can insulate the foil by covering it with masking tape or similar.

You could first try grounding the strings, assuming they are metal.

 

[Jack.Straw]

mneary, thanks for the reply. They are not metal strings... they are some sort of poly i think. I have copper shielding tape i could wrap the preamp in, if it's really needed. Does it mean something that the hum isn't there when the volume is on "10" or 0?

 

[audioguru]

Connect the cases of the controls to 0V.

 

[Jack.Straw]

audioguru, thanks! Unfortunately I'm not sure what you mean by that. Solder a wire from the cases of the pots to... the ground? Where do i get 0v?

Thanks again for all your help... my charger is working great!

 

[mneary]

Now there's no hum at 0. We're making progress. What did you do?

Anyhow, connect the cases of the controls to 'ground' as well as making a shield around what you can, also connected to ground.

You get ground from the shield connection of almost any wire.

 

[audioguru]

You probably cannot solder to the cases. The heat might destroy the pots.
Bolt terminals on the pots and solder wires from the terminals to 0V.

I didn't say "ground' because I didn't want you to run a 2km wire down the street to an earth ground. In this circuit 0V is ground.

 

[Jack.Straw]

Awesome, thanks guys! I did solder to the cases (ooops... acted too fast), so hopefully i didn't hurt the pots too much. I just tested and the hum is gone! Thanks again for your help and time guys!!!

 

[Andrew Leigh]

Hi,

I would still line the cavity with foil and have the cover, if not metal, lined with foil. Wait until you are on stage you will pick up other funnies.

Andrew

 

[Ross Craney]

How do your pickups work with non metalic strings ?

 

[Jack.Straw]

Ross, i've embedded a piezo disk the bridge base. The pickup looks like this:

**broken link removed**