I glued a piezo element inside my ukulele and then built this preamp to boost the sound.
http://www.buildmyelectricguitar.co...ic-guitar/piezo-jfet-preamplifier-mpf102.html
When I plug my ukulele into the preamp and that into my Fender Champ it hums. If I touch the input jack on the ukulele the hum decreases somewhat. If I plug the ukulele directly into the Fender Champ, no hum. If I plug my electric guitar into the preamp and that into the Fender champ, no hum.
Any ideas on what is causing this hum?

 

sounds like a ground loop, and you are coupling 60hz power hum into the pre-amp. check your connections and cables for an open shield.

 

Are you using a battery for power of the preamp?

 

Yes, 9 volts

 

sounds like a ground loop, and you are coupling 60hz power hum into the pre-amp. check your connections and cables for an open shield.

Why would the same cable work with the guitar + preamp and not with the ukulele + preamp using the same cable?

 

Short wires from the piezo to the FET ? and shielded cable to the big amp?

 

Did you connect the jack to the pre-amp correctly? That is, signal must go to tip and 'common'to sleeve. If you send signal to sleeve, you get the hum.

 

Did you connect the jack to the pre-amp correctly? That is, signal must go to tip and 'common'to sleeve. If you send signal to sleeve, you get the hum.

It is connected correctly, the common to the sleeve. But even if I did wire it incorrectly, why would it hum with the uke and not the guitar?

 

You can try flipping the leads on the piezo element.

If that doesn't work, I assume the signal for the UKE must be much weaker than the signal on the guitar (smaller body = smaller dynamic movement). The weaker signal means you likely must turn up the volume on the UKE some (not a lot since it is a log response) and, therefore, amplify ambient noise more.

Also, simple things (really complex once you drill down) the quality of the dielectric of each instrument may play a role. Isolate the 1/4 inch jacks the UKE from the wood and varnish with some polyethylene or polypropylene washers.

 

You can try flipping the leads on the piezo element.

If that doesn't work, I assume the signal for the UKE must be much weaker than the signal on the guitar (smaller body = smaller dynamic movement). The weaker signal means you likely must turn up the volume on the UKE some (not a lot since it is a log response) and, therefore, amplify ambient noise more.

Also, simple things (really complex once you drill down) the quality of the dielectric of each instrument may play a role. Isolate the 1/4 inch jacks the UKE from the wood and varnish with some polyethylene or polypropylene washers.

The uke is acoustic with a piezo element. The guitar is electric with normal pickups so with the power off the uke is much louder. When I tested the uke through the preamp and then the guitar thru the preamp I did not change the volume on anything. The uke hummed the guitar did not. When I play the uke just through the Fender Champ I do have to turn the volume up a lot but there is no hum.

 

That proves your hum pickup is in how you connected the piezo element.

 

That proves your hum pickup is in how you connected the piezo element.

OK, I see your point because I think the wires from the piezo to the jack could be wrong. I'll hook up a new piezo, stick it to the top of the uke and see what happens. What I can't understand is how the uke would not hum when plugged directly to the amp. I'll get back to you tomorrow. Cooking dinner now. Thanks for your help.

 

I tried this about 30 years ago...complete fail!
Apparently the people that do this for a living know things I don't know.

 

I tried this about 30 years ago...complete fail!
Apparently the people that do this for a living know things I don't know.

That's funny. Well I'm just trying this as a hobby. But I guess I should have started learning about electronics when I was seven or eight to hopefully understand it at all now that I'm 67. It just seems like such a simple thing. Just a few parts. What could go wrong? Well, like I said, I'll take another look tomorrow. I cooked dinner, had a friend over who's sister is dying. We drank some manhattans. So there you go. Thanks again for all your thoughts on my dilemma. Talk again in the AM.

 

That proves your hum pickup is in how you connected the piezo element.

Hooked up a new piezo element, made sure the wires were right ......and that did it. No hum. Wow!
Thanks to every one for your time and expertise.