Built the simple Til (Don Tillman) JFET buffer and it's humming, where to look?

[777funk]

I just built thishttp://www.till.com/articles/GuitarPreamp/ and ran the ground across the bottom 3/4" of the perf board. The in and output shields are both attached to this grounding point.

For some reason I'm getting lots of hum with any guitar I plug into it. The only change I made was to use a 10M input resistor since I'll be running piezos besides normal magnetics. Where do I look?

 

[audiomixer]

what power supply do you use?

- Michael

 

[usekgb]

I built one of those last week to amplify some piezo's for a science fair project of my daughter's.  Mine is dead quiet until I plug a cable in to the input.  It seems to be fairly sensitive to different cables as well.  One cable hummed very loudly, while another cable was much quieter.  I also used an unregulated 9V wall wart that was spitting out about 11.5V.  I then ran this in to a 7809 regulator to even the power out.

 

[kato]

Run it from a 9v battery and see if the hum goes away.

 

[777funk]

That's what surprised me, I was running it from a 9V batt. So it wasn't power supply hum. I used cables that are dead quiet even with high gain in an amp. I'm lost as to what's creating the hum. I'll build it on a bread board and try again.

 

[JohnRoberts]

If you short the input does the noise go away?

JR

 

[777funk]

Yes it does (or rolling down guitar volume pot which would be similar).

 

[RuudNL]

At the moment the input is shorted, the drain impedance (6.8 K in parallel with the FET + 2.2 K) is so low, that there is no reason to expect hum, especially if you use a battery as a power source.
So I expect there is something else wrong.
(Did you reverse the signal wire and screen somewhere?...)