JFET Audio Preamp with Piezo Guitar Pickup

I’ve been on a transistor theme here lately and there’s one more I’d like to cover, the JFET. JFET stands for Junction Field Effect Transistor. These transistors are well suited for high impedance inputs and since a Piezo electric device measures in the multi-megohm range it’s a perfect match.

I found the schematic for this circuit on this webpage: Design Guidelines for JFET Audio Preamplifier Circuits By Mike Martell Thanks Mike! Great write up.

After some bread boarding and testing I decided this would be a great preamp for a Piezo pickup mounted inside my acoustic guitar. I salvaged a Piezo element from a Roomba vacuum I had in the scrap pile. They use them as a dirt sensor mounted above the rotating brushes as you’ll see in the video.

I glued the Piezo element inside the guitar on the sound board near the sound hole and connected it to a 1/4″ audio jack that I mounted on the lower body side. It sounds great but there is a bit of 60 cycle hum getting in the signal. I’m assuming that this is coming from the circuit being on a bread board. I plan on mounting the components to a PCB and finding a suitable metal enclosure to put it all inside of. I think the metal will help shield outside noise.

Keep on hackin!

Parts List:

•  – 10 ohm resistor
•  – 470 ohm resistor
•  – 1.2K ohm resistor
•  – 1 Meg ohm resistor
•  – .1 uF polarized capacitor
•  – 2 – 4.7 uF polarized capacitors
•  – 100 uF polarized capacitor
•  – MPF 102 Transistor

Schematic:

Source : http://hackaweek.com/hacks/?p=365