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.

This is a pretty easy project to put together. You can get JFETs from most electronics suppliers. I got mine from Tayda Electronics for $.25 each.

Keep on hackin!

Parts List:

1 – 10 ohm resistor
1 – 470 ohm resistor
1 – 1.2K ohm resistor
1 – 1 Meg ohm resistor
1 – .1 uF polarized capacitor
1 – 2 – 4.7 uF polarized capacitors
1 – 100 uF polarized capacitor
1 – J201 Transistor

Schematic:

4 thoughts on “JFET Audio Preamp with Piezo Guitar Pickup

  1. Hi Dino, I’m the guy that tweeted about using this circuit for a phono pre-amp. First off thanks so much for the post, my neighbor will be really happy if I can make his inherited turn-table work ;)

    I guess if I want stereo I’m going to basically have to build two of these, right?

    In terms of figuring out R3 is there any rule of thumb? If I know the output impedance does that help? Or is this just trial and error?

    Also, in terms of adjusting the amplification is there a way to achieve that by using some sort of variable resistor? I know you said that the optional bypass will achieve max amplification but I’m a bit of a newb so I’m not sure how to adjust amplification.

    Thanks very much!

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