[Dino's] JFET guitar preamp with piezo pickup

This week, with a little help from a Roomba, [Dino] built a guitar pickup and preamp that sounds marvelous. A pickup takes vibrations from the guitar and turns them into an electrical signal which can then be amplified and broadcast. He grabbed a long-dead Roomba which has slowly but surely been donating its organs for his weekly projects. After plucking out a piezo element he grabbed a bag of Junction gate Field-Effect Transistors (JFET) and built a preamp circuit around one of them.

JFETs operate in much the same way as MOSFETs (which we took a look at last week). [Dino's] design adds a few resistors and capacitors to tune the gain and decouple the circuit from the input and power rails. He epoxied the piezo element inside the guitar and connected leads between it and a jack mounted in the body. As always, he does an excellent job of explaining the concepts behind the design and outlining the build techniques that he used. We’ve embedded his video after the break.

Comments

  1. Pedro says:

    Mike, the first link has gone wrong.

  2. cfh says:

    The first link is broken!

  3. Sobachatina says:

    This is awesome and something I very much would like to do. I did wince when he drilled the hole in the guitar though.

    It would be nice if the link to his project worked. I want that schematic.

  4. PocketBrain says:

    I may have to hack like this a little to get an old guitar working, but how is the bass response? The lowest 2 strings sounded a bit quiet. Also, you could source the Piezo (pie-ease-o? pea-ay-zo? peas-o?) element from an old digital watch; those break at a distressingly high rate. (“Sole-der,” according to an old South African friend.)

  5. ChrisInNC says:

    I would have used an endpin jack instead. I’ve seen too many lower bout mounted ones split the bout later. Otherwise pretty killer hack! I may try this for my guitar. Bass response will be very dependent on the location of the piezo on the soundboard.

  6. dendad says:

    Hi. I would have left the shielded lead on and not used the twisted pair. That will be a source of hum most probably. Otherwise, very good. My daughter has a guitar that needs a pickup so I may hack into it if she will let me :)

  7. Mark says:

    My neighbor needs a high gain phono pre-amp. Do you guys think this would be suitable for a record player?

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