Replace the preamp in my acoustic guitar

Pickups, wiring schemes, switch techniques and onboard active electronics for guitars

Replace the preamp in my acoustic guitar

Postby Amynio » 09 Jun 2017, 08:51

My preamp in my acoustic guitar died after I put the battery in backwards. I need to replace it, and fast. Piezos have incredibly high output impedance (5+ mohm) so a buffer circuit is used to knock the impedance down to a sane level.

I tried building this circuit and while it works, it lacks any sort of bass amplification:
Image

I didnt have a 100k resistor, so r3 is 170k. I also didnt have a .1uf, so c1 is 4.7uf. I tried raising the r2 to 5mohm and it sounded the same. v+ is 9v and vb+ is 4.5v from a voltage divider. OpAmp is TL072

All caps are thin film, not electrolytic

The DI Box next in the chain has an input impedance of 1mohm
Any help or alteratives much appreciated!
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Re: Replace the preamp in my acoustic guitar

Postby phatt » 09 Jun 2017, 12:23

The original preamp likely had a buffer input then a gain stage and a tone control stage. The better units even try to wipe off the excess high freq bands as that is a big problem with piezo elements.

All you have built is a buffer and so the tone will sound flat and might be tinny, harsh or even scratchy from too much high freq.
I did build a complete preamp for a chap a while back using an old fet circuit and it came out rather well. If you are interested I'll try to dig through my backups and post the circuit.
Phil.
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Re: Replace the preamp in my acoustic guitar

Postby Nikogo » 04 Sep 2017, 16:18

To have a good bass from piezo pickup the preamp need to have about 5 MOhm input impedance.
I made a simple buffer on a single FET. It is in upper right corner of the guitar schematics.
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Last edited by Nikogo on 04 Sep 2017, 17:33, edited 2 times in total.
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Re: Replace the preamp in my acoustic guitar

Postby Nikogo » 04 Sep 2017, 16:25

Here is the PCB layout for surface mount components:
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