Replace the preamp in my acoustic guitar

Pickups, wiring schemes, switch techniques and onboard active electronics for guitars and basses
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Amynio
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Post by Amynio »

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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phatt
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Post by phatt »

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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Nikogo
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Post by Nikogo »

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.
Attachments
It has two humbuckers and a piezo bridge pickup. For example L. R. Baggs X-Bridge.<br />The pickup selector S5 is a mechanical rotary encoder - a tiny 8 position device that provides all possible combinations for 3 pickups in binary sequence. <br />Switches S1 and S2 give 4 combinations for the neck HB: parallel, 1 and 2 single coil, series.<br />Switches S3 and S4 do the same for the bridge HB.<br />Selector S5 connects:<br />- 1 - neck HB<br />- 2 - bridge HB<br />- 3 - neck + bridge HB<br />- 4 - X-bridge<br />- 5 - neck HB + X-bridge<br />- 6 - bridge HB + X-bridge<br />- 7 - neck HB + bridge HB + X-bridge<br />- 8 - none.<br />Ten position rotary switch S7:<br />- 1 position - no load/ no filters<br />- 2 position - bass cut filter<br />- 3 position - mid scoop filter with some bass<br />- 4 position - mid scoop filter with more bass<br />- 5 position - mid scoop filter with some bass, less treble<br />- 6 position - mid scoop filter with more bass, less treble<br />- 7 position - adjustable top treble cut filter <br />- 8 position - adjustable more treble cut filter<br />- 9 position - adjustable traditional treble cut filter<br />- 10 position - bass cut + treble cut filters for mid boost<br />The S9 switch is a reed switch located under pickguard against 10th position and activated by a magnet (that is installed in the rotary switch knob) only on that position.<br />The piezo pickup is connected through a buffer on a follower J201 with high input impedance 5 M that allows to pick deep bass with x-bridge.<br />The input divider is used to adjust the high signal from piezo pickup to the (not enough) high output humbuckers (for example: Dimarzio Steve's special). The capacitor C10 is compensating loss of trebles in divider.
It has two humbuckers and a piezo bridge pickup. For example L. R. Baggs X-Bridge.
The pickup selector S5 is a mechanical rotary encoder - a tiny 8 position device that provides all possible combinations for 3 pickups in binary sequence.
Switches S1 and S2 give 4 combinations for the neck HB: parallel, 1 and 2 single coil, series.
Switches S3 and S4 do the same for the bridge HB.
Selector S5 connects:
- 1 - neck HB
- 2 - bridge HB
- 3 - neck + bridge HB
- 4 - X-bridge
- 5 - neck HB + X-bridge
- 6 - bridge HB + X-bridge
- 7 - neck HB + bridge HB + X-bridge
- 8 - none.
Ten position rotary switch S7:
- 1 position - no load/ no filters
- 2 position - bass cut filter
- 3 position - mid scoop filter with some bass
- 4 position - mid scoop filter with more bass
- 5 position - mid scoop filter with some bass, less treble
- 6 position - mid scoop filter with more bass, less treble
- 7 position - adjustable top treble cut filter
- 8 position - adjustable more treble cut filter
- 9 position - adjustable traditional treble cut filter
- 10 position - bass cut + treble cut filters for mid boost
The S9 switch is a reed switch located under pickguard against 10th position and activated by a magnet (that is installed in the rotary switch knob) only on that position.
The piezo pickup is connected through a buffer on a follower J201 with high input impedance 5 M that allows to pick deep bass with x-bridge.
The input divider is used to adjust the high signal from piezo pickup to the (not enough) high output humbuckers (for example: Dimarzio Steve's special). The capacitor C10 is compensating loss of trebles in divider.
Last edited by Nikogo on 04 Sep 2017, 16:33, edited 2 times in total.
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Nikogo
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Post by Nikogo »

Here is the PCB layout for surface mount components:
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PCB G3.jpg
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