Created by
Created December 31, 2021
Last modified February 09, 2022
Tags guitar   op-amp   piezo   preamp  

Summary

Low gain electric guitar preamp with input buffering and tone control using OPA 2134


Description

The preamp is meant to accommodate both electromagnetic and piezo bridge pickup's of which the latter especially tend to have very high output impedances, +/- 2M. Therefore it is assumed the pickup selection switch will be prior to the input. The schematic uses 2 LIPO 7.4v batteries for + and - rails because two standard 9v batteries will not fit the single 9v battery cavity inside of the guitar. The bass and treble controls are based on a passive Baxendall tone control network. The 1st op-amp stage is set to a 10M input impedance and gives +/- 3db gain to partially compensate for the 6db loss from the passive tone stack. The 2nd op-amp stage presents a similar 3db gain direct AC coupled high input impedance to the output of the tone stack to preserve linearity. The chosen values of pots are based on what is available specifically for electric guitars and should be audio taper. The pot's used for the tone ideally have a center detent to have a defined linear setting. A full audio 20-20k bandwidth was especially chosen to accommodate piezo pickups. This circuit is gratefully based on the following:

http://www.zen22142.zen.co.uk/Circuits/Music/guitar_tc.htm

Circuit edited following advice on EE Stack exchange


Comments

Space is very limited. Capacitors 1,5,6,7 are MKT. Capacitors 2,4,8,10 are MLCC. Capacitors 3,9,11 are normal ceramic. I used the OPA2134 because it is hi-fi grade and readily available here in NZ

by p-we
January 26, 2022

Suggestions and tips are highly appreciated before I build it

by p-we
January 26, 2022

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