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#1
Preamps and Effects / active guitar
February 01, 2016, 06:57:40 PM
Active Guitar Pre-Amp and Tone Control

This is my first posting here. So I want to contribute a bit from my long lasting DIY-experience tweaking my old strat and other innocent instruments.

Classical wiring of electric guitars was invented in the middle of the last century and addressed tube amps with low gain and high input impedance.  Since then magnetic pick-ups contain many thousands of turns of thin magnetic wire to achieve a maximum output voltage. This approach yields a high circuit impedance with a peak at resonant frequency in the range of 1~4kHz, depending on pick-up.
As a consequence the capacitance of the guitar cable tunes this circuit with audible effect: The longer the cable, the lower the resonant frequency, shifting the  resonant peak from ,,crisp high" towards ,,middle twang".
Furthermore, setting the volume pot to a lower level also reduces treble and makes the sound more dull.

Having said this there are some good reasons to put an active pre-amp into the guitar:
-get full treble response independent of volume pot setting
-get full treble response with any guitar cable
Furthermore an active tone control is added. In contrast to the normally useless factory tone controls this tunes the resonant peak smoothly over the entire range – similar to a guitar cable of tunable length.

Actually a prototype breadboard is working in my strat. The sound varies between very clear strat sound and something close to a humbucker sound. It was interesting to find that output level is nearly constant over the full range of tone pot setting.

The circuit is based on a JFET source-follower acting as a buffer with a voltage gain close to unity. The second source follower is driven by the tone control pot and acts as a variable capacitor tuning the pick-up resonant frequency. I learned this circuit trick by reverse engineering the VOX-Wah-Wah about 1970.

If space permits the circuit can be powered by a 9V-block. In space critical applications two Lithium coin batteries in series will do as well.

Although only tested with my Fender Strat, this circuit should work in any electrical guitar with passive magnetic pick-ups.
More to follow soon...