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active guitar

Started by voltwide, February 01, 2016, 06:57:40 PM

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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...


The TONE pot is 50k linear, the VOL pot 50k log.
C1,C3,C4,C5 are film caps. C2, C6 are ceramic/MLCC.


Measuring The Frequency Range

Using my favourite audio-analyzer program ,,ARTA" and a PC with a sound card (behringer UCA-202) it is quite easy to measure the resonant peak according to the TONE-control setting. Line-Out (right) feeds via a series resistor of 3.3megOhm the pickup. Line-In (left) is connected to the guitar jack.
Starting with the TONE-control in minimum position


Now somewhere in the middle


And finally TONE-contral at max position


In that case the resonant peak can be set between 1,4kHz and 4kHz. The pickup is not the original Fender single coil but some stacked humbucker with 12kOhm DC-resistance.
Adopt this circuit to different pickups by tweaking the resonant capacitor C3 (here 3.3nF).
The JFETs shown are SMD-types provided by ON-semi/Sanyo and can be replaced by 2SK170 or similar.


Hey good effort,  :dbtu:  Will try it when time permits.
I actually have a small preamp that does a similar trick but no honk control.
I'm not sold on having on board preamps and as my pedal board is right at my feet my lead is always short so very little loss to active preamp. That way I can use all my guitars without the need for modification. Phil.


Quote from: voltwide on February 01, 2016, 06:57:40 PMActually a prototype breadboard is working in my strat. The sound varies between very clear strat sound and something close to a humbucker sound . . .
Although only tested with my Fender Strat, this circuit should work in any electrical guitar with passive magnetic pick-ups.

This interests me for a very particular reason: I like to walk around a bit while playing; since I can't afford a wifi connector right now, a long cord is the only way - but then capacitance typically becomes an issue. In my innocence, I had thought I could get away with using a buffer close to the amp & the long cord in front of it, and only lately learned that putting the buffer by the amp is "too late" (though may have other benefits sound-wise). So now instead of being chained to my amp by a short cord I'm chained to my pedals!

I was advised that active pickups might solve this problem & give me back my long-cord freedom; but a preamp might do the same w/out having to switch out my present set of pickups. So I might want to try it at some point.

HOWEVER -  my guitar has humbuckers. You mention it should work w/pretty much any passive pickup. Does that mean the increased signal strength typical of humbuckers should not be an issue? Also are there particular components in the circuit that can be tweaked? I know very little of circuit analysis so my only way way of "analyzing" this would probably be to mock it up in something like LTSpice, though I am not that good at that either.


you are right, placing the buffer at the input of your amp is too late. Nonetheless this can improve sound if input impedance of amp is too small - and that is often the case.
I would expect no problems with the increased output of humbuckers - keep in mind that the voltage gain is less than unity. Using a 9V-Battery allows voltage upto 2Vrms (i.e. 6Vpeak-to-peak) without distortion - and that is pretty much.
The most important part for tweaking is the resonant capacitor which sets the frequency range of the tone control. Depending on the pick-up impedance and your personal taste I would expect anything between 1nF and 10nF.


Thanks, very helpful. This may be my next little project!


Quick question - looking at the schematic you show two grounds coming off R4. Am I correct in assuming this is only a convention to indicate how battery negative is tied in as 0V/ground to existing ground in the wiring harness, e.g. sleeve of guitar jack, strings, etc.?


Yes, this indicates a test point reserved for the wiring, same sort as input and pos voltage supply.


LTSpice is the tool I designed this circuit with. For playing around I will add the simulation file


Please note that the file extension ."txt" should be removed before loaded into LTSpice



I got tired of LTSpice for Mac, poor interface, so yesterday installed it for Windows, much better.