Two-Stage JFET Preamp/Booster

Posted on Feb 5, 2014

This circuit makes a nice, clean preamp or a clean booster pedal. It is not designed for overdrive, but is designed to be overdrivable. If overdriven, it provides the kind of soft clipping one may expect from JFETs, but it is not optimized to be generally used as an overdrive effect. This design started out as an idea for a booster pedal circuit u

Two-Stage JFET Preamp/Booster
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sing low-noise JFETs following the principles of the Fetzer Valve(see ). I wanted plenty of headroom and however many stages I would need to have enough gain for clipping. I did not want clipping, but I wanted all the gain I could have up to the point of clipping. The output levels from different guitar pickups vary, so I figured a preamp that could reach the limits imposed by the supply voltage using the guitars I have could provide enough clean gain for most guitars. Using an 18 V supply allows for more clean gain than one can get with any 9 V circuit. Of course, that also means the output voltage might be higher than can be safely used with some solid state amplifiers and pedals. This design can easily be scaled to a 9 V supply, but might require more stages or higher gain JFETs that might be noisier than the 2n5457. I decided to used Fetzer valve based stages due to the supposed tube like performance of the Fetzer Valve design. I am not sure how well the Fetzer valve emulates a triode stage, but I think JFETs generally do a good job in preamp designs. I like the sound clips on the runoffgroove. com site, and I like the triode-to-jfet conversion designs I`ve heard elsewhere. For example, I think the Dr. Boogie preamp I built (based on the schematic from gaussmarkov. net) absolutely kicks ass. One may argue that the Fetzer design might not be the best for a clean preamp/boost circuit. By omitting the source resistor...

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