Onboard JFET Preamp

Version 1.0 (07/03/2002)

Schematic (GIF 13k)

PCB (GIF 5k)

Components Layout (GIF 11k)





After the unexpectedly big success of my JFET bass preamp I decided to build an onboard version. For a Jazz Bass-like instrument, I had to get along with three controls, maybe some switches if I used push-pull pots. Further, I wanted the functions of the knobs unchanged (vol - vol - tone) and, if possible, use the original pots.
This preamp features independent buffer stages for both pickups, a "Big Muff" style tone control with switchable mid cut and a gain/buffer stage for the output.

(Click on the schematic for a larger version)




The source resistors R2, R5 and R11 have to be selected according to the FET (and the supply voltage). Connect a 5k pot instead of the source resistor and adjust for maximum non-clipping signal at the drain. Then disconnect the pot, measure its resistance and put a resistor with the closest value available into the circuit.
For measuring JFETs please refer to the FAQ section of the JFET Bass Preamp.
The PCB is 2" x 7/10" (5.1 cm x 1.8 cm) in size. The holes fit to a grid of 1/10" (2.54mm). You should get the right size if you print it out with 300dpi.
Switches S1 (Mid Cut) and S2 (Mute) are optional. Feel free to experiment with different values for C3 and C4.
The circuit has an overall gain of +6dB. To increase gain, reduce the value of R12. If you omit R12/C5, you get approx. 0dB (unity gain).
I use this preamp for my acoustic bass to mix two piezo pickups (Underwood/Realist). For this purpose, I changed P1 and P2 to 2M5. First impression: The mid cut function is too strong/not necessary for double bass, and the pickups are out of phase. Thinking about a phase switch. No problems otherwise, works great.



Here are the frequency characteristics of the tone control (simulated with Duncan Munro's Tone Stack Calculator). The curves represent the left/center/right positions of the tone control:
Frequency with S1 as shown (C3=1n+2n2, C4=3n3)
Frequency with S1 switched (C3=1n, C4=3n3)
Pictures of the prototype. I use it on my double bass for mixing two piezo pickups (Underwood and Realist). For this purpose I changed P1 and P2 to 2M5:



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