JFET Bass Preamp

Version 2.1 (07/08/2002)

Schematic (GIF 38k)

PCB (GIF 34k)

Components Layout (GIF 36k)

NEW! Power Supply





I wanted a warm, tube-like sound with as few stages as possible. I believe that part of the clarity and directness of a tube amp comes from the fact that there are so few phase invertions compared to a normal solid state (not to mention the different kinds of unpleasant distortions made by op-amps). So I came up with the absolute minimum setup: Two (inverting) gain stages and one source follower/impedance converter. JFETs, because they behave similar to tubes at overload conditions (of course, nothing but a tube really sounds like a tube...)
The EQ Section was originally planned to be like an Ampeg SVT filter. It sounded good, but I could not get rid of the hum (used a small transformer for the filter coil), also it was rather noisy compared to the rest of the preamp. So I had to make a compromise (op-amps!), but this circuit really works fine, and most important, it can be bypassed. In fact I rarely need it, just use the "Ultra High"- and "Ultra-Low"-switches.
The best sound I got with the "Sensitivity" trimpot turned about 3/4 way right. All the way up gave me a little too much dirt, because the first stage clipped and it was no use turning down the "Gain" pot. Of course, this depends on the instrument used.




07/08/2002 Update V 2.1: Added D2 to protect LED D1 (my first one died after two gigs). Seems like some brands of LEDs cannot stand high reverse voltages. Also updated the PCB. If you already built V 2.0, simply solder D2 to the copper side (like I did).
05/29/2002 Update V 2.0: Omitted former R12, renumbered all other Rs. Added C22 and C23 for better power supply filtering. Corrected and otherwise improved PCB (Error in V 1.0: The taper of P7 must be connected to pin 6 of IC2, not pin7!).
The source resistors R5 and R10 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. Use carbon resistors for vintage sound... 8-)
The PCB is 5" x 2.8" (12.7cm x 7.2 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.
C15 is not on the board (solder directly to S6 and P6). For the correct wiring of the "Pre" and "Post" terminals see here.
For the Midrange-Select option solder the caps to S4, on the board omit C11 and replace C12 with jumper wire.
If you are sure your instrument's output is DC-free, you can replace C1 with jumper wire (e.g. with passive guitars or basses).
For guitar use, omit the Sensitivity pot and put in some more jumper wire... 8-)






Q: What kind of power supply should I use?
A: The power supply can be any stabilized symmetrical source from +/- 9V to +/- 18V (less than 30mA).
Q: Are the pots without a 'log' marker lin types (e.g. in the EQ-section)?
A: Yes.
Q: Is it necessary to build the threshold / tuner part? If I don't put it there, neither of them, will the sound change?
A: You can omit the Tuner Out & Clip Indicator sections without affecting the sound. Same goes for the EQ: If you don't need it, just omit the whole section and replace S5 with a soldered connection.
Q: What are the center frequencies of the EQ-section? I'm esp. curious about the values of the midrange.
A: Bass - 200Hz, Mid - 1000Hz, Treble - 2000Hz. Mid-Switch version: 500 / 750 / 1000 / 1500 / 2000 Hz.
Q: R5 and R10 should be selected to have max. nonclipping. Is this only possibly with an oscilloscope or can I use a DMM and check that the voltage between the resistor and the JFET is at 1/2 V+?
A: This is best done with a scope. If you don't have one (and I guess so from your question), you have to know the Vgs(off) of your FET. It is the voltage the FET needs to "shut off" and varies between -0.5 and -8 V (normally between -2.5 and -5 V). The drain voltage should be half way between the positive rail and Vgs(off).
Example: V(+) = 15V, Vgs(off) = -4.3V
15 - 4.3 = 10.7
10.7 : 2 = 5.35
15 - 5.35 = 9.65
So the drain voltage for this particular FET would be 9.65V DC. Got it?
Q: But how do I get to know Vgs(off)?
A: Here is a quick and dirty method:

Build this "circuit". Apply a negative voltage to the gate of the FET, starting with 0V and slowly going negative. At the same time, measure Vd with a DVM. Stop turning the pot when Vd reaches V+ (or more than 99% of it, say 14.99V...). Now measure the negative voltage at the gate (Vgs in the picture), and here you have Vgs(off)!
This method may not be 100% accurate, but works well enough for this purpose.