This is the basic output stage of a typical wah-wah pedal. You can see that the pot that controls the bandpass frequency is connected not only to the output jack but also to the input of a buffer transistor that feeds back part of the signal to the first stage.
This will make the wah sensitive to the load that is placed on its output. It can also cause an adverse reaction with a low impedance input of the next stage, such as with a fuzzface type cicuit. The FF can break into oscillation with the frequency changing as the wah treadle is moved.
One suggestion to rectify the impedance mismatch is to add an output buffer:
The simple jfet source-follower presents a high impedance (light load) to the wah pot and drives the output with a low impedance signal that isolates it from the next pedal in line.
This works fairly well but has one problem. The bias on the jfet is usually small, and depends on the gate-source characteristics of the individual fet, which limits how much input signal can be applied to the buffer before it begins to distort. Since the frequencies around the center of the wah’s bandpass are boosted, they will cause the jfet buffer to clip and a bit of distortion is added to the wah signal. While it may be possible to hand select a jfet that will perform well in the circuit, another solution is possible:
An extra resistor has been added to the jfet gate to set a higher source voltage. This allows a larger input signal before the onset of distortion resulting in a clean wah filtered output. Problem solved!
The improved wah buffer is identical to the jfet buffer on stripboard that was shown previously in this blog. The 10M (R1) resistor is not needed since the circuit is permanently wired to the wah and not switched. If you do not like stripboard, this circuit can be constructed using one of my Multi-Purpose pc boards.
Another option would be to use the leaky germanium transistor buffer. This is an especially good option since I’m offering free AC128 transistors that can be used for this purpose. The Ge buffer can also be built on stripboard or on the Multi-Purpose pc boards. Full details are included when you buy the pcb.
I just wanted to say thank you Jack for sharing your knowledge and experience. You have made this a much more enjoyable pursuit for us all!!
I did that buffer for my wah pedal , and imho it sounds better or even more wah effect in general, but with distrortion pedal after wah i hear little noise probably from circuit.
Here in your web http://www.muzique.com/lab/buffers.htm
i saw another buffer based on TL071 chip , so i thinking making one for testing with wah pedal – will be there less noise or not?
Did you try it before?
Thank you for that useful and interesting web.
You can change C2 for a 10uF
It drives the fuzz better.
Thanks for your cool website.
Why did you choose an electrolytic cap for C2? Wouldn´t a film cap be better?
Is it possible to use a 2N5457 as a replacement for the J201? I just can’t seem to find them where I live. And, if so, would it require me to change the value of any of the other components?
Uh… I finished reading the AMZ Simple Jfet buffer article… Sorry, redundant question… And by the way, thank you, Jack. I’m currently and extensively using a Mosfet booster that I made out of veroboard, with the Mil bypass (can’t find 3PDT’s either, so…) and couldn’t be happier.