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.
I finally found some j201’s and got to build the Wah Pedal Buffer. I hot glued it to one of the inside walls of the pedal.
However, during assembly, I noticed that the circuit already had a buffer at the input of the effect. It’s a late 90’s Dunlop GCB-95, and I guess it’s basically the same circuit they’ve been using to this date.
Since I can do without this buffer, is it possible to reroute it and place it between the effect’s output and the on/off switch?
the gcb-95’s have an input and an output buffer, emitter followers. the output buffer you put in is redundant. if i were you i would replace the bipolar emitter follower with a jfet source follower right on the board; removing the existing components and replacing them with mr. orman’s since they will have the same topology (maybe except for the input voltage divider) and jfet’s clip more tastefully.
Jack does the buffer have to have an input cap at all? I see there is a 220n cap in front of it.
Thank you very much, i also had troubles with the distortion and had to change J201 to MPF102.
Yes, it needs the cap or the DC from the buffer bias will flow through the wah-pot and could make it noisy… also the 100k pot would then be in parallel with R3 and change the DC bias.
I added this to my wah but it didn’t last? For some strange reason the FET went? I had this glued onto the inside of my rev. D Crybaby, it was working fine but last night at a friends place it was no go? Any reason why it would go belly up?
Measure the voltages on the jfet pins to see if they are correct. Should be 9v on the drain, 4.5v on the gate and around 3.8v on the source.
regards, Jack
Hi, Great blog and information. Thanks for providing all the info. I want to add this buffer to a wah pedal I have modded. I’m using
a 3pdt switch with an led for bypass on the wah. I would like to have this buffer as a switchable option on the wah using a toggle. How should I wire this up? Thanks for all your help!!
Hi, Jack! Thanks a lot for your articles. They are really perfect! Why we can’t adjust this buffer to receive 4.5V on source. And please tell us why many famous manufacturers put 20u coupling capacitor in the output by the time RC-calculation shows that lower value (e.g. 1u ) will be suitable?
Is it possible to have the output variable to make fine tweaks in the output level from the wah?
Sure. just add a volume pot before the output jack… 10k to 100k should be good