Wah Wah Mods analisys



A brief history

A couple of years ago I (re)started my guitar playing only to put it aside (dam Battlefield :-P). In that intense rebirth of the guitar passion a acquired a couple of guitar pedals and was even building one of my one.
One of the pedals I bought was a Wah Wah Dunlop GCB 95 pedal with the clear mission of modding the hell out of it.
I never even got it out of the box until recently...lol.
Since I'm into a slow comeback to the electronics field I decided to resume this small project.

The Wah FX

I’m not going to explain how the Wah filter works. You can read it in this nice article, but in resume it’s a low pass filter with a peak on the cut-off zone of the response frequency curve (bode diagram). This reproduces somewhat a kind of vocalization of a “wwaaahhh” sound.
The wah circuit is pretty much the same across all pedals, with slight component variations. It uses a kind of LC filter with a circuit that reproduces a variable capacitor (what makes the filter move forward and backward is this C variation).

Here are some pictures of my GCB 95 circuit board (this is the I version and I believe the latest).

PCB front	PCB back
PCB overlay

The Mods

There are a lot of mods on the web so I decided to analyse them before grabbing the iron.

Here are the more popular:

True bypass mod
Vocal mod
Gain and volume mod
Mid range mod
Sweep range mod

Here are other not so popular:

FatWah mod
Transistor mod
Buffer mod


I’m using LTSpice to simulate de circuit. I’ll be basically be looking at the bode diagram (frequency response) of the circuit. I will not show how the mods are done since you can Google it. There's tones of articles explaining what to do. Next I'll present my findings.

The sound changers

Note: All the following diagrams will be present in relation to the original circuit. This means all the mods a showed alone (one at the time). At the end I'll combine the ones I'll most probably implement. All the simulations where done with the pedal value at one of the ends (heel down - the woo part of the wah sound).

True Bypass (Must have or not depends on you)

This actually is the only I did not simulate. It made some confusion in my mind for the first 2 minutes but after reading this article it all became clear.
Since the signal is not totally disconnected/separated (just the output is routed via a SPDT) you will end up with and filter in the way of guitar signal. That filter produces the effect known as “Tone-sucking”. Basically it filters a portion of the original sound muting some of the frequency components of the sound (usually the low and/or high end of the signal’s frequency).

The solution?

Install a good input buffer or true bypass the signal.
Input buffering resides in putting a high/input and low/output impedance between the signal to allow the effect circuit to draw more current from an outside source and spare the weak signal from the guitar. This is applied not only for isolation of circuitry but also to “give new life” to the original signal, when this runs through long cables (the longer the cable the more resistance exists which makes the signal weaker). See good buffers here (JFET are my favourite It makes the guitar very alive).
True bypass is just that The signal is completely shut of from the effect pedal, by input and output routing. There are several articles about doing this, some removing the input buffer and other keeping it. I chose to keep it because of the next mod.
So this is one I should do.

The FatWah (I’m pretty sure it’s a must have)

This is not very known and probably one of the most dramatically changing mods for wah pedals (from an analytical point of view), strangely not mentioned in many mod articles probably because it requires the buffer stage used in the GCB 95(luck me :-D).
First lets take a look at the original frequency response of my GCB 95.

Original GCB 95 bode diagram

As you can see the response is very more or less what you expected (if you read the wah explanation article). The only thing odd is the lower end of the frequency seems to be quite low (-16Db @ 110 Hz – A2 note playing a loose 5th string guitar in standard tuning I believe). The peak sits near 440Hz (A4).
Doesn't seem so much like the theoretical wah filter showed in the article right? Actually it's no wonder this happens. When you plug you wah pedal the sound of your guitar gets "thinner". This happens because the lower frequency spectrum of the signal is being getting attenuated.

So what does the Fatwah does to the signal. Lets look.

FatWah mod bode diagram

Amazing!! The lower frequency range got a nice bost. The down side (from the graphical analysis) is that the "fatness" came at the cost of some vocal strength of the wah due to the reduction in the Q.

never the less the sound should become much more "full".

The vocal mod (also a must have)

Vocal mod bode diagram

This mod changes the Q of the filter making the "hump" of the signal more pronounced. This makes the wah more vocal.

The mid range mod (already factory available)

I will not apply this one for a single reason. It's already in the GCB 95. Changes the Q less then the vocal mod but is not negligible. The wave signal change is similar to the vocal mod but less strong.

The sweep range mod (if detuned wah or instrument adapting)

This actually makes the "hump" slide in frequency. This is the tuning point for your wah. I read in a post in a forum that the sweet spot for a wah pedal is between 440Hz (A4) and 1760Hz (A6) (to octaves apart. This may be needed if your wah sounds dull. The inductors mounted on these puppies suffer from large factory tolerance (ranging from 300mH to 650mH). To tune it you can unsolder the inductor, measure it, calculate/simulate the best value for this capacitor and change it (by adding parallel or series to the original if needed). This is also used to change (a higher value) the wah to a bass wah.

The more subtle mods

The transistor mod (a maybe)

I've simulated with a pair of BC109C. This changes the signal slightly. Makes the Q higher but the gain is smaller. Making a smother more vocal wah.

The gain/volume mod (will not do it)

That's it. it makes the signal more stronger. if you don't remove the input buffer it's not needed. Can make the pedal pick up noise.

The output buffer mod

This is a double mod. This mod is designed to make the wah immune to the fuzz pedal which allegedly kills the wah effect. I have not given this subject much study (the big why) since I'm not a fuzz man my self. This supposedly suppresses the need of an input buffer (I'll be keeping mine for the FatWah mhuahahahah). If a JFET buffer is performed the changes to the sound will be minimal. The output will be something like output of the signal without the buffer multiplied by a 0.98-0.95 gain (will lose around 2-5% in volume).

My Mod selection

Applying the FatWah + Vocal + Middle (already present) I'll get this

My selected mods bode diagram

Seems a lot like the theoretical wah doesn't it? ;-P

Will give it a try and post the results later.

Note: There's also the fasel red/yellow mod. But I'll get to it in a later post.

Cya next time.