EQWHA-2  Reverse Engineered And Pimped
Making A State Of The Art Wah Out Of A Surplus Tinkerer´s Object
last update: Feb 29, 2020

Copyright 2020 by H. Gragger. All Rights Reserved. All information provided herein is destined for educational and D.I.Y. purposes only. Commercial re-sale, distribution or usage of artwork without explicit written permission of the author is strictly prohibited. The original units  with their associated  trade-names are subject to the copyright of the individual copyright owner. The Author is by no means affiliated with any of those companies. References to trade names are made for educational purposes only. By reading the information provided here you agree to the Terms of Use.
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Index

Abstract
Differences And Peculiarities
Modding
Verdict
Reference


Abstract

EQWHA modded
Quite a while ago I bought a wah pedal from tubeampdoctor, called the EQWHA-2.

They sell it as non-working, tinkerer´s object, but mine does work. It goes quite cheap, it has a reasonably well made case and appears a good foundation for building all sorts of wah pedals.

Mine has a silvery grey matte surface, but I read it was sold in chrome too. I see Ruby Tubes sell it as EQWAH too.
Here you see it with the mods already in place.


I sounds much like the "Jimi" setting on my Clyde Deluxe, which means it is a Cry Baby type.

Incidentally, it uses exactly the voicing cap (10nF) that makes up a Cry Baby wah.
A look at the innards however did not make me much wiser since they all look the same. Luckily, they have printed component designators onto the PCB, which unluckily do not match any schematic available. So I had to reverse engineer the pedal. Took me a few hours, but indeed it is an almost exact clone of the GCB-95, which by itself goes back to a schematic from the sixties.

Now instead of gutting the pedal, one might as well use it as a foundation for minor changes, since virtually all boutique wahs are based upon this godfather.

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Differences And Peculiarities

As customary, it has an input buffer to palliate tone sucking caused by a permanently attached load. Of course, it is made so true to the original that it even copies the inferior bypass switching using a 1PDT footswitch. Do they never learn from mistakes?

The input buffer is a bit different from the GCB-95 in that it uses a darlington input transistor arrangement made out of two discrete transistors. Most of the vintage schematics have a 1k resistor at the collector, which this one does not have.
Although the biasing scheme for this type of buffer is quite basic, it will work perfectly since there is no big signals to be expected (something we cannot guarantee later down the circuit).

They positioned a 100nF filter cap directly at the input buffer supply, which is not a bad idea.

R9 (at the inductor) sets the bias for Q4, they decided to have 100k there (normally 82k).

The power supply is marginally different, an additional filter resistor and an IMHO undersized clamp diode against reverse power.

Nothing is known about the inductor.

This altogether is exactly what is there in a boutique wah, and indeed it sounds like a Cry Baby. Actually not my favorite.

They chose to connect the board to the outside world with an 8-pin connector. This may be proprietary and different from other wahs.

Audio and power connectors  are mounted directly, which brings down assembly cost. Everybody seems to take this route nowadays. This is another good reason that speaks for modding the existing board, since it is all there.

download-icon
Download the schematic

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Modding

Now we know what does what on the EQWHA-2, we can start optimizing. Virtually all inductor wahs I found are 100% clones of this circuit. Most of them potentially make variable what has been a fixed value originally,  thus provide variety that leads people into believing in innovation.


"A friend of mine has just bought a XY signature cry baby wah. I have a cry baby original but his XY sounds so much nicer than mine. Is it possible to convert it?"

- Anonymous voice of a musician mislead by targeted disinformation and hype

"As one might expect, there is very little new in electronics, at least in analog circuitry. (...) Sometimes the innovators try to make their transfer of known electronics methodology from one discipline to this one seem revolutionary. The revolution is not actually the technology, but rather its revelation to the rest of the world."

- Kevin O´Connor: The Ultimate Tone Vol 4, Power Press Publishing, Thunder Bay, CN, 2006, p. 4-15f
  • true bypass switching: really a stock 3PDT footswitch bypass scheme should be employed, does the annunciation LED bit into the bargain
  • fuzz friendly output buffering
  • R5 and / or R12: input gain. Changes bass response too. Can be internal or external.
  • R7: "vocal mod" changes the Q of the circuit and makes the effect more pronounced.
    Can be internal or external.
  • R6: "mid range", as is says. Can be internal or external.
  • C8: "voicing", makes some drastic changes to the voicing from bass wah to Shaft and all colors inbetween. Usually realized with a rotary switch or a 3 position toggle switch. This makes 90% of the "sound" of a particular wah type.
  • wah pot: changing this is not really a "mod", rather a question of preference. And of course, once the pot is worn.
  • Inductor: hmm. Hard to say what people hear as a difference on different inductors.
  • Cantrell mod: a 5k (trim-) pot in series with the ground leg of the wah pot. Adjusts the travel of the wah pot[3]. Other series resistors (pot lug 3) work too.
Input buffering:

Agreed, using a buffer to alleviate bad input switching for the sake of saving a decent footswitch is a ridiculous idea. This may have been plausible 60 years ago when those parts were not available or incredibly pricy.

However, impedance matching is a good idea. As mentioned, the buffer that is there is perfectly fine and transparent, and it only has to be relocated slightly (see pictures below). Read what I have to say about buffering a wah in general in my Clyde Deluxe article.

A fuzz friendly output buffer (i.e. impedance matcher) is then the only major change which I would add to this pedal. The one shown at home-wrecker
[1] would be perfectly fit for the job, but I came to dislike n-channel mosfets due to their potentially high noise levels.
Cheap enough, a fixed gain op-amp will serve perfectly as it does on my Clyde. Maybe it is plenty enough to provide an internal trim pot for setting output gain and leave it like that or using a unity gain buffer.

Them Changes:

changes
                          to the pcb, top
Changes to the PCB (click enlarge):

On my PCB, I removed all those notoriously tweaked resistors and caps and solder posts onto the board instead, so that I can tweak them with top- or remote mounted components, without having to ever access the bottom of the board again.

As depicted, voicing cap, vocal range (=Q) and mid range are prepared for external access. I added a bass/gain pot lateron, since I felt it needed more body.                     

changes to the pcb, bottom
Changes to the PCB (click enlarge):

On the bottom you see additional supply leads for the external output buffer board. Note the cut trace that permanently connects the wah input to the input jack. Now, it connects to the 3pdt footswitch only,  while the orange wire provides a return path going into the buffer.

The original input buffer is thus always in front of the wah circuit, but the whole electronics is bypassed in bypass mode, as is customary today.
                         

wah
                          inside changes
Inside peek (click enlarge):
  • Top left: 3pdt retrofit full bypass switch
  • Bottom left: (wrapped) simple buffer with output impedance ("imp").
  • voice toggle switch (center off): toggles between stock 10nf (cry baby) 20nf (Zakk Wylde/JH-1) and 15 nF inbetween mode.
  • Q, Mid and Bass pots, note the mark that designates original values
  • LED, with inline dropper resistor.
see[2] how to do all this, particularly how to prepare the pots.

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Verdict

How does it sound? Well, like any of them, you choose the flavor. With the voicing switch you can dial in anything from bass wah to JH-1 /  Zakk Wylde to Shaft. This has always been just a question of a single, cheap capacitor.
Is it a great wah? I don´t know, but it certainly is no less good sounding and useful than the competitors. The feeling of the pot is different. Maybe they use a different law, but it looks very sturdy. And the case and mechanics are very robust. Not welded steel like my Clyde Deluxe, but not cheesy plastic either.
I refrain from making sound demos, because they are all alike. There is nothing new under the sun.

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Reference

[1]
Home Wrecker: wah mods: http://www.home-wrecker.com/wahmods.html
[2] Andreas Möller (Stinkfoot): diverse excellent wah modding articles in one placehttp://stinkfoot.se
[3] Zakk and jerry wahs, https://www.diystompboxes.com/smfforum/index.php?topic=91315.0

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Update History
  • Feb. 29, 2020  first release
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