The Shin Ei WT-1 Wah Pedal - "Inductor" Study
Last updated 02/04/23
By Paul Marossy
Since this wah pedal sounds SO different from the original circuit designed & patented by Brad Plunkett in 1967, I thought it would
be interesting to model it in LTSpice to see it we can learn a few of its secrets. We'll also compare some other well known circuits to see
how they differ. First, let's take a look at the Vox "Clyde McCoy" circuit vs. the Shin Ei WT-1 "Companion" circuit.
Here is the Vox "Clyde McCoy" circuit which many wah pedals are a variant of / direct copy of. Notice the arrangement at the inductor.
There is a 500mH inductor, an 33K "Q resistor", and an 82K resistor & 4.7uF cap to ground. There is a feedback resistor between Q1 & Q2,
with the 0.01uF "sweep cap" in series. This is pretty standard arrangement in an inductor based wah pedal circuit.
Here is the Shin Ei WT-1 "Companion" circuit. Notice how the circuit differs in the area where the "inductor" is. First of all, it's not really an inductor - it's a small transformer that is wound with only a primary winding. So it does function as a inductor as it provides the inductance (750mH) necessary to make it a resonant circuit, but does it do anything unique? The inductor is grounded through a 10uF cap and a 100K resistor. The feedback arrangement is also slightly different - the feedback "sweep" cap is 0.033uF instead of the typical 0.01uF. This will effectively make it more like a Bass Crybaby. Finally the input cap, at 10uF, is quite large. So it's not exactly just another straight copy of the classic wah pedal circuit.
Curve tracer testing is an interesting way to determine the characteristics of an inductor. When looking at the "inductor" used in the Shin Ei WT-1, shown on the left, it shows the same characteristics as most other inductors - no saturation evident, or any other anomolies. On the right is the Dunlop Red Fasel, which does show some saturation at certain frequencies. The original Fasel used in my Italian Vox wah from the 60s exhibits the same characteristics. The inductor in the Dunlop GCB-95 circuit surprisingly exhibits the same characteristics as the red Fasel. The inductors in the Schaller "Bow Wow Yoy Yoy" and the El-Rad inductor used in the Maestro Boomerang and other wah pedals of the same era look very similar to the curves observed by the Shin Ei "inductor". Inductors are generally designed specifically NOT to saturate, but in the case of the Fasel it appears that it was a happy accident that arose perhaps from cost cutting measures, or simply by accident.
So what else is unique about the Shin Ei WT-1 wah pedal? The one that is most evident is the range of frequencies where the resonant peaks occur. Below are some
comparisons of wah pedals that I possess and have tested.
Here is the Vox V846 and Dunlop GCB-95 circuits. The resonant peaks occur between approx. 440Hz and 2.3kHz. Notice how similar they are.
Here is the Maestro Boomerang BG-2. The circuit it utilizes is a bit different than the "Clyde McCoy" topology. According to LTSpice, the resonant peaks occur at a much higher frequency, which probably accounts for the much less "peaky" sound that it has.
Here is the Schaller "Bow Wow Yoy Yoy". This circuit is rather unique in the wah pedal world, and it utilizes two inductors of different values to achieve the "bow wow" and "yoy yoy" sounds. The resonant peaks occur in a similar frequency range as the Maestro Boomerang BG-2 but are not as spread out.
Finally, here is the Shin Ei WT-1 "Companion". Here is where the differences are pretty self-evident. The resonant peaks occur between 200Hz and 500Hz, squarely in the middle of "typical" guitar frequencies, which is much lower than any of the other ones we have looked at previously. This is likely what gives the Shin Ei its unique sound quality. This seems to be largely due to the size used for the "sweep cap" and possibly also due to the high inductance of the "inductor". No magical qualities to be found by using the lopsided transformer in lieu of a bonified inductor.
I hope you found this quick study to be interesting. I'm no great expert at using LTSpice, but it can be a useful tool for circuit analysis, and it this case it explains some of the reasons why this Shin Ei wah pedal sounds so different than the rest of the wah pedal pack.