Wah Pedal Guitar Effect Circuit

This is a design project for guitar effect called Wah Pedal Effect. Wah Pedal has been desing around since at least the early 60’s. The Vox is one of brand guitar effect which  is give beautiful sound and simple, only a couple of transistors and an inductor. The really pertinent question that had puzzled many people - including especially me - is how do you get a moving resonant frequency out of a fixed inductor and a fixed capacitor? How does that silly two-transistor Wah Pedal circuit get a moving band pass out of a circuit that changes neither the inductor value or the capacitor, but only what amounts to a volume pot? This  is the figure of the circuit;

It took me a while, but the trick is - the wah pot, the second transistor and the fixed capacitor implement an electronically variable capacitor. The inductor is and remains fixed, and the capacitor is electronically varied and so the circuit has a variable tuning LC filter to cause the effect. To get down to how this works we'll disassemble a two-transistor wah of the classic Vox style and learn how everything in there works, and have a good time with the circuits on the way. This is how the effect work. The first transistor is a straight forward feedback amplifier. Ignore for t7he moment the parts separated by the dotted lines. These are separated from the first transistor by capacitors and so cannot participate in DC biasing. The transistor is biased into linear amplification by the voltage on its own collector which feeds current to the 470K resistor, some of which is shunted to ground by the 82K resistor. The rest of the current through the 470K goes ti the base the inductor and the 33K resistor which parallels it and the 1500 ohm resistor leading to the base. The inductor’s DC resistance is quite low compared to any of the other resistor (typically 40-75 ohm), so the base current is determined primarily by the 470K and 82K resistor and the 1500 ohm resistor. In fact, the 1500 ohm resistor is small compared to the 470K, so we will ignore it for a moment. We can ignore the inductor, 33K and 1,5K resistor. This is one form the classical voltage feedback biasing arrangement, and the values are chosen to give a reasonable llinear range of swing on the collector.

Source: http://www.geofex.com/article_folders/wahpedl/wahped.htm