Upgrade: in my spare time (in the breaks & stops taken whilst driving across France ), I worked out what was wrong with the circuit. @hurz pointed out some major defects including the fact that the circuit first presented, attenuates the signal by 40db
NOTE: The current design does not suffer from this problem!
The current representation of the circuit does not solve all the problems which I believe is impossible with a single stage design but it does provide a nice tuneable tight band pass which elevates the input signal up to 10db above the zero volt line!
Having researched this, I see that almost all current thinking on this type of circuit moves away from the original single stage idea toward either op amps or Jfet inputs stages. This reflects the need to pump the volume up a little before hitting the lossy filter. Indeed a filter by it's nature is a lossy mechanism. Amplification is the only way around this.
In the future, I may design something using filters in the feedback path of op amps which is quite a powerful mechanism and although I do not like the sound of op amps, I will admit they have their place in audio circuitry!
I have played around with the capacitor and resistor values so as to provide an effect that sounds great and is easy to build. (no huge rf receiving and therefor noisy inductor! )
The circuit is a tight bandpass filter with control over the centre frequency of the bandpass (wha ) and also control over the depth of the filtering action.
Mechanics: The simplest way to create a foot pedal wha control is to use a LDR (Light dependent resistor ) and a LED.
Additionally I would add a mosfet buffer on both the input and output sides of this circuit. This will prevent loading down the electric guitar and will allow the filter circuit to function better.
