Inductor-less Wah/Crying Baby/Quack Effect Using Operational Amplifier

Figure 1. Wah Pedal Circuit Schematic Diagram

A type of guitar effect pedal that produce “wah”- or “quack”-like sound, which is similar to the crying sound of  a baby is called  wah-wah pedal, or just  wah pedal. The effect is produced by filtering the musical instrument through a band-pass filter, which the center of its peak frequency is swept from low to high, mimicking  audio tonal transition of human voice pronouncing “wah” or “quack”. The control of the center frequency is done by a potentiometer, which is connected to a tough foot pedal. Some publicly available wah (crying baby/quack) effect circuit design from old circuit is designed using relatively big inductor in their design. Some “auto-wah” pedal circuit are using op-amp and doesn’t use any inductor, and use only resistors and capacitors as the frequency-determining components. Here I present a modification of an “auto-wah” circuit, from a popular publicly available schematic diagram labelled as Electro Harmonix Dr. Q circuit, to become a “manual-wah” pedal. Figure 1 shows the schematic diagram of the modified circuit. The modification is very simple, by removing the envelope detector circuitry and replacing the frequency-control-transistor  with a potentiometer. Note that the second op-amp is used only to stabilize the internal ground reference (symbolized with horizontal strips), which is actually the half point of the voltage between external ground (symbolized with vertical strips) and positive terminal of the power supply. See how it sounds like in the video below: