This sound effects circuit is designed to work as a signal distorter. If used with an electric guitar, it allows the production of special sound effects. The signal distorsion is done by substracting and mixing the normally clipped input signal with a processed noise signals.
The gain is dependent on the setting of P1 and the chosen effect degree. It is between 10dB and 30dB.
Opamp A1 is an impedance converter and signal buffer. Opamp A3 amplifies the signal constantly by a factor or 10. D1 and D2 clip the signal which is then fed to A4. The noise signal comes from the junction of R9 and R10. Opamp A7 is another impedance converter. A8 is used to drive a VU meter.
Sound effect generator usage
Set the input level with P1 until the VU meter points between 40 and 75% when a guitar string is struck. Select the distorsion level with P2 from 0% to 100%. You can now select the waveform with P3.