EarthWorm OPA

Virtual Analog model of guitar distortion pedal with selectable Op Amp models

EarthWorm OPA is a virtual analog model of a guitar distortion circuit with selectable Operational Amplifiers. The four available Op Amps are selected from well-known guitar pedal circuits and give the user a variety of sounds to choose from.

The overall topology of the EarthWorm OPA is based on the ProCo RAT distortion pedal. With the exception of a simplified power supply, the entire circuit has been emulated on a component by component basis. Building off of my own research into Op Amp emulation, this plugin models the slew rate, gain bandwidth product and clipping behavior of the four included Op Amps. The RAT topology was chosen because the available gain is more than enough to push the Op Amp and expose the non-linearites of each model.

Additionally, a "Symmetry" parameter has been added to alter the biasing of the signal entering the gain circuit. This allows the user to adjust the balance between odd and even harmonics. Each Op Amp model responds a bit differently to the biasing control.

At higher frequencies (above ~2k), some of the Op Amps can begin to display instability and generate digital artifacts. The actual culprit is the diode emulation, which can struggle to keep up with high frequencies at the amplitudes the current model creates. This mirrors the physical phenomena my Op Amp model is attempting to capture. As such, while digital distortion may not be to everyone's taste, I included this behavior, rather than altering the clipping portion of the circuit.