Circuit bench
A pedal circuit solved part by part, the way a SPICE simulator does it, while you listen and watch. Turn the knobs, change any part, or tap a wire to put a probe on it.
Peak voltage from the pickup for each note. A single coil strummed hard makes a few hundred millivolts.
Input
pickup, unloadedat the pedal input
Output
at the amp input
Probe
Spectrum
Frequency response
The ink curve is the small-signal response from the generator to the probe: every tube, transistor and diode linearised at its bias point, which is what a very quiet signal sees. It follows the knobs and the probe as you move them. The blue curve is measured with pink noise at the level you set, the way an audio analyser does it. When stages start to clip, the blue curve pulls away from the ink one and the grey coherence line (right-hand scale) drops below 1: at those frequencies the circuit is no longer acting like a filter, and no single curve describes it. Both use the bench generator and its output resistance, whatever the Source switch is set to.
Output
Bias
What this means at the pedal.
How it works. Every part becomes an equation, and the circuit is solved many times per sample, 192,000 times a second while you drag and 384,000 once you let go, so the distortion doesn't fold its own harmonics back into the audio. Transistors use the Ebers-Moll model with gain, leakage, Early effect, base resistance, junction capacitance, and temperature. Their charge storage is set from the bias point, which is close for small signals and approximate while they switch hard. The transistor presets are representative values, not measurements of particular parts. Tubes use the Koren model with each preset fitted to its datasheet operating point, plus a grid-current diode; they are representative of the type, not measurements of any tube. Capacitors are ideal, and the battery or HT supply is a voltage behind a resistance.
Part of the reference section at swellamps.com.