How to Simulate Guitar Pedal Circuits
Simulate a guitar pedal circuit before you build: how to set up the input source, which analyses to run (DC, AC, transient), and what each one tells you.
Simulate first, solder second
Simulation isn't a perfect replacement for breadboarding — real germanium transistors are still going to surprise you, and no SPICE model captures the full weirdness of a 60-year-old Fuzz Face. But it gets you maybe 90% of the way there in 1% of the time. Bias mistakes, obvious frequency-response problems, gain stages that won't behave — you catch all of that in minutes instead of evenings.
The workflow is: draw it, simulate it, fix what's obviously broken, then breadboard it knowing the basic topology works.
Set up a realistic input
Don't simulate from a perfect voltage source. A guitar pickup has roughly 5–10 kΩ of impedance and outputs around 100 mV peak for normal playing (more for a hot humbucker, less for single coils).
A solid generic 'guitar source' for simulation:
- AC sine source, 100 mV peak, 1 kHz (for AC sweeps make this an AC source with magnitude 1).
- 10 kΩ resistor in series with the source — this is your pickup impedance.
- Connect to the pedal's input as if from a cable.
For pedals that interact with input impedance (Fuzz Face most famously), this matters a lot. Skip it and your simulation will sound nothing like the real pedal.
The three analyses you'll run constantly
Each tells you something different. Run them in this order:
- DC operating point first. This tells you every node's idle voltage. Look at op-amp outputs (should sit at Vref ≈ 4.5 V), transistor collectors (should sit near half-supply), and the Vref divider itself. If anything is wildly off, fix it before doing anything else.
- AC sweep next. Plot the frequency response from 10 Hz to 20 kHz. This is how you see what your tone stack actually does, where you have unwanted high-frequency rolloff, and whether bass is being lost in a coupling cap.
- Transient analysis last. Apply a small sine wave (or guitar-like input) and plot the output waveform in time. This is where you see clipping shapes, asymmetry, and whether the circuit oscillates.
Knobs and sweeps
The killer feature of simulation is parameter sweeps. Don't just simulate one value of a resistor — sweep it. Pedal Bench lets you mark any component as 'sweep R from 1 k to 100 k in 10 steps' and overlay all ten plots on the same axes.
Sweep the feedback resistor in a Tube Screamer to see how the gain curve changes. Sweep the tone-pot position to see the EQ at every setting. Sweep a clipping diode's model between silicon and germanium to see how the waveform corner changes. You can answer 'what if?' questions in seconds.
What simulation gets wrong
Worth knowing the limits. SPICE models for op-amps are good but not perfect — some chips (notably old 741s and 4558s) have a 'character' that doesn't fully show up in simulation. Germanium transistor models vary wildly in accuracy, and leakage current is hard to model. Saturation behavior at extreme drive levels is approximate.
So use simulation to design and to catch problems, but always confirm on a breadboard before you etch a PCB. The simulator gets you to a circuit worth breadboarding; the breadboard tells you if it actually sounds good.