Clipping Diodes 101
How clipping diodes shape distortion: silicon vs germanium vs LED vs MOSFET, hard vs soft clipping, and how to pick the right Vf for the sound you want.
Distortion is just clipped sine waves
Your guitar's signal is a smooth-ish wave wobbling between, say, ±0.5 V. A diode conducts as soon as the voltage across it crosses its forward voltage (Vf). Wire a pair of diodes in opposite directions across the signal, and they chop the peaks flat the moment the wave gets too tall.
Flattened peaks are mathematically equivalent to adding a stack of new harmonics — and harmonics are what your ear hears as 'distortion,' 'fuzz,' or 'crunch.' That's the whole trick. Everything else is flavor.
The four diodes you'll actually use
There are thousands of diodes in the world, but pedal builders mostly reach for four:
- Silicon (1N4148, 1N914): Vf ≈ 0.6 V. Bright, aggressive, the sound of the Boss DS-1 and the ProCo Rat. Cheap, indestructible.
- Germanium (1N34A, OA90): Vf ≈ 0.3 V. Lower headroom = more compressed and 'fuzzy.' Warm but noisier and temperature-sensitive.
- LEDs: Vf ≈ 1.7 V (red), higher for blue. Loads more headroom — the wave goes louder before it clips. Open, dynamic, touch-responsive. Used in the Marshall Bluesbreaker and many modern ODs.
- MOSFETs (gate tied to drain): Smooth, asymmetric, vaguely tube-like. The OCD's signature.
Hard clipping vs soft clipping
Where you put the diodes matters as much as which ones you pick.
Hard clipping shunts diodes from the signal node straight to ground (or the rail). The waveform smashes into a wall the moment it hits Vf — sharp corners, lots of odd harmonics, that fizzy buzzsaw thing the DS-1 and Rat do.
Soft clipping puts the diodes inside an op-amp's feedback loop. The op-amp tries to push the output past the diode threshold but the diodes 'leak' more and more current the harder it pushes, so the corners round off gradually. Result: a smoother, more touch-sensitive distortion. This is the Tube Screamer / Klon family.
Asymmetric clipping
Use, say, two diodes one way and a single diode the other way and the wave clips at +0.6 V but +1.2 V — chopped on one half and taller on the other. That asymmetry is rich in even-order harmonics, which a lot of players describe as 'tubey.' The Klon Centaur is the famous example; one side has two silicon diodes, the other has one germanium.
Try it without unsoldering anything
Pick a Tube Screamer in the Library, jump into the schematic, double-click the clipping pair, and swap them for LEDs or germaniums in two clicks. Hit Simulate and look at the output waveform. You'll see the corners change shape before you've heard a single note. That's a lot cheaper than ordering four diode varieties from Mouser.