Clippers

Reading the curve

The transfer curve is the output against the input, held at each voltage in turn. A straight line means no clipping. Where the curve bends over, the diodes are conducting, and the flatter it gets, the harder the clipping. The dashed line is the clipper with the diodes taken out, and the circles mark where the output falls 1 dB under it, which is where clipping starts. The blue stretch is the part of the curve the sine moves over.

Each diode starts to conduct at its knee, about 0.6 V for silicon, 0.2 to 0.3 V for germanium and Schottky diodes, 1.6 to 2 V for a red LED and more for green and blue. Diodes in series add their knees. The dashed lines across the curve show the voltage where each string passes 1 mA. Germanium and LED knees are softer, so the curve bends over a wider range of input.

Uneven pairs. With different diodes on each half, one side clips at a higher voltage than the other. The wave turns lopsided and even harmonics appear (orange), which usually sounds rounder. With a coupling cap ahead of the diodes, the cap charges up until the current through it averages out, which moves the whole wave a little toward the side that clips later.

To ground. The series resistor and the diodes make a divider that changes with the level. Below the knee the output follows the input. Above it the diodes pull more current and hold the output near their knee, so the clipping is hard. A larger resistor lets less current through the diodes, so the output levels off slightly lower. A cap across the diodes rounds the edges. A germanium pair passes some current even at rest, which trims the level slightly before it clips.

In op-amp feedback. Below the knee the stage has its full gain, 1 + Rf / Rg. Once the diodes conduct, they sit across Rf and the gain falls toward 1, so the output can’t get much more than a diode drop above the input. The clipping is softer than diodes to ground, and the clean part of the signal still comes through on top. With Cg in the gain leg, the gain falls toward 1 below its corner, so the low end clips least.

In series. The diodes pass nothing until the input is over their knee. Small signals vanish, and every cycle loses a slice around zero.

How this tool works. Every curve comes from the bench’s engine and its diode law, an exponential with each preset’s saturation current and ideality factor, typical of the part, not measured. The MOSFETs are wired gate to drain, as in some pedals, and fitted as diodes. We solve the clipper at each input voltage for the transfer curve, with the coupling cap and Cg holding the voltage they have at rest and the caps across left out. That’s the curve in the middle of the band. The sine runs 60 ms (at least 12 cycles) through the whole circuit with its caps, so they settle, and the last four cycles are kept. In op-amp feedback the sine rides on a stiff half-supply reference, and the op-amp is the bench’s model, with its gain-bandwidth, slew rate and output limits. For the gain stage around the diodes, try the transistor and op-amp stage tool.

Spot an error, or want a circuit added? Email info@swellamps.com. A schematic or a clear photo of one helps. Anything you send may be used to improve these models and our desktop app.

© 2026 Swell Amplification LLC. All rights reserved. The simulator and tools, their circuit models, drawings, presets and text are the property of Swell Amplification LLC and may not be copied, reproduced, redistributed or used to build another product without written permission. The original circuits are historical designs documented in public sources, and no claim is made to them. Everything here is provided as is, without warranty of any kind, for study and personal use. Results are simulations of typical parts, not measurements of any real unit. Don’t rely on them for the safety of any build or repair. Tube amplifiers carry lethal voltages.

Product and company names are trademarks of their owners. They are used here only to identify the circuits these models are based on; this site is not affiliated with or endorsed by any of them.

From Swell Amplification, swellamps.com.