Peppermill

The Peppermill is a DIY guitar overdrive circuit from Runoffgroove, the collective known for free, openly published amp-in-a-box designs like the Supreaux. Where most overdrive pedals clip the signal with diodes or an op-amp running into the rails, the Peppermill gets its grind from field-effect transistors arranged as a MOSFET driving a JFET, a topology chosen to behave more like a tube gain stage than a typical pedal stage¹².

The circuit has no commercial pedal form. It lives as a published project that home builders put on stripboard, perfboard or a third-party PCB, the best known being GuitarPCB's Blues Power v2, which the maker describes as "The Perfect Peppermill" with added clipping options³.

Where the Peppermill came from

Runoffgroove (often written R.O.G. or ROG) publishes complete schematics, part lists and build notes for its circuits free of charge, and its designs circulate widely in the DIY community. GuitarPCB, a PCB vendor, credits Runoffgroove directly in its own documentation: the Red Special v4 project notes thank "Runoffgroove" for original circuit design, and the Blues Power v2 page states plainly that "The circuit is based off the R.O.G. Peppermill but with additional clipping options to allow the circuit to exhibit more" tonal range³⁴.

So the family tree runs one way: the Peppermill is the original design, and the Blues Power v2 is a derivative based on it that adds additional clipping options. That matters to builders comparing boards, because the stock Peppermill and the modded version do not behave identically³.

How it works: a MOSFET driving a JFET

The Peppermill's defining feature is its gain stage: a MOSFET (metal-oxide-semiconductor field-effect transistor) driving a JFET (junction field-effect transistor). Both transistor families turn a voltage at their gate into a change in current flow, which is closer to how a triode tube amplifies than the voltage-to-voltage behavior of an op-amp. That is the whole reason FETs appear so often in DIY amp-in-a-box projects; runoffgroove chose them for the Peppermill for their tube-like sounds¹².

The character difference between the two device types is real and documented elsewhere in audio gear. Pendulum Audio, whose PL-2 compressor lets the user switch between JFET and MOSFET limiting, describes the distinction in its manual⁵:

In a compressor that difference shows up as grab; in an overdrive it shows up as how the stage saturates as you push it. Other makers have made the same choice for the same reason. General Guitar Gadgets' Improved EA Tremolo swaps the runoffgroove.com MOSFET first stage for a JFET because "The JFET stage (first transistor stage in the circuit) we use sounds great"⁶, and Flower Pedals built the Cream side of its Lupine Dual Fuzz on a JFET transistor topology⁷. The Peppermill uses both devices in series, the MOSFET doing the driving and the JFET doing the amplifying, to get tube-like soft saturation instead of the hard clipping of a diode-to-ground design.

For contrast, Warm Audio describes its ODD Box V1 as "a hard-clipping, op-amp-based overdrive inspired by an iconic pedalboard staple"⁸. That is the conventional approach: an op-amp with diodes that slice the waveform's peaks off abruptly. The Peppermill's FET stages instead round the waveform over as they run out of headroom, which is the mechanism behind its amp-like reputation.

The sound: a cranked Fender-style overdrive

The Peppermill is voiced as a cranked Fender-style overdrive, the sound of a blackface or silverface amp pushed hard rather than a British crunch box. Fender's own take on that territory, the Competition Drive, is described in RockBoard's PedalPedia as creating "a warm overdrive reminiscent of classic '60s and '70s rock and blues tones" with "harmonically rich crunch and sustain"⁹. That is the general territory: warm, sustained, dynamic, not high-gain metal.

A useful point of comparison is Strymon's Iridium, whose Round Amp setting is "Based on* a Fender® Deluxe Reverb" (Normal channel)¹⁰. The Iridium is a full amp modeler: it reproduces the whole amp, its clean headroom and its breakup. The Peppermill takes the opposite approach, one gain stage that goes after the breakup character only, and it sits in front of whatever amp you already own. A player who wants the complete cranked-Deluxe experience in a box is looking at amp modeling; a player who wants a single Fender-flavored drive stage to push their existing rig is looking at a circuit like this.

Compared with the classic diode-clipping overdrives, the Tube Screamer family, the FET approach trades the midrange hump and compressed squeeze for a more open, touch-responsive breakup. Whether that is better is a matter of taste; it is simply a different clipping mechanism with a different feel.

Building one: parts, voltages and PCB layout

Because Runoffgroove publishes the circuit freely, the practical build path for most players is a third-party PCB. The main one is GuitarPCB's Blues Power v2, sold as "Blues Power, Modded Peppermill w/ Clipping Options"³.

What the Blues Power v2 adds over the stock circuit:

• Switchable clipping options, so the builder can select between clipping characters on the fly.
• LED clipping among the options, which GuitarPCB says provides "a much warmer tube-style tone"³.
• Everything else from the R.O.G. Peppermill design as published³.

Builders who prefer other formats are in the same ecosystem as the wider DIY world: vendors like Griffin Effects ship project files containing a single-sided PCB layout, stripboard and perfboard layouts, a schematic and a true-bypass wiring diagram for their circuits¹¹, and Runoffgroove's own project pages are the reference point for the schematic and original build notes.

Selling builds and licensing

Runoffgroove's circuits are published for personal building, but many builders eventually wonder about selling finished pedals. The answer depends on whose board you used. As an example of how these licenses work, Madbean's PepperSpray manual (a different circuit, based on the Harmonic Percolator) states:

That is a common pattern in the DIY world: small commercial use of purchased boards is allowed, reselling the boards or kitting them up is not. Before selling anything built from the Peppermill, read the license that comes with the specific PCB you built on, since each vendor sets its own terms.

Pedals on this page

Flower Pedals Lupine

Flower Pedals · Fuzz

$199 list price

Warm Audio ODD Box

Warm Audio · Overdrive

$129 list price

Strymon Iridium

Strymon · Speaker simulation

$399.99 list price

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