Open Source › Hardware

The Reference Box

Six small circuits in one box, selected one at a time by a rotary switch, chosen because each one's distortion follows from resistor arithmetic and Fourier theory: a divider, a one-pole low-pass, a symmetric and an asymmetric diode clipper that differ by one diode, a precision full-wave rectifier, and a unity follower driven into class-B crossover. It exists to give the instrument known answers. It is measured, never played — nothing about it was chosen for how it sounds, and it is not a pedal or a product. The published Reference Box library was made with this box, and each measurement on that page sits beside the number that was computed first. Build one, measure it, and the check is yours to repeat.

The finished Reference Box from above: a bare aluminium enclosure labelled PEDALSCOPE REFERENCE BOX, with an ON/OFF toggle, a green power LED and a black rotary knob with positions P1 to P6. — The box as built: power switch, power LED, and the six-position selector.

The Reference Box opened: two 9 V lithium batteries across the top of the enclosure, the switch and jack harness in multicoloured wire, and the perfboard with its two op-amp sockets mounted on the lid. — Inside: two 9 V lithium batteries for the split supply, the harness, and the perfboard on the lid.

An oscilloscope capture of P5: a sine wave on the top trace, and below it the full-wave-rectified output, a train of rounded humps meeting in sharp cusps, repeating at twice the input's frequency. — P5, captured 10 August 2026: a sine in (top), and its full-wave-rectified output (bottom), which repeats at twice the input's frequency.

An oscilloscope capture of P6 at 10 kHz: a sine wave on the top trace, and below it an output that follows the sine except for a short flat step where it crosses zero, the step marked by two cursors 2.80 microseconds apart. — P6, captured 10 August 2026 with the 1 kΩ load the box is built with: a sine in (top), and an output (bottom) with a short flat step where it crosses zero; the cursors mark its width, 2.80 µs. The input is at 10 kHz on purpose. The step lasts a fixed time, so it is a visible part of the cycle only at high frequencies; at a guitar note's frequency it is far too small a part of a cycle to see on a scope. PedalScope still measures the harmonic distortion it produces there.

Reference Box signal path: the input jack, the two-pole rotary switch, the six positions P1 to P6 with their as-built predictions, the output jack, the split-rail power and the build order. — The signal path and the six positions, each annotated with its as-built prediction. Open full size.

Schematics of P5, the precision full-wave rectifier, and P6, the crossover stage, with measured part tags, the meter checks and the bench note on P6's input bias offset. — P5 and P6 in full — the two op-amp positions, with the parts tagged by their measured values and the meter checks to run before the chips go in. Open full size.

Perfboard layout of the Reference Box: component side and solder side, the off-board harness table and the meter pre-checks. — The perfboard layout, both sides, with the harness table and the meter pre-checks. Open full size.

Power wiring of the Reference Box: two 9 V batteries in series with the centre tap as ground, a double-pole switch breaking both rails, and the indicator LED. — The split-rail battery supply. Open full size.

The files

Build plan	build-plan.md — the build narrative, position by position, with the as-built predictions and what each one validates. Read it first.
Bills of materials	bom-abra.md (a shopping list) and bom-mouser.csv (a distributor-import shape).
Photos	The four images above, in photos/: the box from above and opened, and the two oscilloscope captures.
Schematics	The four SVGs above, and the scripts they are generated from: diagram.gen.py, p5-p6-schematic.gen.py, board-layout.gen.py, power-wiring.gen.py. The schematics are generated, never hand-edited; a change is a change to the script and a rerun with a stock Python 3.
Everything, zipped	PedalScope-Reference-Box-plans-8f30a0444715.zip — every file in this table and the photos above, plus the licence and notice. SHA-256 `e6df1d73614559b5465db4643896597366afd3c1b84b0f588afc884cc7967884`.
Revision	`8f30a0444715` — a content hash of the published files (full value `8f30a0444715948ab0dbaf435410b66a5f9ad8c2b148a143f55508efe2b98fa6`). It changes only when one of the files does, so two downloads with the same revision are the same plans.
In the repository	hardware/reference-box/ — the same files, with the commit that published them.
Licence	CERN-OHL-P-2.0, the CERN Open Hardware Licence, permissive variant — build it, change it, sell what you build, with attribution. The notice carries the copyright line. The photos are covered as documentation.

The plans describe the box as built — the one that made the published library — and its schematics carry the as-built predictions. The bench records that measured it are not published: they are the library's provenance and travel with it.

The PedalScope name and mark are not part of the grant. Building the box, reproducing a method or redistributing a library does not make the result a PedalScope product, and none of the licences above conveys a right to the name.