Anthracite

A MIDI-controllable analogue fuzz pedal.

Fuzz pedals are awkward to live with: the sound changes when you swap guitars, a wah in front of one loses its range, and few switchers can drive one. Most of that traces back to a single cause — the input impedance of a fuzz is low, and it is fixed. Anthracite lets you choose the load your pickup sees, and keeps the whole signal path analogue: it is switched by analogue switches rather than mechanical contacts, and the microcontroller only moves those switches and never touches the audio.

Designed and built by Trope (山本回路設計).

Specifications

Power	9 V DC, centre-negative, 5.5 / 2.1 mm barrel. No battery compartment
Operating voltage	8.4 – 11.5 V
Recommended operating voltage	8.6 – 10.0 V
Current draw	About 70 mA at 9 V with the effect on (measured 67 – 74 mA depending on settings; about 4 mA less in bypass)
Adapter rating	200 mA or more
Input impedance	About 13 kΩ (Single) / 24 kΩ (Hum) / 44 kΩ (Wah) at 1 kHz, effect on
Input impedance, bypassed	About 1 MΩ from 100 Hz to 1 kHz, measured
Output impedance	About 1 kΩ at 1 kHz
Bypass	Buffered. Switched electronically — no mechanical contacts in the audio path
MIDI	3.5 mm TRS, MIDI Type-A, 31250 baud. MIDI IN, and a MIDI OUT (Thru) jack that doubles as an external-switch input
Microcontroller	STM32F030C8T6 (Cortex-M0, 48 MHz, LQFP48)
Enclosure	73.5 (W) × 57.5 (H) × 128 (D) mm including knobs and jacks, 268 g

The pedal monitors its supply and flashes the LED if the voltage drops below about 8 V or rises above about 12 V. The exact trip points vary slightly from unit to unit, but the warning does not appear inside the operating voltage range. Do not run the pedal outside that range even if no warning is shown.

The effect-on input impedance figures come from a SPICE model of the input circuit and agree with measurements on a v1.0 unit. The impedance stays roughly flat at low frequencies (about 24 / 34 / 55 kΩ at 20 Hz) and falls gradually towards high frequencies. That roll-off is deliberate: a heavier load pulls the pickup's resonant peak lower, and the Input switch selects between three loads.

Boards

Board	Size	Layers	Copper
Top (analogue)	60 × 102 mm	4	35 µm (1 oz)
Bottom (control)	60 × 73.5 mm	4	35 µm (1 oz)
Footswitch	14 × 16.15 mm	2	—

All boards are 1.6 mm FR-4.

Controls

Four potentiometers and three three-position toggles.

Control	Range
Volume	50 kΩ, log (A taper)
Fuzz	10 kΩ, reverse log (C taper), wired as a variable resistor. Roughly 39 dB to 65 dB of small-signal gain in the fuzz stage
Bass	20 kΩ, linear (B taper). Low shelf: up to about −25 dB of cut and +13 dB of boost at 20 Hz. The half-way point moves between about 170 Hz and 390 Hz with the knob
Treble	20 kΩ, linear (B taper). High shelf: up to about −26 dB of cut and +14 dB of boost at 10 kHz. The half-way point moves between about 700 Hz and 1.7 kHz, so it also acts on the midrange around 1 kHz

Toggle positions are listed from top to bottom.

Toggle	Positions	What it changes
Bias	Skew / Offset / Center	Operating point of the output-side fuzz transistor (3:1, 2:1, 1:1). More asymmetry means more even-order harmonics and less gain. Second harmonic moves from −16.7 dBc (Center) to −9.5 dBc (Skew) at 400 Hz / 0.1 Vpp, Fuzz 50 %. With DIP-SW2 on, the toggle selects Offset / Skew / X-Skew (4:1) instead
Response	Gradual / Medium / Steep	Series resistance at the fuzz input, which sets how abruptly the pedal cleans up as you roll back the guitar volume. THD at 400 Hz / 1 Vpp, Fuzz 50 %: 65.6 % / 59.7 % / 49.3 %
Input	Wah / Hum / Single	Input filter, which sets the loading your pickup sees (see the impedance figures above)

MIDI

The pedal receives; it never sends control messages of its own. MIDI IN is active whichever way DIP-SW3 (MIDI mode / external-switch mode) is set.

CC	Function	Values
81	Bypass / engage (channel 12, with DIP-SW4 off as shipped)	≥ 64 engages, < 64 bypasses
83	Input	0 = return to the physical toggle · 1–31 Single · 32–63 Hum · 64–95 Wah · 96–127 All-OFF (input filter disconnected)
84	Response	0 = return to the toggle · 1–31 Steep · 32–63 Medium · 64–95 Gradual · 96–127 no change
85	Bias	0 = return to the toggle · 1–31 Center · 32–63 Offset · 64–95 Skew · 96–127 X-Skew

CC 83/84/85 are received on the same channel as the bypass / engage message. Sending 0 to any of them hands control back to the physical toggle, so a MIDI override never becomes something you cannot undo from the panel.

MIDI-Learn stores a channel and CC number for the bypass function in EEPROM; with DIP-SW4 on, the pedal uses them instead of channel 12 / CC 81. Units ship with channel 6 / CC 1 already stored. Many MIDI foot controllers send their expression pedal on CC 1, so with DIP-SW4 on, an expression pedal on channel 6 will switch the effect as it crosses value 64; learn a different CC or change the controller's CC. If nothing is stored, DIP-SW4 on disables MIDI control entirely rather than falling back to channel 12 / CC 81. Do not learn CC 83, 84 or 85. Active Sensing (0xFE) is emitted about every 250 ms, and received messages are passed through to MIDI OUT unmodified, both only in MIDI mode (DIP-SW3 on, as shipped). With DIP-SW3 off, the MIDI OUT jack becomes an external footswitch input.

What is in this repository

hardware/     KiCad projects for the three circuit boards (top, bottom and
              footswitch) and the front-panel artwork, with the symbol and
              footprint libraries they use
enclosure/    Enclosure 3D model (STEP), hole-layout and assembly drawings
              (PDF), front-panel artwork for laser marking (DXF, SVG, PNG).
              The Fusion 360 source is planned
firmware/     STM32 firmware, CMake build. For now only the build and
              flashing guide; the source is still to be added
docs/         Schematics (PDF, plus a PNG of each sheet), bills of materials
              (CSV), 3D models of the assembled boards (STEP)

Manufacturing data (Gerbers, pick-and-place, JLCPCB-format BOM) will be attached to each release rather than kept in the tree, so that what you download always matches a specific version. No release has been published yet.

The user manual, measured data, and circuit explanations live at https://docs.trope-oshw.org — they are not duplicated here.

Building the boards

The KiCad projects are self-contained: every custom symbol and footprint is in the project's own libs/ directory, so you can clone this repository and open them without installing anything else.

Ordering parameters used for the production run will be recorded in ordering-notes.md attached to each release. They matter — a board made to different stackup or copper weight is not the same board.

Firmware

See firmware/README.md for how to build it and how to write it to a finished unit. You do not need to touch the firmware to use the pedal; the instructions are there because being able to replace the software on hardware you own is part of what this licence is for.

Licensing

What	Licence
Hardware (hardware/, enclosure/)	CERN-OHL-S-2.0
Firmware (firmware/)	GPL-3.0-or-later
Generated design documents (docs/: schematics, bills of materials, board 3D)	CERN-OHL-S-2.0
Text (this README, CHANGELOG, ERRATA, CONTRIBUTING)	CC-BY-SA-4.0

Full texts are in LICENSES/; per-file assignments are declared in REUSE.toml following the REUSE specification. See COPYRIGHT.txt for the notices.

Parts of the firmware are generated by STM32CubeMX or supplied by third parties and keep their own licences (ST HAL: BSD-3-Clause, ARM CMSIS: Apache-2.0). Likewise, some footprints in hardware/*/libs/ are derived from the KiCad standard libraries and stay under CC-BY-SA-4.0 with the KiCad libraries exception. The symbol libraries mix symbols derived the same way with symbols drawn for this project, so both licences apply to them; REUSE.toml lists the files.

Trademark

The design files in this repository are open source under the licences above. The Trope and 山本回路設計 brand names, the Trope logo, and the product name "Anthracite" are not. They are trademarks of 山本回路設計 and are not licensed to you by any of the licences here. See also section 8.2 of the CERN-OHL-S v2.

You are free to build, modify, and sell hardware derived from these files. When you do, please give it your own name and your own branding. A modified version could be called "Foobar Audio — Charcoal Fuzz"; it should not be called "Anthracite" or "Trope Anthracite".

You may state truthfully that your product is based on the Anthracite design by Trope. You may not use the Trope logo, or a name so close to Trope or Anthracite that a buyer could confuse the two.

All Trope branding has been removed from the published files, both the panel artwork and the circuit-board silkscreen, so hardware made from these files carries no Trope marks by default. Please keep it that way. Instead of a product name, the top and bottom boards carry the board version (V1.0), the licence (CERN-OHL-S-2.0) and where the source lives (github.com/trope-oshw/anthracite).

Boards from the first production run carry Anthracite V1.0 on the silkscreen instead. That text is the only difference: electrically they are the same as the published boards. Units built by Trope after that run use the published board files as they are.

Questions, or a request to use the marks: trope.oshw@gmail.com — ask first and the answer is almost always yes.

Versions

The product and the boards carry a vMAJOR.MINOR number, silkscreened on the PCB. The firmware carries its own semantic version. They move independently: fixing a firmware bug does not change the boards, so the board version stays put.

See CHANGELOG.md for what changed in each release, and ERRATA.md for problems found in hardware that has already been built. If your unit behaves differently from this documentation, check the errata first.

Buying one

Units are built and tested by Trope, and every one ships with its own test report. Price and availability are not settled yet; when they are, they will be announced at https://trope-oshw.org/anthracite/#buy, together with the cost breakdown and the formula the price is derived from. Until then, ask at trope.oshw@gmail.com.

Buying one is what pays for the next design being opened.

Contributing

Issues are welcome; pull requests to the hardware are not, for reasons explained in CONTRIBUTING.md. Firmware and documentation PRs are welcome.

Disclaimer

This design is distributed as-is, with no warranty of any kind, express or implied, including merchantability and fitness for a particular purpose. See sections 6.1 and 6.2 of the CERN-OHL-S v2 for the full disclaimer.

If you build, modify, or sell hardware based on these files, you are the manufacturer of that hardware. You alone are responsible for its safety and for its compliance with every regulation that applies where you place it on the market. Any certification, testing, or conformity marking obtained by 山本回路設計 applies only to units built and sold by 山本回路設計, and does not transfer to your build.

Use a regulated, correctly-polarised 9 V DC supply. Incorrect power can damage the pedal and whatever is connected to it.