JC-303

Unlock the iconic sound of Roland's TB-303 with JC-303, a faithful emulation of the legendary acid house and techno synth, available free of charge as a VST2, VST3, LV2, CLAP, and AU plugin for Linux, macOS, or Windows DAWs.

JC-303 UI

Key Features

Authentic TB-303 sound engine — based on Open303, over 15 years of community reverse engineering and development
AI Overdrive section — GuitarML-driven pedal and cabinet emulations using machine learning models
Devilfish-inspired mods — select parameters from Robin Whittle's legendary 1993 modification
Open-source — all DSP and GUI source code freely available under GPLv3
Cross-platform — Windows, macOS, and Linux
Plugin versatility — VST2, VST3, LV2, CLAP, and Audio Unit formats

Download

Visit JC-303 Download Page to grab yours.

Installation

Each platform ZIP contains folders per plugin format. Copy the desired format(s) to your system's standard plugin directory.

Windows

Format	Default Path
VST2 (.dll)	`C:\Program Files\VSTPlugins` or `C:\Program Files\Steinberg\VSTPlugins`
VST3 (.vst3)	`C:\Program Files\Common Files\VST3`
CLAP (.clap)	`C:\Program Files\Common Files\CLAP`
LV2 (.lv2)	`%APPDATA%\LV2` or `C:\Program Files\Common Files\LV2`

macOS

Format	Default Path
Audio Unit (.component)	`/Library/Audio/Plug-Ins/Components`
VST2 (.vst)	`/Library/Audio/Plug-Ins/VST`
VST3 (.vst3)	`/Library/Audio/Plug-Ins/VST3`
CLAP (.clap)	`/Library/Audio/Plug-Ins/CLAP`
LV2 (.lv2)	`/Library/Audio/Plug-Ins/LV2`

macOS De-Quarantine

macOS Gatekeeper blocks unsigned plugins. De-quarantine after copying:

# Audio Unit
sudo xattr -rd com.apple.quarantine /Library/Audio/Plug-Ins/Components/JC303.component

# VST3
sudo xattr -rd com.apple.quarantine /Library/Audio/Plug-Ins/VST3/JC303.vst3

# VST2
sudo xattr -rd com.apple.quarantine /Library/Audio/Plug-Ins/VST/JC303.vst

# CLAP
sudo xattr -rd com.apple.quarantine /Library/Audio/Plug-Ins/CLAP/JC303.clap

# LV2
sudo xattr -rd com.apple.quarantine /Library/Audio/Plug-Ins/LV2/JC303.lv2

Linux

Format	Default Path
VST2 (.so)	`~/.vst`, `/usr/lib/vst`
VST3 (.vst3)	`~/.vst3`, `/usr/lib/vst3`
LV2 (.lv2)	`~/.lv2`, `/usr/lib/lv2`
CLAP (.clap)	`~/.clap`, `/usr/lib/clap`

After copying, rescan plugins in your DAW.

Sound Engine

The JC-303 synthesis core is based on Open303, an open-source digital emulation of the TB-303 that began in 2009 when Robin Schmidt posted a call for collaborators on the KVR Audio forum.

Over 15 years, a community of TB-303 enthusiasts dove into oscilloscope measurements, circuit reverse engineering, and deep analysis of the original hardware's quirks and character. The result is one of the most accurate open-source TB-303 emulations available.

Step Sequencer

JC303 ships with a full acid step sequencer ported from Aciduino V2. It is a 16-step monophonic sequencer clocked sample-accurately from the audio thread, playing the built-in 303 engine and sending MIDI out to the host. Note length is the authentic 303 half-step (50%) gate, and slide/tie bridge correctly from the last step back to the first. The sequencer panel and manual below target the midilabv2 GUI theme.

Layout

Each of the 16 steps is a column of controls:

Row	Control	Meaning
`step`	LED	Selects the step to edit; also shows the playing step and the active pattern length
`rest`	toggle	Note ON (toggle up) or rest (toggle down)
`accent`	micro-toggle	Step plays with accent
`slide`	micro-toggle	Step slides into the next one
`tie`	micro-toggle	Step ties into the previous note

Below the grid are the transport buttons (PLAY/STOP, REC, REST, CLEAR, gen), the generative mod knobs, and the on-screen octave keyboard.

Editing steps

Left-click a step LED to select the step to edit. The selected step blinks; the step currently playing is highlighted. Clicking a step at or beyond the active pattern length snaps the selection to the last active step.
Note/rest toggle: each step is a note by default; toggle it off to make it a rest, on to re-activate it.
Accent / Slide / Tie are per-step toggles. A tie carries the previous note over, and handles wrap-around from the last step to the first.
The keyboard always shows the note of the selected step.

Setting the pattern length

Right-click any step LED to set the pattern length from 1 to 16 (the clicked step becomes the last step of the pattern). Steps beyond the length are kept but stay silent.
Length can also be set with the Length item in the SEQ menu page.

Seq keyboard

Click a key to set the note of the selected step.
Mouse wheel over the keyboard transposes the note of the selected step by one octave per notch (the octave of the keyboard follows the note).
In record mode the rec cursor is the target for both actions.

Record mode

Turn REC on to enter serial (step-by-step) entry. The record cursor starts at the currently selected step and advances one step per entered event, wrapping at the pattern length.

Keyboard: press a key to record its note and advance. Existing accent/slide/tie flags on the step are kept.
External MIDI: velocity 100 or above records the step with accent; striking a second note while one is still held records a slide (legato); tapping the sustain pedal (CC64, 127 on) records a rest.
REST button: in record mode it records a rest and advances the cursor; outside record mode it turns the selected step into a rest and moves the selection to the next step.
CLEAR button resets the whole track to rests (all accents, slides and ties removed).

Generative

The gen button generates a random pattern across all 16 steps using:

Knob	Range (default)	Meaning
`FILL`	0–100 (80)	% chance each step is ON (not a rest)
`ACC`	0–100 (50)	% chance of accent on an ON step
`SLIDE`	0–100 (30)	% chance of slide on an ON step
`TIE`	0–100 (100)	% chance of tie on a rest step
`TONES`	1–12 (4)	Number of pitch classes to snap to (1 = single note)
`LOW`	0–127 (20)	Lowest MIDI note in the random pitch range
`RANGE`	0–127 (48)	Range above `LOW` of the random pitches

The sequencer is briefly muted while a pattern is generated so no stray notes play. Generating keeps the pattern length intact but overwrites the steps within it.

Transport and clock

PLAY/STOP starts and stops the sequencer.
The SEQ menu page in the LCD exposes:
- Length — pattern length (1–16).
- Tempo — internal BPM (20–300); active only in Internal sync mode.
- Sync Mode — Internal (free-running at Tempo), Host (follows the DAW transport position and tempo), or Midi Clock (follows external MIDI Start/Stop/Clock messages).
- Start Mode — Transport (starts on host transport or MIDI Start) or Note Trigger (starts automatically on the first incoming note, even while stopped).

AI Overdrive Section

JC-303 integrates GuitarML technology, which uses machine learning to capture the sound of real overdrive pedals and guitar cabinets. The trained models recreate the response of actual analog hardware in real time.

Community-driven project with a growing library of models
Sheer unlimited tonal possibilities beyond the original TB-303

Devilfish-Inspired Mods

Created by Robin Whittle in 1993, the Devil Fish modification is one of the most sought-after third-party modifications of the original TB-303. JC-303 brings select Devil Fish parameters into the plugin, expanding the sonic palette beyond the stock 303 architecture.

Plugin Formats

Format	Windows	macOS	Linux
VST2	✅ (self-build)	✅ (self-build)	✅ (self-build)
VST3	✅	✅	✅
LV2	✅	✅	✅
CLAP	✅	✅	✅
Audio Unit	—	✅	—

VST2 binaries require a Steinberg VST2 SDK license and must be compiled from source.

Building from Source

JC-303 uses CMake and JUCE. Clone the repository and generate build files for your platform.

Dependencies

Ubuntu / Debian:

sudo apt install build-essential gcc cmake libx11-dev libxrandr-dev libxinerama-dev libxcursor-dev libfreetype6-dev libasound2-dev

macOS: Xcode command line tools

Windows: Visual Studio 2022

Generate Build Files

git clone https://github.com/midilab/jc303.git
cd jc303

macOS (Universal binary):

cmake -B build -G Xcode -D CMAKE_OSX_ARCHITECTURES="arm64;x86_64" -D CMAKE_OSX_DEPLOYMENT_TARGET=10.13

Windows:

cmake -B build -G "Visual Studio 17" -A x64

Linux:

cmake -B build

Compile

cmake --build build --config Release

GUI Themes

Choose a theme at CMake configuration time:

cmake -B build -D GUI=midilab

Available themes: amadeusp (default), midilab

VST2

Copy the vstsdk2.4/ folder to the project root before running CMake to enable VST2 builds.

Roadmap

✅ Binary releases for macOS, Windows, Linux
✅ Graphical user interface
✅ Internal engine tuning parameters (Devilfish inspired)
✅ Overdrive section
✅ Step sequencer
🔄 Step Sequencer Patterns
🔄 Preset support

Community

JC-303 is a community-driven project open to contributions in coding, testing, bug reporting, design, and ideas.

License

JC-303 is released under the GNU General Public License v3. The Open303 engine portion is additionally licensed under the MIT License.

Roland and TB-303 are registered trademarks of Roland Corporation. JC-303 is not affiliated with or endorsed by Roland Corporation.

Key Features​

Download​

Installation​

Windows​

macOS​

macOS De-Quarantine​

Linux​

Sound Engine​

Step Sequencer​

Layout​

Editing steps​

Setting the pattern length​

Seq keyboard​

Record mode​

Generative​

Transport and clock​

AI Overdrive Section​

Devilfish-Inspired Mods​

Plugin Formats​

Building from Source​

Dependencies​

Generate Build Files​

Compile​

GUI Themes​

VST2​

Roadmap​

Community​

License​