Audio Tools — legendary synthesizers, rebuilt in the open

A filter that screams like the original.

The MS‑20 gets its character from two small, misbehaving filters. AT20V models them from Tim Stinchcombe's circuit analysis and Korg's service manual, and solves their nonlinear feedback loop sample by sample instead of approximating it.

It was built with measurements, not guesswork: every figure below was checked against the numbers in the original documentation.

Low-pass filter: 12 dB per octave, with the diode clipping that limits the resonance
High-pass filter: 6 dB per octave, the real slope of the MS‑20, not the textbook 12
Calibration: cutoff at 5 and peak at 10 self-oscillates at 500 Hz, as Korg calibrated it
Filter revisions: Rev 1 (Korg35): grit grows with the mixer level. Rev 2 (LM13600): clean passband
Oscillators: band-limited, with the integrator curve, ring modulation as XOR and PW that fades to silence
Electrical conventions: Hz/V keyboard, 1 V/oct and 2 oct/V modulation, S-trig triggers, high-note priority
Oversampling: the whole voice runs at 4x or 8x

The AT20V rendered in 3D. The sound is the engine itself playing Random Bleeps, a self-playing preset.

Thirty-five jacks, all of them live.

Every input and output of the original patch panel works, including the hidden default connections: plug a cable and it replaces the internal route, exactly like the hardware. Cables hang with real physics, outputs take stacked plugs, and you can build feedback loops anywhere.

The External Signal Processor follows the pitch and level of a guitar or a voice and turns them into control voltages, so another instrument can play the synth.

Close-up of the AT20V patch bay with yellow, red and blue cables

AT20V front panel with patch cables connected — The front panel and patch bay. Every knob and jack was rendered in Blender from measurements of the original instrument.

AT20V keyboard with three keys pressed, the control wheel and the momentary switch — The 37-key keyboard with the control wheel and the momentary switch, both patchable.

What the MS‑20 never had.

A second page adds what a modern studio expects, drawn in the same visual language as the original panel.

Vocoder. 8 to 20 bands. Sing into the audio input and play chords; formant shift, resonance and sibilance included.
Polyphony. Mono like the original, or up to 12 voices with up to 4-voice unison, detune and stereo spread.
Tempo sync. The modulation generator locks to your DAW, so patches can play themselves in time.
MIDI out. Thru, or Patch mode, which turns the triggers of the patch bay into notes for other instruments.
Delay and reverb. A tempo-synced ping-pong delay and a plate reverb for the ambient presets.
AT20V FX. An effect version for audio tracks: put it on a vocal and drive it from a MIDI track.

AT20V Extensions page with vocoder, polyphony, delay, reverb and system knobs — The Extensions page: vocoder, polyphony, delay, reverb and system settings.

From the 1978 factory charts to patches that play themselves.

14

Factory

Patches transcribed from the setting charts of the original owner's manual.

13

Classics

Basses, screaming leads, drums and effects.

11

Self-playing

Generative patterns synced to your DAW. Hold a key to transpose them.

9

Ambient

Slow polyphonic pads with delay and reverb.

4

Vocoder

Robot voices, choirs and whispers for the audio input.

Low-angle view of the AT20V keyboard and front panel

Download AT20V 1.0.0.

Free, no account, no activation, no telemetry.

macOS

macOS 11 or later. Apple silicon and Intel in one universal binary.
VST3, Audio Unit and standalone app. AT20V-1.0.0-macOS.zip, 140 MB.

Download for macOS

Unzip the file.
Copy AT20V.vst3 and AT20V FX.vst3 to ~/Library/Audio/Plug-Ins/VST3.
Copy the two .component files to ~/Library/Audio/Plug-Ins/Components.
Rescan plugins in your DAW. In Ableton Live: Settings, Plug-Ins, Rescan.

Windows

Windows 10 or 11, 64-bit.
VST3 and standalone app. AT20V-1.0.0-Windows.zip, 56 MB.

Download for Windows

Unzip the file.
Copy AT20V.vst3 and AT20V FX.vst3 to C:\Program Files\Common Files\VST3.
Rescan plugins in your DAW.
AT20V.exe runs without a DAW.

The builds are not code-signed yet. macOS may refuse to open them until you run xattr -dr com.apple.quarantine on the installed plugins, and Windows SmartScreen may ask you to confirm the first time. Both packages include an INSTALL.txt with the exact steps.

AT20V running on Windows — The same plugin running on Windows.

Why open source.

Synthesizers are cultural instruments. Records were made with them, sounds were designed on them, and people learned music on them. When a commercial plugin is discontinued or stops running on a new operating system, the songs and presets built with it can disappear too.

Open source is how an instrument outlives the company that made it.

Preservation

How a classic circuit behaves is knowledge that belongs to musicians. Written down in code that anyone can read, it cannot be locked away.

Learning

Every filter equation, calibration constant and design decision is visible. A student can follow how an analog circuit becomes software.

Trust

No accounts, no license servers, no telemetry. You can check exactly what runs in your studio.

Longevity

Anyone can rebuild the plugin for the next operating system, in ten years or in thirty.

Collaboration

Owners of the original hardware can measure, compare and correct the model. Each improvement is shared with everyone.

AT20V is free software released under the GNU Affero General Public License v3, the same license as the JUCE framework it is built on. You can use it, study it, change it and share it. If you distribute a modified version, you share your changes under the same terms.

The source code will be published here soon.

One synth at a time.

AT20V is the first of a series. Each new instrument gets the same treatment: the original documentation, measurements, and a faithful model that anyone can study and improve.

Legendary synthesizers, rebuilt in the open.