CA-72: a free, open-source Minimoog Model D modelled from its schematics
Idle Foundry releases CA-72, a free GPLv3 plugin modelled on the Minimoog Model D, in VST3, CLAP and AU, with a polyphonic mode.
Idle Foundry released the first version of CA-72 on October 4, a free, open-source synth plugin modelled on the 1972-73 Minimoog Model D, the one with the so-called “old” oscillator board built around CA3046 chips. It runs as VST3 and CLAP on macOS, Windows and Linux, and as an Audio Unit on macOS since version 0.1.2. The code is released under GPLv3 on Idle Foundry’s GitHub repository, now at version 0.1.5.
Modelled from the schematics
The developer describes an unusual method for a free plugin. Every board of the instrument was transcribed from its service documents into SPICE netlists, then simulated in ngspice. SPICE is an electronic circuit simulator: it computes a schematic component by component, far too slowly to play, but accurately. The plugin’s real-time models were then derived from those simulations and tested against them, factory calibration procedures included. Speaking to Bedroom Producers Blog, the developer said Claude, Anthropic’s assistant, helped build the synth from the schematics and service manual.
Who it is for
- The musician who wants a Minimoog sound for free: monophonic basses and leads, with the original panel, its three oscillators and its ladder filter.
- Linux users, often left out by commercial emulations.
- Anyone learning subtractive synthesis: the panel follows the instrument from oscillators to mixer, filter and contours, the Minimoog’s name for envelopes (see our guide to ADSR envelopes). It is also a good testbed before a first hardware synth.
What it costs
Nothing, with no account or sign-up. The macOS installer is signed and notarised. The Windows one is not signed yet, so on first run Windows shows a warning you get past with “More info”, then “Run anyway”. Since 0.1.5 a zip archive also allows a manual install.
What it adds to the original
The Minimoog is monophonic: one note at a time, lowest note first. CA-72 keeps that behaviour by default and adds four controls below the panel that the instrument never had:
| Control | What it does |
|---|---|
| POLY and VOICES | 2 to 10 voices; each note plays a whole instrument, with its own keyboard circuit, contours and glide. |
| ENTROPY | 0 to 100%: each voice gets small offsets in tuning, filter, contours and glide, as if built from a different batch of parts, plus slow drift. |
| SPREAD | Places polyphonic voices across the stereo field. |
| FEEDBACK | Recreates the Minimoog owners’ trick of patching the phones output back into the external input to overdrive the preamp and mixer. |
POLY changes the nature of the instrument: our explainer on polyphonic, paraphonic and monophonic voice counts covers what each extra voice costs. The plugin also ships 24 factory presets (basses, leads, pads, percussion), a browser with search, tags and favourites, and MIDI Learn since 0.1.3.
A week of releases
| Version | Date | Main change |
|---|---|---|
| 0.1.0 | October 4 | First release |
| 0.1.1 | October 6 | Windows crashes fixed, update check added |
| 0.1.2 | October 6 | Audio Unit for macOS |
| 0.1.3 | October 6 | MIDI Learn |
| 0.1.4 | October 7 | Unresponsive controls in Cubase 15 fixed |
| 0.1.5 | October 8 | POLY presets no longer overload 3 and 4-core machines |
The limits of a 0.1 release
- High voice counts and FEEDBACK can cause dropouts depending on the computer and audio buffer size; FEEDBACK costs about a fifth more CPU per voice.
- MIDI Learn only takes absolute 7-bit controllers: no relative encoders or 14-bit messages.
- The update check only runs when you click it, and the plugin sends no data, according to the developer.
What it competes with
Minimoog emulations are plentiful, but they are paid products: Arturia’s Mini V, Softube’s Model 72 or Cherry Audio’s Miniverse, from the company that also just revived the Gleeman Pentaphonic. They bring maturity, sound libraries and support. CA-72 answers with a zero price, open code, a Linux build and a documented modelling method. Whether a 0.1 release holds up against them, by ear and over time, remains to be seen.
Hear it
Idle Foundry has published an official demo:
Source: Idle Foundry, CA-72 GitHub repository. More free tools are listed in our Free & Open Source section.