Bastl Instruments has released the Citadel Alchemist, new DIY kits, and has partially open-sourced the Kastle 2 and Citadel products.
Last October, Bastl Instruments did something many had been asking for with their Citadel modules: ported the fun and inspiring Kastle 2 desktop devices to Eurorack, keeping all the same functionality.
Shortly before Christmas, the Alchemist was released, another Kastle 2 synthesizer that revives the original Kastle 1 ARP but in a significantly expanded form. And now it’s also Eurorack compatible. Bastl Instruments has introduced the Citadel Alchemist alongside DIY and open-source news.
Bastl Instruments Citadel Alchemist
The Citadel Alchemist (16HP) is a Eurorack reimagination of the Kastle 2 Alchemist semi-modular Synthesizer. Like the Citadel Wave Bard and Citadel FX Wizard, it is cross-compatible, allowing you to flash it with another firmware.
It offers the same multi-synthesis melodic engine (filter, FM, supersaw…) as the desktop version, but comes with added Eurorack connectivity. Full details of the engine here.
It has a 3.5mm patch bay that offers the flexibility needed in the eurorack format while maintaining the strengths of the Kastle 2, allowing it to stand on its own, and yet let it truly shine when patched with
The module features new switches for hands-on control of the built-in pattern generator.
Alongside this, it includes everything you need to integrate with other modules: headphone output, TRS MIDI input and sync, a tempo generator with divider/multiplier, USB MIDI, stereo mixing, and more.
Open-Source (Partially)
The Bastl Instruments Kastle 2 and Citadel platforms are very community-oriented, based on the popular RP240 chip and using an optimized fixed-point DSP library.
One highlight is the ability to easily swap firmware, allowing all three firmwares to be used with a single desktop unit or module. That’s a brilliant idea, making these modules and synths super versatile.
Their goal is to share as much as they can with the community without compromising their business model – enough for people to learn from the designs, repair the hardware, or hack new functions into it
The developers are now taking an exciting step further and partially open-sourcing the platform. Here is what will be open-sourced:
- Most of the code (core, Wave Bard and FX Wizard apps, DSP library, V/Oct calibration) – MIT
- Schematics as PDF (Kastle 2, Citadel), Documentation – CC-BY-SA
- Panel templates (without proprietary fonts and graphics) – CC-BY-SA
- 3D case (without graphics), pin header holder – CC-BY-SA
- TTS Voice recordings (Version, Calibration, Test Mode) – CC0
- Web Editor apps source code – MIT License
And here’s what stays proprietary and won’t be open-sourced
- Alchemist app (at least for now – reserved for future projects)
- Board layout, production gerber files (we make a living by selling the HW)
- Test jig & production procedures
- final graphics, our custom fonts, including anything that incorporates them
- Factory sample banks (available for download, but not licensed for redistribution in derivative products, etc.).
DIY Versions
This third news will make many modular soldering enthusiasts happy. Bastl Instruments is going back to its roots and offering the Kastle 2 and Citadel, including Alchemist, as DIY versions.
Each kit includes a factory‑tested SMT‑assembled PCB, all necessary hardware components (pots, jacks, etc.), and the enclosure/faceplate. They don’t provide technical assistance or remote troubleshooting for DIY Kits.
However, you can book a paid repair service called Come to Daddy (similar to Befaco). It costs 30€ + VAT for either Kastle 2 or Citadel, but without shipping.
First Impression
It was only a matter of time before the Alchemist was released as a Citadel module. The open-source version is very welcome, and I’m eager to see if we’ll see new non-Bastl firmware or even more.
Bastl Instruments Citadel Alchemist is available now for $319/235€ + VAT. The new DIY kits for the Kastle 2 are 128€ + VAT, and Citadel is 194€ + VAT. Alternative faecplates are also available for 20€ and 25€ + VAT.
More information here: Bastl Instruments
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