The Kastle 2 boxes from Bastl Instruments are wonderfully weird little devices that have no right to be as fun as they are, considering their fiddly interface. It is with some relief to my fudgy fingers and poor eyesight that these little synthesizer and processing festivals have made it into decent‑sized Eurorack modules as the Citadel range. There’s the FX Wizard multi‑effects processor, the Wave Bard sample‑playing groovebox, and now we have the Alchemist — all firmwares are interchangeable.
The Citadel Alchemist is a multi‑algorithmic, hybrid‑synthesizing, melody‑making, patchable instrument. True to its name, it treats synthesis and timbre like chemical infusions, stirring up textures in a cauldron and serving them up as a potent transformation. Or, in other words, it’s a bit like Mutable Instruments Plaits’ weird cousin who’s definitely off their meds.
On the menu, we have five synthesis modes that draw on three oscillators, waveform mixing, FM with feedback, ring modulation, noise, track and hold, an LFO, an envelope and transient shaper, and a stereo filter. There’s a pattern generator to feed gates and CV into the synthesis engine, and some effects you can apply at the output. With a nod to its desktop box origins, there’s also a headphone output and a MIDI input with full MIDI control. Alchemist can be completely self‑contained, but the 24 patch points suggest it also enjoys external interference.
Filter mode offers three‑oscillator subtractive synthesis with sawtooth and square waveforms. The Timbre knob controls the cutoff of a resonant low‑pass filter, while Shift+Timbre detunes and mixes the waveforms. The waveforms are fizzy and thick when you push the detuning, and the filter is just the right side of gnarly.
- FM mode attaches two modulator oscillators to a main carrier oscillator for a range of sounds from wooden bonks to bell tones via a good bit of clanging dissonance. Your controls are FM depth and ratio/detuning.
- Supersaw mode is next, which gives you a more chilled filter on the first control and sawtooth detuning and intervals on the second parameter. It’s a good classic sound that gives a decent contrast to the Filter mode.
- Hypersine mode is a classic Kastle‑synth digital tone. It combines ring modulation with track and hold modulation with detuning to give a smooth transition from sweet sine waves to glassy tones to corrupted wobbles.
- Glitchnoise mode is wildly unpredictable and a bucketful of fun. You’ve got some control over FM feedback, filtered noise and oscillator ratios. The result is a bubbling mixture of metallic and soft tones fighting over a bag of sherbert.
The sounds have unique characters and an edginess that can bring a lot of different tones to your rack.
The sounds have unique characters and an edginess that can bring a lot of different tones to your rack. I confess that having both controls on a single knob feels like a misstep. I want to lean into both of those parameters when I’m playing with the sounds, and having side‑by‑side knobs would make a whole lot more sense. However, Bastl have decided that the main focus of the knobs should be modulation and the larger Envelope knob in the middle of the faceplate.
The envelope is more fascinating than you’d normally expect. Turn to the right, and you get a sharp, transient‑focused envelope that expands into a drone. Turn to the left, and you get a slower body‑focused envelope with an increasing attack into reverse‑type sounds before falling into a drone function that also enables morphing between the modes. This gets really interesting when you modulate the Mode input: if you turn right, you get instant switching between tones, but if you turn left, the Alchemist slides from tone to tone, making bizarre transitions through melody, noise, and back again. You can effect a huge amount of change with a twist, and you start to realise why Bastl hold the envelope in such high regard.
Shift+knob turning aside, at this point, I’m really happy with the Alchemist’s spicy flavours of sound generation. It’s not well behaved or particularly elegant, but there’s a lot of fun and fiddling to be had, especially when you push in modulations with the three knobs on the left, do weird things to the pitch range, patch in the LFO, and drop on some effects.
There’s another aspect that’s easy to miss in a Eurorack synth‑voice scenario, and that’s the humble pattern generator. The pattern generator kicks out CV and gate patterns based on pretty much every other control. Pitch Mod adds more pitches, Envelope plays with density, the LFO can arpeggiate, while other Shift+knob combos deal with scale, root note, and gate pattern. The patterns are chirpy and give off a slightly unsettling funfair vibe, which you can subvert using the inversion and randomisation switches. You can also plug the module into your computer and fiddle with the patterns and scales if you so wish.
The Alchemist is about as playful a synth as you are ever going to find.
The Alchemist is about as playful a synth as you are ever going to find. The individual modes are vigorous and engaging, the knobs are all vying for attention, and the modulation within and without will keep you busy with the patch cables. It can veer unexpectedly from a mish‑mash noise machine to spacy reversed pads, then back to ear‑tinkling melodies. And all of it is self‑inflicted. To put it in remarkably appropriate modern‑day language, Alchemist cooks.