There’s a whole contingency of teeny tiny synths out there, devices that look like toys but are powerful little things that can rival a room of normal-sized synths, playable as an electronic music studio that will fit in a shoebox, but there aren’t many that are smaller in stature but as big in terms of power and features as Bastl’s Kastle synths. Joining the Kastle 1.5 synth and the Kastle Drum (and I think their Bestie mixer fits in here, too), Bastl’s new Kastle ARP fills out a gap in the series, offering up a box for generative patterns and melodies that can run off of batteries or USB, so you can pull it out of your pocket, hook up some headphones, and patch around. With the same sort of minuscule and machine pin jumper wires for patch cables as the previous Kastles, ARP’s 65mm x 55mm frame might deter those with spotty dexterity and hammy fingers (no shame in that), but if you can deal with those patch wires, you will find rewards and riches in this Kastle.
So I might seem a bit jokey about the diminutive stature of Kastle ARP, but it’s no joke. The pin jumper patch cables are small, even finicky, but they’re not annoying as I would have predicted, even given how cramped ARP can be once all patched up, and patched up it should be. ARP is the first Kastle designed for melodic creation and it works great for generative arpeggios, noisy Atari punk console type of sounds, chords, slow notes, and chaotic blasts. ARP is born in the key of C and is quantized—a key to its constant in-tune melodic nature, but the key can be changed by entering a top secret (not really) mode. You can, of course, also change the pleasant in-tune nature by modulating ARP to the hilt, to make it not so in-tune.
Kastle ARP’s architecture is driven by two ATtiny chips, one for serving the LFO (taken from the Kastle Drum), and the other for everything else. The pattern generator, with the possibility for endless variations, is inspired by Rob Hordijk’s Rungler and is at the heart of the synth, and is what moves the sine-based VCO along. In terms of movement and time, there’s a Tempo control for setting the time, and a Tempo Mod with CV for creating shifting tempos so it’s possible to make it where the motion is constantly changing. You can also patch an external clock into the TRS 3.5mm I/O jack (which lets you connect to external sources via the stereo jacks two connections) and patching the jumper from the Input of that into the Clock in, unless you have a 3.5mm to pin jumper cable on hand :). I found that ARP was most predictable with the internal clock once I patched in an external clock the CV modulation with the internal LFO changed from its triangle core to combine with whatever was driving it. For example, with the Tempo turned all the way down, a square wave input for the clock source created a square wave front with a stepped modulation on the backside at the LFO (as per the Mordax DATA). Changing the Tempo control changed the LFO and the results were pretty cool as it mixed in more of ARP’s internal triangle, with cool results in the modulation and more complex patterning available. If, like in my example, the Tempo control on ARP is slower enough than the external Clock in, the Tempo runs at its own pace and still modulates slowly and the clock will only trigger the pattern generator. Whether this is a byproduct of the design or intentional, it opens up a lot more unsynchronized options, perfect for this little synth.
The Timbre control is a digital waveshaper and has the Timbre Mod control with CV in, where you can go from a subtle filtered sound to absolute chiptune blasphemy, and can be modulated by the internal LFO or an external signal via the assignable I/O. The Decay lets you choose when you want notes to retrigger and goes from spacey, long drawn out notes where it skips notes due to the decay length, to busy, cramped patterns where note retriggering happens with every clock signal, and can be modulated. The range of notes is also more than enough to generate interesting patterns with the base Note control determining the central pitch and the Note Mod control, which can be modulated, determining the range in octaves of notes in the chosen chord that is to be arpeggiated. As for choosing chords, there are three different CV inputs for different chord variations.
Kastle ARP, though small, has some low end, that of the sine variety, and you can mix that into the output by patching the Bass into the ARP input (the ARP input goes directly to ARP’s main output), which adds the root chord to the output. There’s no way to mix this to taste, but overall it works pretty well, and the sine wave doesn’t obfuscate the main melody line too much, and in fact turns Kastle ARP into a duophonic synth. Normally I prefer square wave basses for more meat, but I think the sine works well here, filling out the sound.
Bastl really knows how to milk things. Their Pizza series modules, with the dual function knobs and sliders, assignable ins, outs, and routing…they really leave no potential stone unturned, and this helps make devices like ARP way more than you’d think. When I first unboxed ARP I thought it would be a fussy little thing that might get dusty quickly, but it turned out to be really fun, surprisingly useful, and even more surprising, a truly versatile instrument. I can foresee a whole setup of small synths covering only a portion of a table top, and being way more powerful than it should.

Price: $132
bastl-instruments.com