Mod Squad: Bastl Instruments Kastle

A fistful of synth that won't empty your wallet
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It seems like a new portable synth or drone machine hits the scene every day. But one recent instrument truly stands out from the crowd—not just for its sound but for its feature set and implementation.

Kastle, from Bastl Instruments ($79;, is a digital instrument that is surprisingly versatile for its size, thanks to a clever arrangement of parameters and patch points. It also runs on three AA batteries and fits in the palm of your hand.

Whether you use it on its own or combine it with other synths, Kastle has a sound and potential that belie its small size.

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The instrument is based on a pair of ATtiny85 microcontrollers that offers a pair of oscillators and an LFO with stepped waveforms. Moreover, Kastle is an open-source product, so if you are Arduino-savvy, you can load your own programs into it and take advantage of the ample hardware configuration. You can even swap out the chips if want to customize it further.

Kastle provides a pair of 3.5mm jacks—one, a headphone output (with no level control); the other for routing a pair of signals (audio, CV, or clock) in and out of the instrument. This means you can easily integrate Kastle into a larger configuration of modular synths despite the fact that it uses tiny, single-pin cables for patching (which you may already be familiar with from products such as the Moog Werkstatt-01). Although the single-pin cables take a bit of getting used to, it’s definitely worth the effort because their diminutive size allowed Bastl to maximize the patching capabilities of Kastle.

The instrument, however, is semimodular, so you can play it without patching. In its basic state, Kastle uses phase modulation synthesis, but it also offers phase distortion and track-and-hold modes. Remarkably, you can change synthesis modes using a modulation signal, which is one of the most exciting aspects of the instrument.

The top panel has knobs for controlling the pitch, modulation level, and pulse width of the main oscillator; the pitch of the modulation oscillator and the incoming modulation level; and the LFO rate and the level of modulation it receives. Modulation sources include the main oscillator output (2 patch points, which can also be used as inputs for passive missing), oscillator pulse outputs (3 patch points), LFO triangle wave and LFO pulse wave (3 points, each), and a stepped signal output (3 points). The latter is inspired by Rob Hordijk’s Rungler circuit and gives you 8-, 16-, and randomly stepped voltage output depending on how you patch it.

The range of outputs makes sense considering the number of modulation inputs the instrument has: pitch, timbre, and LFO rate (3 patch points, each); LFO reset and Bit input for the stepped generator (2 points, each); synthesis mode (2 points); and I/O (2 points). Together, this wealth of patch points makes Kastle much more than a noisy drone machine. It’s easy to coax bell-like tones from it, as well as compelling, glitch-like sequences using the stepped generator to change synth modes along with other parameters. And you can take things even further by implementing feedback paths.

Kastle’s portability and wide range of timbres came in very handy during a recent tour, and it fit in comfortably with the various Eurorack systems I worked with. It was especially handy when used with the Koma Elektronik Field Kit, which also uses tiny patch cables.

Priced at $79, the Bastl Kastle is nearly an impulse buy and difficult to resist, but more importantly, it gives you a lot of synth for the money.