I was lucky enough to bump into the guys from Bastl Instruments at Ableton’s Loop Summit. They were running workshops building their Kastle micro modular synth in the space I was using after them. I’ve bumped into these guys once before a few years ago at Musik Messe, but this time I got to spend a bit more time with them, and they’re really cool.

But enough of that for now. What I did do is get a chance to take a longer look at the Kastle synth and have a really long play with it.

The Kastle synth is tiny. I mean really tiny. It’s footprint is the same as 3 AA batteries. You can plug it into headphones and it sounds awesome. The real power of Kastle is its tiny patch cables, like patching any other modular but on a very small scale. It’s tiny patching capability gives it enormous power and verstility, but much more importantly it makes it loads of fun and that’s what’s really attracted me to it.

So let’s find out a bit more about what Kastle is from the Bastl Instruments site:

Kastle is a mini modular synthesizer with headphone output, 2 in/out ports for interfacing other gear and it runs on just 3 AA batteries. It is DIY friendly and ideal for beginners in modular synthesis, but it will add quite some unique functionality to any modular synthesizer systems. It delivers the fun of modular synthesis at cost and fits into your pocket so you can play it everywhere!

It has unique digital lo-fi sound and it can be melodic as well as very noisy and drony, soft or harsh. It is designed to be fun on its own but it is most powerful when combined with other modular gear.

Kastle is an open source DIY project which runs on two Attiny 85 chips that can be reprogrammed with an Arduino (google: “programming Attiny 85 with Arduino”). One chip is dedicated to sound generation while the other handles modulation. Several firmwares for the Attiny chips are available.

The Synth version combines complex oscillator and LFO with stepped waveform generator.

The oscillator section has 3 sound parameters pitch, timbre and waveshape – all under voltage control and with 3 different synthesis modes. It has a main output and a square wave output. Both can be used independently or combined. The 3 synthesis modes are phase distortion, phase modulation (also known as FM) and track & hold modulation. Each mode utilizes two oscillators. The Pitch controls the main oscillator, the Timbre sets the pitch of the modulating oscillator and the waveshape depends on the synthesis mode. The waveshape also controls the pulse width of the square wave output from the main oscillator.

The voltage controllable LFO has a triangle and square output and a reset input. The stepped waveform generator is inspired by the Rungler circuit by Rob Hordijk. It can produce 8 different voltages either in random order or in 8 or 16 step looping patterns depending on how the BIT IN is patched.

Here are the main features of the Kastle:

  • complex oscillator
  • 3 synthesis modes: phase distortion, phase modulation and track & hold modulation
  • pitch control with offset and CV input with attenuator
  • timbre control with offset and CV input with attenuator
  • waveshape control with offset and CV input
  • voltage controllable LFO with triangle and square outputs and reset input
  • stepped voltage generator with random, 8 step and loop 16 step mode
  • 2 I/O CV ports are available and can be routed to any patch point
  • the main output can drive headphones
  • 3x AA battery operation with power switch
  • open source
  • possibility of exchanging different LFO and OSC chips
  • the pattern on the sides changes and every unit is an original

From using the Kastle for a couple of weeks now I can safely say that it fantastic and enormous fun to play with and experiment with. I’d highly recommend it to anyone.

If you want more information about the Kastle you should check out the Bastl Instruments site, and you buy one right here.


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