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Bastl Instruments Kastle v1.5 - Mini Modular Synth


Updated Mini Modular Synth

Kastle is a mini modular synthesizer with a 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 some quite unique functionality to any modular synthesizer system. It delivers the fun of modular synthesis at a low cost and fits into your pocket so you can play it everywhere!

It has a 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 one handles modulation. Several firmwares for the Attiny chips are available thru gitHub.

The v1.5 version combines a 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 6 different synthesis modes. It has a main output and a secondary waveform output. Both can be used independently or in combination. The 3 synthesis modes available thru the main output are: phase modulation, noise mode and track & hold modulation.There are 3 more modes on the secondary OSC output: phase distortion, tonal noise mode and formant synthesis. Each mode utilizes one or two oscillators. The PITCH parameter controls the main oscillator, the TIMBRE sets the PITCH of the modulating oscillator and the WAVESHAPE depends on the synthesis mode.

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. 


  • Complex oscillator
  • 3 synthesis modes on the main output: phase modulation, noise mode, track & hold modulation
  • 3 more modes on the secondary OSC output: phase distortion, tonal noise mode, formant synthesis
  • 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 16 step loop mode
  • 2 I/O CV ports that can be routed to any patch point
  • The main output can drive headphones
  • 3x AA battery operation or USB power selectable by a switch
  • Open source
  • Possibility of exchanging different LFO and OSC chips
  • Durable black & silver PCB enclosure


          4.83 Average

          6 Reviews

          Jon W

          Small and unique piece of synth hardware. Here is my first patch after unboxing sequenced with a Korg SQ-1.


          Ooh man, I love this little synth. It’s a great addition to my tiny synth team and brings some new and exciting sounds.

          Ryan E

          This little synth from Bastl is SO COOL. I chose this as my first synth because it was billed as a good intro synth for people without much experience, and it is totally stoking more flames for synthesizers. As someone without much synth knowledge, it took a little poking around and a little YouTube-ing to get a good grip on the oscillators and everything else -- but I feel like I have a better understanding every time I play around with it. A great first synth. I also love that is has a headphone jack and is battery-powered, so as I'm commuting or traveling I can play with it to pass the time. 10/10 would recommend to anyone interested in electronic music.

          James S

          This thing is sweet. It makes all the best noises and also the worst. You don't even have to really know what's going on so definitely buy this.


          Super fun little toy. You can make some beastly sounds with this thing. Very fun to play with, tricky to master!

          Jesse A

          I've only had a few days with this piece, but I actually ended up using it several times in a very unexpected way. While it's great as a very complex noise maker, I found that by setting the LFO to a very slow rate and getting the pitch to be somewhat consistent, you can make really really interesting evolving pad sounds with this guy by sampling 10-15 seconds of a more tame sound and stretching it over the keys. I love it!

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