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An open source eurorack sample streaming and sound synthesis module.
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CTAG Strämpler

Strämpler harware UI

What it is:

  • Half streamer, half sampler, therefore called Strämpler (close to German word of Strampler meaning romper suit).
  • Allows streaming of large audio files from SD card (limit 2GB file size due to FAT32 used).
  • A eurorack modular synth module with 22 HP width and an internet connection.
  • A bridge to connect to, to play with samples within your modular synth setup.
  • Allows tweaking of your sound just like you'd do with a sampler + a modular synth.
  • Allows to parameter tweak and modulate sounds by control voltage (CV), employing a complex modulation matrix.

Why it is:

  • A group of audio enthusiasts enjoying coding and hardware making.
  • The sexiness of sampling for sound design and synthesis.
  • The (subjective) need to have more sampling modules in the eurorack modular synth domain.
  • The (subjective) need to make eurorack more compatible with internet of things.
  • Build a platform to learn, build and practise skills, and engage students.
  • To allow anyone to understand technology by offering open access.
  • To benchmark the capabilities of the WiFi/BLE enabled Espressif ESP32 platform and get a deep understanding of it.
  • To squeeze and optimize code so that it can work on a small embedded system.
  • Because we can.
  • Because of some inspiration of the Elektron Octatrack.


  • 2 voice eurorack sample streaming module.
  • Streaming of large sound files from SD-card (limited by FAT32 2GB).
  • Internet connection to, allows to download files through api onto SD-card of module.
  • One ADSR per voice to control sample amplitude.
  • One band pass filter per voice with controllable base, width and Q. Can be used as low / high pass (biquad implmentation).
  • Arbitrary playback speed adjustment (+/- 100%).
  • Pitching +/- 12 halftones, samples can be played e.g. using external gate / CV keyboard.
  • Distortion per voice (tanh() saturation).
  • Delay as send effect (stereo, ping pong, max delay time 1.5s).
  • External stereo input, with delay send, mix with voices.
  • Modulation matrix, where many parameters can be modulated using external CV.
  • Gate and latch modes for sample playback.
  • REST-API for user file upload (service discovery by MDNS / bonjour).
  • 44.1kHz, 32 bit float internal resolution, 24 bit codec resolution, 12 bit CV input resolution sampled at 2KHz (yes, modulation in the audio range is possible to a certain extend), approx. 1ms DSP buffer latency (32 words per channel).
  • Approx. 100mA +12V / 10mA -12V power draw.

Potential new features / current limitations / work to be done:

  • Sampling of external input.
  • Upload to
  • Improved sound browser, browse by tag, browse by search.
  • More testing of modulation.
  • More effects.
  • More performance optimization.
  • Automatic voice alternation.
  • Bug identification and fixing.
  • Code refactoring to make things more beautiful.
  • More user friendly interaction.
  • Documentation / tutorials.
  • Your ideas?

How to engage yourself:

  • Join the enthusiastic developer team on Github.
  • Help build and spread the hardware module (and the word).
  • Use Strämpler to create cool sounds and music and share them (with a link to us).
  • More ideas?

Words of caution:

  • CTAG Strämpler does contain bugs, it comes without warranty of any kind, none of the authors are liable to damages arising by the use of it.
  • CTAG Strämpler is an intermediate to advanced project, both in terms of hardware and software design:
    • In order to build the hardware you need intermediate to advanced SMD soldering skills and respective tools. I.e. the PCB contains many 0604 SMD components as well as TSSOP packages and a QFN. However, with a bit of practise you will be able to DIY build your own module. Dare and be rewarded, tackle the frustration on the path, it's worth it. The idea of the platform is also to boost your soldering skills, it CAN all be done by hand.
    • The software is built using Espressif IDF using the C programming language. Intermediate knowledge of C is required to understand the code. Furthermore, some basic DSP algorithms are applied. A DSP newbie, however, could take the project to really get rolling and build up on DSP capabilities. CTAG Strämpler is ideal to try out and play with your own DSP algorithms.
  • The Espressif ESP32 platform used for CTAG Strämpler is, with regard to its computational power and I/O capabilities, hard at its limit. We're squeezing the platform here and are already surprised, what one can get out of $5 chip in the year 2019. Surely, other DSPs / microcontrollers could do a better job, but do they allow for internet of things as easily?

How to get started / build instructions / user manual:

See the Wiki pages of this project; where in particular build instructions can be found.



Who made this happen:

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