A DIY module for the hands-on hardware enthusiast

After releasing SALT and SALT+ we had a lot of requests for a DIY synth kit, so we developed PEPPER for people who like to build things themselves.

PEPPER is a DIY kit for experimenting with Bela and your modular synth. PEPPER is a bare PCB that comes with a list of common parts, and a faceplate for mounting PEPPER in your modular setup. Source the parts, solder the whole thing together, connect your Bela unit, and get started writing patches in C++, Pure Data, Supercollider and more, or use the pre-loaded example projects.

Table of contents

  1. What is PEPPER?
    1. The PEPPER PCB
    2. The PEPPER faceplate
  2. Technical notes
    1. PEPPER compatibility
    2. PEPPER vs SALT

What is PEPPER?

PEPPER is a DIY kit that consists of a custom-designed PCB and faceplate. PEPPER comes with a list of common through-hole components for you to source, and instructions for putting it together. Attach PEPPER to your Bela system and you can start prototyping and experimenting with Bela and your modular synth.

Once assembled, PEPPER adds all this to your modular setup:

Width: 18HP

I/O (1/8” jacks):

  • 2 audio in (AC-coupled, 10V pk-to-pk, 44.1kHz, 16bit)
  • 2 audio out (DC-coupled, ±5V, 44.1kHz, 16bit)
  • 8x CV in (with pots attenuating, 16bit, 0-10V). Optionally, up to 4x of the CV in sockets can be swapped for trigger ins, leaving the pots connected directly to Bela’s analog inputs.
  • 8x CV out (16bit, 0-5V)


  • 8x pots (attenuation of CV in)
  • 4x buttons
  • 10x LED output

The Bela unit (sold separately) consists of a Bela cape and a BeagleBone Black, and custom Bela software. Once attached, Bela provides the following processing power:

  • 1 GHz ARM Cortex-A8
  • 512Mb RAM
  • Sub-millisecond round-trip latency


The core of PEPPER is bare PCB. We don’t currently ship the parts because these are relatively easy to source components which you can buy from your favourite hardware shop. (Click here for the full BOM, or bill of materials.)

Once you have your parts collected, solder them to the PEPPER PCB. Click here to see the build guide on the PEPPER wiki.

The PEPPER faceplate

The PEPPER kit also includes a faceplate, so you can include PEPPER in your Eurorack case. It’s made out of black PCB board with silver solder mask.

The finished PEPPER module looks like this:

You’ll need the components, the PCB, and a soldering iron. Then, just follow the step-by-step instructions for putting PEPPER together that can be found on the PEPPER wiki.

Technical notes

PEPPER is a passive breakout interface for Bela. It uses passive components to protect Bela’s I/Os and amplification only on the audio output.

  • Only positive voltages (0-12V) can be read from the analog and digital inputs although PEPPER is +/-12V safe.
  • PEPPER’s external voltages (analog, audio, digital) are scaled down to fit the input range of Bela.
  • Analog outputs are not amplified and therefore 0V-5V.
  • Analog outputs have ~1k output impedance, in order to protect from damage from other outputs that might be plugged in inadvertently. This means that using passive multiples could lead to voltage drops.
  • When powered from 5V (i.e. being used standalone and not taking power from a Eurorack case) the potentiometers will not cover the whole range of the analog input when there is no external input signal connected.

PEPPER compatibility

Please note that PEPPER is compatible only with Bela, and not with Bela Mini - this is because Bela Mini does not have analog outputs.


SALT (currently not in production) and PEPPER are two ways to bring Bela into the modular domain, but there are important differences between them.

PEPPER is a DIY approach that you build yourself. It has limited input and output ranges, with 10 input channels and 10 output channels. Because you have to assemble it, PEPPER lends itself to learning about electronics, as well as hacking to customise it to your own specific needs.

An important consideration for PEPPER is that you will have to determine a way of connecting the USB ports of Bela to the outside world, because the USB ports from the Bela unit are not immediately accessible from the front panel of a Eurorack. We supply a smaller breakout board with the PEPPER faceplate to help with this, so you can buy breakout cables and attach them to this second faceplate.

In contrast, SALT is a fully-fledged Eurorack module. It comes fully assembled and includes an embedded Bela unit. This unit is composed of a BeagleBone Green and Bela cape (BeagleBone Green is used to reduce vertical space). SALT features full-range voltages, as well as 14 input and 14 output channels. Because SALT arrives expertly assembled and and tested, it is a music machine that is ready to go - just plug in and start programming.