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Anyere edited this page May 15, 2019 · 8 revisions

Pepper is a DIY, low-cost solution for integrating your Bela board with the world of Modular and Eurorack synths. Pepper is a custom-designed, bare PCB that comes with a list of common through-hole components for you to source, and instructions for putting it together. Attach Pepper to your Bela board and you can start prototyping and experimenting with Bela and your modular synth. Pepper is NOT compatible with BelaMini, or the CTAG series.

You can buy the Pepper PCB now on our shop. We will soon make a faceplate for Pepper available as well.

Once soldered up Pepper will add all of this in your modular set-up:

  • I/O (1/8" jacks):
    • 2 audio in (AC-coupled, 10V pk-to-pk, 44.1kHz, 16bit)
    • 2 audio out (DC-coupled, 1.8V pk-to-pk, 0.4V offset, 44.1kHz, 16bit)
    • 8x CV in (with pots attenuating, 16bit, 10V safe). 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, 0V-5V)


  • Width: 18HP
  • 8x pots (attenuation of CV in)
  • 2x buttons
  • 10x LED output

Components needed

We do not have yet a full list, but to give you an idea here is a summary of the parts required:

  • 20x 3.5mm mono jack sockets such as this one
  • 8x potentiometers such as this one
  • if you are powering from the 12V rail of your Eurorack system, you will need a switching 5V regulator, such as this one.
  • Common value resistors, capacitors and diodes
  • 2x tactile switches such as this one
  • 1x 10 segment LED bar-graph such as this one or 10 stage DIP switch
  • A handful of 2.54mm headers: 4x1 female, 3x1 female, 4x1 male, 2x30 male, 2x26male, 2x16 male(optionally boxed)

Notes on inputs and outputs

Pepper is a passive breakout interface for Bela, with passive components used to protect Bela's I/Os, but no amplification.

  • Only positive voltages (0-10V) can be read from the analog and digital inputs although Pepper is -5V safe.

  • External voltages (analog, audio, digital) are scaled down to fit the input range of Bela.

  • Outputs are not amplified. Therefore:

    • audio outputs are 1.8V pk-to-pk, DC coupled, with a 0.4V offset (that is, you can achieve values between -1.4V and 2.2V).
    • analog outputs are 0V-5V
  • Analog outputs have ~1k output impedance in order to protect from damage from other outputs being plugged in inadvertently. This means that using passive multiples could lead to voltage drops.

  • When powered from 5V, the potentiometers will not cover the whole range of the analog input when there is no external input signal connected (see Customisation below).

Powering Pepper

To keep Pepper flexible, we have designed it so it can run off many different power supplies:

  • For best results Pepper should be connected to a Eurorack 12V power rail via a ribbon cable. This will allow for the full-scale range from the pots and requires the 5V regulator to be installed.

  • Pepper can also be powered directly off a 5V Eurorack PSU, assuming there are 400mA available on the 5V rail.

  • Just like the original Bela board, Pepper can also be powered via USB power, or 5V barrel jack into the Bela board.

Note that the PSU in use does not affect the I/O voltage ranges at the jack sockets, only the full-scale range you will get from the pots when nothing is plugged into the input.


  • In the default configuration the 8 pots act as attenuators for the 8 CV-in sockets before they are connected to Bela's analog inputs. These can optionally be re-configured so that up to 4 of the CV-in sockets are repurposed as trigger inputs, connected to Bela's digital inputs, while the corresponding pots are still controlling Bela's analog inputs. This is selected through jumpers on the back of the PCB.

  • When powered from 12V via the Eurorack connector, adding a Zener diode and a resistor allows to have the full voltage range available via the potentiometer input when no external jack is connected.

  • In order to use Bela's DC coupled audio output, you have to install a 1x4 socket header on J10 on your cape (see here), otherwise you can use the headphones output.

  • Pepper's default configuration uses a 10-segment LED bar connected to Bela's digital outputs. This is great to visualize sequencer steps, levels and much more. Optionally this can be replaced by an array of 10 DIP switches. As the digital outputs can be repurposed as inputs in software, this would then act as 10 discrete digital controls into Bela.

Pepper vs Salt

Salt and Pepper are two ways to bring Bela into the modular domain.

Pepper is a DIY approach, with limited input and output ranges, 10 input channels and 10 output channels, and that you build yourself. You also have to figure out how to connect the USB ports of Bela to the outside world. It is open to hacking to customize it to your needs.

Salt is a fully-fledged Eurorack module, with full-range voltages, 14 in and 14 out channels, and that comes to you pre-assembled and tested. It is a music machine ready to go. Just plug in and start programming.

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