Modular OpenTheremin

The OpenTheremin is a stand alone Theremin with built in oscillators. Through the control voltage outputs (CV) an existing modular system can be connected and controlled easily. However the design of the OpenTheremin version 4 allows the electronic circuit of the OpenTheremin to be built into a modular Theremin system itself. This page describes a possible arrangement of such a built.

OpenTheremin Modular

The dimensions of the OpenTheremin cirucit fit with the Eurorack format. However with the antenna and signal connectors there is some tinkering needed. The circuit does not feature a standard Eurorack connector yet and so we will have to be creative there.

Here is how it can be done.

1. The modular case


For this build I decided to go with the FRAPS TOOLS PLUS modular case. The case 42HP case is made from extruded aluminium with two wooden sides. The case would not need to be metal and in fact a wooden case may make the isolation of the antenna wire even easier.

2. The power supply

The OpenTheremin circuit needs to be powered with a stable 5V DC (max 100mA). So any rack power supply that also offers 5V should be fine. I decided to build one that can be powered through a simple USB-C connection. For this I used a cheap symetric +/-15V step-up converter module followed by two linear regulators.

As a commercial alternative I want to recommend the psu II – 1hp power supply solution by our friends at seismic industries. This super slim USB-C power module provides +/- 12V and 5V you need for the Theremin.

My DIY power module was then wired up to a USB-C break-out board that screws to the case.


 3. Antenna connectors

Theremin Antenna connectors BNC

For the modular theremin I choose a common BNC connector to mount the antennas to the wooden sides. The aluminium tube connects to the outside of the connector as it does not actually need the co-axial connection. It just makes for a solid and easily removable connection.

The nut on the BNC connector had to be drilled slightly bigger to fit the 6mm aluminium tube. The tube was then just glued in with epoxy and the electrical connection made with a bit of conductive silver glue.

A stiff wire connects the antenna terminals with the screw terminals on the theremin board. The wire is fixed with plastic wire mounts to prevent the antenna-wire from touching the metallic case.

3. Signal Beak-Out Board.

Next the output jacks need to be wired up. All the signals can be found on soldering pads at the edge of the board.I made a small front pate with holes for the jacks and labelled them accordingly. The gate pad is quite close to the edge and I had to be careful for it not to touch with the rack frame.

I then screwed everything in and I think it is looking great. The audio signal can be used in combination with the control voltages and the gate.

4. Choosing Modules to combine with the Theremin

Finally the most creative part is choosing Eurorack modules to combine with the OpenTheremin.

OpenTheremin Modular

To emulate the sound and functionality using the control voltages a VCO (Voltage Controlled Oscillator) and a VCA (Voltage Controlled Amplifier) are needed. I build and experimented with the modules from MRG Synthesizers based in the UK.

For a wider variety of sounds and the possibilities to experiment with gate and envelopes I also put it the new Behringer Brains module. This module offers I wide variety of sound generators and has several CV inputs to control pitch, volume and parameters of the sound. The trigger input can be connected to the OpenTheremin gate to play percussion sounds. The Behringer Brains is a derivate from the famous Mutable Instruments Plaits and is based on its open source code. Unfortunately Behringer decided to not make their code available openly.

A small mixer in the setup proofed to be very handy when combining the different audio sources and to adjust the final volume. The perfect fit for me was the simple ABC Six Channel Mixer from Bastl Instruments. I had the chance to meet Václav and Ondřej form Bastl Instruments several times and they are just great.

So this is it. Time to patch all up and have fun.
OpenTheremin V4 boards are available at

 Modular OpenTheremin V4.