DIY 4-Voice Europoly overview

I'm setting this blog up to document the work I'm attempting on a DIY 4-voice Eurorack keyboard synth. It's early days yet and all I have is a plan, a 3D model in Sketchup and the beginnings of the Arduino code.

The keybed will be a 44 key Fatar TP9S which I've ordered on ebay. The brain that will scan the keybed and output CV and gate will be an Arduino Due. I chose the Due as it has a lot of ins and outs and so will be expandable later if I want to add features to the electronics. The Due will be outputting gates on its digital pins and CV via an Analog Devices AD5628 Digital to Analog Converter. Running the AD5628 at 5V will give me 5 octaves of CV output, which means the 3 1/2 octave keyboard will be able to be transposed one octave.

Four DAC channels will be used for CV initially while the other four will be left unused. In time I intend to use the other four for a software envelope output per voice, but that will not be part of the initial spec.

The two DACs on the Due will also be used for modulation outputs in time (most likely a quantised sequencer output) but at this stage will remain unused. These outputs run at a max of 3.3V which limits their usefulness.

Voice modes under consideration are Poly 4, Poly 3 (to leave a voice free to be sequenced), Unison 2 voice (A&C, B&D), Unison. A Poly 3 + Sequence mode could be written which would enable the keyboard to be used to input notes to the sequence when a button was held down, when the button was released the sequence would play and the keyboard would play the remaining three voices. Alternatively eight potentiometers could be used, hooked up to eight analog input pins for a simple sequencer like the Oberheim Two Voice.

Physically the synth will be constructed of 12mm marine ply covered in tolex and will have two rows of 150HP vector rails for mounting of Eurorack modules. At this stage 4x Pittsburgh Modular Synthesizer Blocks are envisaged for the core voices, with additional modules to be added later for additional functionality. A key aim for this synth is to have stereo outputs with stereo spring reverb. Voltage controlled panning of individual voices is also an aim, particularly for use with a quadrature LFO for 90 degree out of phase swirling voices.