Following the success of the monophonic 'ItsyBitsy M0 Sigma-6' synth, it seemed inevitable that a  polyphonic version would be developed. You could house the parts in a small enclosure for connection to an external MIDI keyboard, or you could build the synth into the same enclosure as a keyboard. 

There is a multitude of old keyboard instruments which could be given a new life paired with a Sigma-6 Poly-synth. And the keyboard doesn't need MIDI connectivity... the Sigma-6 Poly will have provision to interface the keyboard contacts directly to the micro-controller board, via a simple interface board.

A unique feature of the Sigma-6 tone-generation scheme is its ability to modulate the signal amplitudes of six oscillators dynamically in real time, using an additive synthesis technique. Click here for details...

View/Download doc explaining the Sigma-6 synthesis method
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The "Sigma-6 Poly" synth comprises several Sigma-6 "voice modules" connected via a MIDI bus to a "Master Controller" board, the function of which is to interpret MIDI messages from a polyphonic controller (keyboard or sequencer) and allocate keyed notes to the voice modules. The Master Controller will support a front-panel user interface, so the individual voice modules will not need a control panel.

The voice module circuit can be reduced to a "bare bones" design comprising just the SAMD21 micro-controller and an audio DAC. The MIDI IN receiver doesn't need to be opto-isolated, because it is driven from the Master Controller, not from the outside world. Here is the schematic of the "Sigma-6 Poly Voice". Several of these will be combined onto a single PCB.

The Robotdyn 'SAMD21 M0-Mini' MCU board costs about $5 (USD) on AliExpress. The SPI DAC (MCP4921) costs about $3, so the parts cost for the voice modules is minimal. A complete Sigma-6 Poly-synth will be very inexpensive to build. 

You may be wondering... why not use a single high-powered micro-processor, e.g. a Raspberry Pi, to implement all voices and the master controller functions? Well, there are benefits, both technical and musical, to dedicate an independent MCU for each voice, as opposed to implementing all voices in a single high-end micro-processor. (Also, I don't like high-end 3rd-party operating systems!)

For example, the audio dynamic range is enhanced by adding the (analog) outputs from each voice using an analog mixer circuit. With 8 voices, the signal amplitude can be up to 8 times the output from any individual voice. Hence, the effective audio dynamic range is increased by a factor of 8. Sure, you could use a 16-bit DAC with a single MCU, but there are challenges to extract the full benefits from a 16-bit DAC -- not to mention the higher cost. Also, there may be more subtle benefits to using independent voice circuits, such as the tuning being slightly different in each voice.

Here's a block diagram showing a poly-synth system with 6 voices:

User Interface - Control Panel

The control panel of the polyphonic synth will have a bank of 6 slider-pots allowing the user to set the output levels of the six oscillators, i.e. to change the mixer input levels defined in the selected preset.
When any slider is moved, all six oscillator mixer input levels will be set according to the slider positions, over-riding the levels defined in the preset last selected.

The sliders provide similar functionality to the "draw-bar" mechanism in Hammond organs.

To facilitate fast change of preset during a live performance, a row of 8 push-button switches will allow selection of a "Favorite Preset". The 8 "favorites" will be user-configurable from a greater number of instrument presets stored in flash memory. The switches have an LED to indicate the selected preset.

A graphic OLED display, with 3 buttons and a 'Data Entry' pot, will provide various other user-interface functions, e.g. to set up configuration parameters, to select a preset (other than favorite) and to adjust active patch parameters. Further down the track, the firmware will be extended to provide 'User Presets', i.e. the ability to save and load the active patch as a user-defined preset (stored in EEPROM).

Below is a schematic of the Master Controller board. This sketch doesn't show connections to the front-panel controls and indicators, OLED display, etc. These connections are shown on the next sketch following.

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... to be continued ...

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Printed Circuit Boards

Project collaborator and DIY synth enthusiast Jean-Pierre (in France) has already designed the PCBs.
Here's a picture of his "Poly-6 Voice" board...

Using 2 of these boards, you can build a poly-synth with 8, 9, 10 or 12 voices.

And here's a pic of the "Master Controller" board...

(Gerber files will be made available as soon as the designs are verified.)

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Last update: August 2025
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