qtpy_synth

A simple QTPy RP2040-based synth to experiment with synthio.

Features

• Mono audio output circuit, converting PWM to audio, as per RP2040 design guidelines
• Optoisolated MIDI Input via MIDI TRS-A 3.5mm jack
• Two pots for controlling parameters
• One switch for controlling parameters
• Four capsense touch buttons for synth triggering
• USB MIDI in/out of course too

Software

Some programs written specifically for this board:

• hwtest - test out the hardware with a simple synth

• simpletouchsynth - play with filters using touch sensors. Uses intial cut at a "qtpy_synth" hardware library.

• dronesynth - 8-oscillator drone synth

• wavesynth - larger general two-osc synth that can also do wave-mixing and wavetables. Has many rough edges but can sound great!

  • early video demo:

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  • another video demo:

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See also:

• https://github.com/todbot/circuitpython-synthio-tricks
  • Contains many other synthio examples that can work with this synth with minimal changes

Building it

The Tindie version is a partially-assembled PCB.

The completely assembled SMD sections of the board are:

• audio output circuit
• TRS MIDI input circuit

The parts you need to add are:

• QTPy RP2040
• 0.91" I2C OLED display (w/ pins in Gnd/Vcc/SCL/SDA order)
• Two 10k potentiometers, 9mm (example pots) and knobs
• One tact switch, 6mm (example switch)
• Headers and sockets to mount QTPy & display, to taste
  • This male & female header kit looks pretty good, as I prefer to put female headers on the PCB and male headers on the QTPy and OLED display. You may need to trim down your headers to be the right size.
• Feet. I like M2.5 nylon standoffs (e.g. M2.5 standoff asst kit) but feel free to do what you like, including making an enclosure!
• Or 3d-print a simple case for it! I also like these 3d-printed knobs

Pins used

The board uses all of the QTPy RP2040 pins:

• board.A0 - left knob
• board.A1 - right knob
• board.TX - middle button
• board.RX - TRS UART MIDI in
• board.MOSI - PWM Audio Out
• board.SCL & board.SDA - I2C bus for OLED display
• board.A3, board.A2, board.MISO, board.SCK, -- touch pads 1,2,3,4

For more details see hwtest/code.py.

Schematic