Introduction: Digital Synth VRA8-Px for Arduino Uno

Made by ISGK Instruments


  • 3 Voice Paraphonic Synthesizer for Arduino Uno
  • A Variant of Digital Synth VRA8-P


  • 3 Voice Paraphonic Synthesizer (Pseudo Polyphonic Synthesizer), MIDI Sound Module
  • Serial MIDI In (38400 bps), PWM Audio Out (Pin 6), PWM Rate: 62500 Hz
  • Sampling Rate: 15625 Hz, Bit Depth: 8 bits

Demo Audio


  • Exhibited in Ogaki Mini Maker Faire 2016
  • Exhibited in Maker Faire Tokyo 2017, Analog Synth Builders' Summit 17

VRA8 Series

Step 1: Wiring

Required Hardware

  • A1: Arduino Uno
  • U1: 3.5 mm Audio Jack
  • R1: 150 ohm Resistor
    • or 140 ~ 160 ohm Resistor (e.g. 100 + 47, 100 + 27 + 27)
  • R2: 100 ohm Resistor
  • C1: 100 nF Capacitor
  • Wires


  • See the image

Step 2: Software Installation

  1. Download Source code and extract it.
  2. Download Hairless MIDI<->Serial Bridge and extract it.
  3. Download loopMIDI and install it.

Step 3: Start Synthesizer

Explanation for Windows

  1. Write DigitalSynthVRA8Px.ino to Arduino Uno and quit Arduino IDE.
    • CAUTION: Use Arduino IDE 1.8.1 or later.
  2. Start loopMIDI.
  3. Start hairless-midiserial.exe (Hairless MIDI<->Serial Bridge).
    • Set [File] > [Preferences] > [Baud rate] to 38400 bps.
    • Select Arduino Uno (COM*) on Serial Port.
    • Select loopMIDI Port on MIDI In.
  4. Open vra8-px-ctrl.html (VRA8-Px CTRL) with Google Chrome.
    • Select loopMIDI Port on MIDI OUT.
    • Select a MIDI controller on MIDI IN (if you have it).
  5. Click (or touch) the Software Keyboard, and you can hear the sound.


  • Click sounds may occur when you connect the audio out to an amp/a speaker or reset the board
  • Click sounds may occur when you change the controllers (especially AMP EG and FILTER CUTOFF)
  • Low FILTER CUTOFF with high FILTER RESO can damage the speakers
  • The Arduino PWM audio output is a unipolar LINE OUT
    • Please connect this to a power amp/a headphone amp (not to a speaker/a headphone directly)