Pro-One Modifications


Detailed below are some modifications I've made to my Sequential Circuits Pro-One synthesizer. In the early 1980s, I had defaced my Pro-One by adding a PAIA Veloci-Touch controller (details given in the February 1983 issue of Polyphony). I was never all that happy with the Veloci-Touch, and so I decided to replace it with something more applicable to my needs that made use of the holes already drilled in the panel. At the same time, I installed a Kenton MIDI interface (which works great) and made a small enhancement to it as well.

Modification Summary:

  • Replaced the white noise source with a new circuit that provides white, pink, red, and infra-red noise. A rotary switch selects the noise colour.
  • The new noise source can act as a modulation source in addition to its role as an audio signal. A pot sets the noise level fed to the buses. A rotary switch selects whether the noise signal feeds the Direct or Wheel modulation buses.
  • Added a Sample & Hold circuit. The new noise source is the signal input to the Sample & Hold. The LFO square wave is the clock. The Sample & Hold output can be used as a modulation source.
  • The pulse width of the LFO signal can be adjusted from 0% to 100% with a pot.
  • The LFO signal can be inverted. A rotary switch selects LFO non-inverted, LFO inverted, or Sample & Hold as a modulation source.
  • Added a triangle wave for oscillator A. A pot allows manually mixing between the triangle and sawtooth waves.
  • Filter resonance can now be used as a modulation destination. A rotary switch selects from either the Direct or Wheel modulation buses as a source.
  • The filter envelope can be inverted. Inverted and non-inverted envelopes can be applied to filter frequency and the Direct or Wheel modulation buses via two slide switches.
  • The filter frequency signal from the Kenton MIDI interface can be assigned to the Direct modulation bus. A pot sets the signal level fed to the bus.

The circuitry fits on one tiny Rat Shack board. The pots, rotary switches, and slide switches are mounted on the plastic casing between the main panel and the keyboard.


I've split the details and schematics onto four pages.

You'll need the Pro-One schematics to make sense of these mods.


The two boards near the back of the synth are for the Kenton MIDI interface. The ugly board nearest the keyboard is the Rat Shack board I used to build these mods. I had to lower the rear of the keyboard so that the contacts from the rotary switches would not touch the metal supports at the back of the keys. I did this by replacing the two rear keyboard standoffs with ones that were an eighth of an inch shorter.

I added a panel legend to the two slide switches for the filter envelope inverters, but never got around to adding a legend for the 8 pots and rotary switches. Here's a cheat sheet that I use to remember what the knobs are for. I used Letraset press on lettering to create the borders and the panel legend. (Ignore the legend along the left side, that is leftover from the Veloci-Touch.)

proone1s.jpg proone2s.jpg
proone3s.jpg proone4s.jpg

Sound Samples

Here are some sounds that make use of these modifications. A splash of reverb has been added to each track.

  • Sample & Hold - Using the new Sample & Hold to modulate filter frequency and filter resonance.
  • Triangle Lead - Using triangle waves on both oscillators with the filter wide open to create a ripping lead sound. Recognize the riff?
  • LFO Downsaw - Using the inverted LFO to modulate filter frequency with a downward sawtooth wave. Oscillators tuned a fifth apart.
  • Noise Mod - Using the new noise generator to modulate oscillator frequency (triple tracked).