OMS-301 One of Three VCO 

One Of Three VCO Mk1

VCO Mk1 

One of Three VCO Mk2

VCO Mk2 

VCO Front panel 

You can find the UK parts listing of VCO Mk2 in PDF form here. 

VCO Mk2 Project Page 


Pulse output (814kB) with manual manipulation of the pulse width pot. 

Sawtooth output (560kB) 

Triangle output (690kB) 

Sine output (605kB) 

Sync  (2.07MB) Example of two VCOs synced. Master tri to slave sync input. Both vcos triangle outputs mixed together. slave initially 1 octave above master. sound starts with no sync, then gradually sync goes from soft to hard. Finally the slave vco course frequency control is adjusted to produce changes in timbre. 

VCO Tuning Procedure: 

This is based on the 'Trimmers' section of the User's guide V1.4. I must say that when I built the VCO Mk1, I was a little baffled about the tuning procedure, and it took me about an hour to get the thing in tune, and even then, I wasn't sure how I'd done it. Tony updated the procedures, and included some hints to make things easier. So here is the procedure I use. It may not be the best way, but it works for me! First I hook up my keyboard to the MidiDac, then take the pitch output to the VCO V/Oct input. I use a Fluke 8060A multimeter set to read frequency on the VCO triangle wave ouput. 


1. Turn the HFT to it's minimum setting. To do this, as per the user guide,  set the VCO to play about 3kHz, and turn the HFT trimmer until the pitch starts to drop, then keep turning until the pot reaches the end of its travels. 

2. Play the lowest note on the keyboard and set the VCO COURSE and FINE controls to give 100Hz. 

3. Play a note 2 Octaves above the last and check the frequency. If it's flat, adjust the V/Oct trimmer to make it flatter still. If it's sharp, then adjust the V/Oct trimmer to make it sharper still. 

4. Play the lowest note again and readjust the COURSE & FINE controls to set 100Hz. 

5. Repeat steps 3 & 4 to give a precise 2 octave interval. 


1. Play a note 1 Octave from the top of the keyboard and set the COURSE & FINE controls to give a frequency of 8kHz. 

2. Play a note 1 octave above the last and set HFT to give 16kHz. 

3. Play the first note again and adjust COURSE & FINE controls to give 8kHz. 

4. Repeat steps 2 & 3 to get high frequency interval correct. 

Play the lowest note on the keyboard and set the COURSE & FINE controls to give 100Hz, then recheck for 400Hz, 2 octaves above this. 


1. Set the COURSE & FINE controls to middle position and trim TUNE control to match the frequency to any existing VCOs. 


Page last updated: 26th April 2003