CEM3340 VCO (with Self-Tuning, Octave Switch & Wave Mixer)

Module features:

  • VCO with 3 wave outputs; square, sawtooth & triangle 1v / OCT CV input control
  • Mixer for the 3 wave outputs with one sole output jack to minimize front panel connections
  • 5 octave selector switch
  • Self-tune feature which covers exactly 1 octave range, accurate up to 1 cent
VCO Main Image
Now available to purchase in Eurorack format on the shop!

This module was born from the frustration of having to tune my old CEM3340 VCO every time I wanted to use it, sometimes even in the same session. I initially used the Look Mum No Computer VCO from one of his “How To Build A Modular Synth” with just the sawtooth & triangle outputs (but also FM inputs).

I’ve designed this module from the datasheet, along with some well known parts from other VCO designs, and some of my very own designs which I’ve made from scratch.

To begin, I trimmed down every part of the datasheet which I thought was unnecessary & what I’d found myself to have never used in my previous VCO. So I removed FM inputs, HF precision tuning & put in a fixed pulse width.

There’s just one (1v/oct) CV input & a built in mixer of the 3 wave forms so that I’d only need 1 output jack. No tuning knobs to adjust, and no jacks to change over if you want a different waveform, just adjust the mixer.

5 Octave Selector Switch:  Inspired by the MFOS 1v/oct octave switch which I noticed Look Mum No Computer has featured in his Kosmo Performance VCO, it is in essence, just a very accurate voltage divider. For this part of the circuit it is crucial that you select +/-0.1% resistors. You can specifically order these from the internet, but it’s cheaper (and quicker) to just measure some of the ones you have and choose 4 that are closest together. I’ve also used a similar circuit as LMNC here by using the LM4040AIZ (4.1v regulator), though I’ve adjusted it with a 68k resistor to ground to reduce the variation that VR1 gives you. 

You’ll need to adjust VR1 to give you an output of exactly 4.0v – this is the reference voltage used for the octave selector circuit & also forms the basis of the 1v reference used for the auto-tuning circuit.

Wave Mixer: For this part of the circuit, I’ve used the same idea as the 4 channel mixer module I’ve previously posted. The only differences between this one and the complete module are that I’ve removed the master gain, and the fourth channel. 

Auto-Tune VCO: This part was much easier than I’d first anticipated. The idea is to replace what would normally be an analogue potentiometer that you would adjust to tune, with a digital potentiometer (MCP41100) which is controlled by an Arduino. The arduino counts the frequency of the CEM3340, then adjusts the digital potentiometer accordingly. 

I’ve made it so that it tunes on start-up, or on a button press. All you need to do is press C once, press the button & then it’ll tune. It’s accurate to +-0.2 cents and holds its tune for as long as I can test for – though this is helped by a stable temperature within the enclosure.

You can see the video of this in action over on my Instagram account.

Stripboard Layout for the Self-Tuning VCO

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