Doboz VCO340

The Doboz VCO340 is a DIY VCO built around the CEM3340. This is a VCO on a chip that was used in many famous synthesizers back in the 80s. Here’s what the Electric Druid has to say:

The 3340 is the oscillator chip found in many of the famous polysynths of the analog era; the Sequential Prophet 5 and T8, the Oberheim OB-Xa and OB-8, the Roland Jupiter 6 and MKS-80, and even the MemoryMoog. It also turns up in rarer synths like the Crumar Spirit and the Synton Syrinx.

Electric Druid

Here is a demo:

There are several Eurorack modules built around the CEM 3340. The Doboz VCO 340 looks well designed and offers a few features the others don’t in a neat and compact package.

My plan was to build two of these and have most of a complex oscillator in 16hp. The plan is to put this in my battery powered case.

The DIY Doboz VCO340 comes as two PCBs and an aluminum front panel. Everything was packed in a padded envelope and the boards were individually wrapped in plastic bags. Shipped on 8/1/2022 from Slovakia and received 8/8/2022 in the USA.

The parts are all through hole. Most are common and easy to get. There are a few that are harder specialty items. There is a bill of materials with a full list of parts and part numbers from suppliers.

I ordered most of the parts from They had 90% of what I needed but a couple things had to be back ordered and won’t be available until November! 🫤

The original CEM3340 chips are available. There are also some newer chips like the AS3340 and V3340. Seems these are very close but there are some subtle differences mostly related to the PWM output.

Here is a video talking about the differences in newer chips and original CEM chips:

While we wait here are some resources:



Video Demos






2 responses to “Doboz VCO340”

  1. DJ Soulfeggio Avatar

    This is cool. I was unaware of this module. On my bench I have a Kassutronics VCO, an AISynthesis VCO and already built GGS. I was actually looking for one that has an octave knob with detent, is that what SW1 is?

    1. admin Avatar

      I have not heard of those VCOs you mentioned I’ll check them out. I’m becoming a fan of classic VCOs. I also built the Dannysound EN129 Thru Zero VCO

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