VCO 3340

The CEM3340 VCO chip is one of the legendary chips designed by Doug Curtis that played an important role in the analog polysynths of the 1980s. By integrating a complete analog VCO on a chip, it made more compact and cost-effective synth designs possible.

Together with the classic analog polysynths themselves, the Curtis chips have long been out of production. In 2018 however, thanks to the new boom in analog synthesis, the 3340 VCO chip is once again available, from no less than three sources. Curtis' own company OnChip Systems re-issued the CEM3340 Rev. G, and cloned/compatible devices are available from CoolAudio (v3340) and from the Latvian company Alfa (AS3340).

The new availability of these chips triggered my interest in building a VCO around these chips. I chose to use the AS3340, which is the cheapest of the three options. The 3340 VCO is known for good tracking and temperature stability. It has the exponential converter and temperature compensation built in, which makes it very easy to build a decent VCO. For the module, I wanted to stick with that idea: an easy to build VCO with good stability and basic features. After a few revisions, the result looks like this:

Features

• Pulse, Saw, Triangle and Sine outputs 
• Big frequency control + fine-tune control
• PWM control, serves as attenuator for PWM input or as pulse width control when no PWM is connected 
• Exponential (V/Oct) and linear FM inputs
• Switchable hard and soft sync
• 9mm pots, Thonkiconn jacks and all throughhole components on a single PCB 
• 8hp Eurorack format

About the design

The 3340 chip is a triangle core oscillator, and in addition outputs saw and pulse wave forms, the latter with PWM. The waveform outputs levels are not standard eurorack levels and all different, so external scaling is needed to bring them to the standard 10Vpp.

Since it is a triangle core oscillator, it is a great starting point for glitch-free sine waves. I added the same JFET-based triangle-to-sine converter that I use in my other VCOs.

The 3340 chip has some unusual implementations of "hard" and "soft" synchronization. They give slightly different results, and the results also depend on the amplitude of the sync signal and whether it has positive, negative or both positive and negative transients. To keep the module simple, I decided to expose the sync inputs (switchable between hard and soft) directly via only an AC coupling capacitor. The user can experiment with different transients by feeding a saw, inverted saw or square/pulse wave to the sync input. The amplitude can also be tuned if desired with an external attenuator.

The module has exponential and linear FM inputs, that can also best be used with external attenuators. For Pulse Width Modulation an attenuator is integrated into the module. If no PWM signal is plugged in, the PWM control simply sets the pulse width manually.

Build documentation

I offer the PCB for this module for sale. The build documentation below contains everything you need to know to complete a successful module. Please read it through carefully before starting, and especially check the notes in the Bill of Materials when ordering parts.

• All documentation can be found on github.

Demos

Here is a short demo of the basic functions of the VCO. There is some clipping or other issue in the audio track, so it sounds grittier than in reality:

Molten Music Technology has made a much nicer demo of the VCO 3340:

He also has a complete build video of this module, based on the kit sold by Infinite Machinery.