This VCO started out life as a replacement for the original VCOs in my '73 Serge, so it replicates a lot of the functionality of that module. Of course there have been numerous enhancements to the design as well, making it right at home in a 1 volt per octave synthesizer. The core chosen is the well known Electronotes ENS-76 VCO Option 1 as used in the ASM-1, Modulus, etc. This core was an obvious choice due to the amount of information, lists of suitable substitutes and so on that are available on the web.
It has AC and DC linear CV inputs as well as 1 V/oct. inputs and scalable/reversible CV inputs. It has sawtooth and pulse/square outputs, as well as a waveshape that is variable between sawtooth and a "sine-like" shape, and a fixed "sine-like" output too. It also includes a simple sub-oscillator for those who wish to experiment with that.
Note that the "sine-like" waveform is not a pure sine wave. There is a significant glitch in it that is almost impossible to trim out. As this is the recreation of a module that had such a waveshape, this is appropriate. If you require an accurate sine wave, this module is unsuitable for your purposes.
Please also note that there have been many variations of this PCB, and I no longer have copies of the artwork of the older variations. The variations are minor. I have no doubt there will be more variations as the module continues to evolve.
A little on how it works:
The schematic of the VCO. Click here for a larger version of this diagram.
Power rail decoupling for the VCO.
Take the time to look at the ASM-1 page where you will find suggested alternates for various components such as the dual transistor and the FET.
The sub-oscillator is based on that from an LM3900 app. note (NS AN-72), and will not cover the full range available from the VCO core itself.
The very top of the circuit diagram shows the inverting/non inverting scaling mixer as used in the original Serge VCO, while the bottom left hand corner shows the Saw to "Sine" converter from the same source. Some guestimation was needed when developing these sections of the circuit, as my original Serge VCOs are potted in epoxy.
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