Homebrew Synth VCO

DIY Voltage Controlled Oscillator

Courtesy/Thanks to: Midwest Analog Products

Description: Read more about this VCO on Scott Stites site.

This is the first in a series of DIY projects that will start you on the road to building your own modular analog synthesizer. If you've been dismayed by the exorbitant prices analog synths fetch nowadays, you'll like what's ahead. The goal was to come up with modules offering stable, reliable performance, at a fraction of the cost of what's seen elsewhere. You might not find all the exotic or seldom-used features of the big boys, but you will get a circuit which is musically useful, can be relied upon to stay in tune and costs under twenty bucks!

Since the voltage controlled oscillator is the heart of an analog synthesizer, we'll kick things off with it. The photo above shows what the completed project looks like, and its list of features appears at the right. This circuit has been dubbed the VCO-1 to distinguish it from VCOs which might appear here later.

The VCO-1 circuitry is quite straightforward. It's based upon well worked out principles that have been kicking around for more than twenty years; be sure to see the acknowledgments below. And if you'd like to understand its theory of operation, please refer to the two guides sold by Magic Smoke Making Music With the 566 and Making Music With The 3080 OTA which explain how the exponential current source, the oscillator, the sine wave shaper, the pulse width modulator and other sections work in detail. In any event, these two handbooks should be part of your documentation since a fair amount of light is shed on how to best employ the VCO-1's features.

Sheet One, shows the current source and oscillator portions. Notice especially R8, Q5 and Q6. The two transistors are cemented together in thermal contact, along with R8 which is a +3500ppm/°C temperature compensating resistor.