Fascinated about musical analog synthesizers, I decided to make a very special one. It is an analog synthesizer based entirely on vacuum tubes. I started using a very nice project that Eric Barbour has done (web copy here) but added my taste, to extend the functionality and features of the synthesizer.
In my opinion, the most important thing that Eric's synthesizer lacks, is the polyphony and tonal range. Both are because of the VCO system used. In order to figure out how the VCO performs, I constructed it and I found that by replacing the 100K anode potentiometer of the 2D21 with a bigger value one, I could tune the VCO in a much greater tonal range. In fact I was able to tune it in the whole 5-octave range.
Eric used a single VCO do demonstrate how it works, so the synthesizer is monophonic (one note at a time). He has provided a way to sync multiple oscillators for a more "fat" and filled tone. I wanted to extend his synthesizer to a polyphonic one. There are two ways of doing this. The one is to use an X-CV-to-MIDI controller and "X" VCOs to produce a X-polyphony synthesizer. The problem with this is that the tube VCOs do not comply with the 1V/Octave scale so this cannot be easily done. My proposition, which is the other way to go, is to use 60 such VCOs, one at each note (key), to produce a 60-polyphony synthesizer without using a CV-to-MIDI controller. Using that way, all 60 keys can be pressed simultaneously without colliding the synthesizer. Also there is no need to worry about 1V/Octave or linearity of the tubes any more. Each VCO is driven by a CV produced from a keyboard controller key. At the other end, all the VCOs output audio signals are connected together into a single point, which is the common audio output. To compensate for the differences in audio volume between VCOs, a potentiometer has been added in each VCO audio output.
Such a 60-VCO synthesizer is very costly and quite difficult to construct. To minimize cost, some changes were done in the original VCO.
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