CS-80 style VCO

[elektrouwe]

here is a schematic for a CS-80 style VCO. It is only a LTSPICE simulation until now. A guy asked for help on a german forum ( http://www.sequencer.de/synthesizer/viewtopic.php?f=13&t=78218 ) and I found it interesting to help him, because Yamaha used a closed loop VCO in their Chip. Although closed loop VCOs typically have better linearity than our "bread & butter discharge cap. with switch"-VCOs they are not popular in music electronics, because the rising ramp has a slope with fixed rise time and the amplitude is not constant over CV-range.
Rise time at low freq. does not play a role, but for frequencies in the kHz range , waveform changes towards an asymmetrical triangle. I guess that is the reason why Yamaha added a waveform shaper that simply masks the rising slope with a constant amplitude "punch" from the closed loop one-shot.
the short (30us ?) punch does not add audible harmonic content and, I guess again, is "grinded" away in the (bandwidth limited) signal path.
Please comment

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CS-80_VCO.jpg
Description:	CS-80 style VCO schematics
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[steffensen]

I have nothing to add here, only that im glad someone finally picked this up. Will follow with interest!

[keyman2]

Looks deceptively simple...What would be required to make this a 'practical circuit' in terms of compensation for temperature or other variations which might alter tuning/performance?