[sdiy] EMS VCO expo

mark verbos markverbos at gmail.com
Tue Jul 11 16:43:09 CEST 2023


That’s a classic exponential converter from the first generation of VCOs. Q63 is acting as a zener or voltage reference and Q61 is like an inverter to make the span of voltages for the top and bottom of the pair. It’s what Jürgen Haible did in his “Living VCO”. and you can find something like it in early Buchla VCOs. Later designs use an opamp to replace that, but there’s something really fun about the instability of this discrete solution. 

Mark



> On Jul 11, 2023, at 4:22 PM, Tony Allgood via Synth-diy <synth-diy at synth-diy.org> wrote:
> 
> Can anyone explain the function of Q61 and Q63 in the exponential convertor of the EMS Synthi VCO?
> 
> The schematic for the CV summing, expo and integrator can be found here:
> 
> http://www.hinton-instruments.co.uk/ems/oscexp.gif
> 
> Many thanks,
> 
> Tony
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