[sdiy] Yusynth VCO

[Oren Leavitt]

Neil Johnson wrote:
> Hi,
> 
> Walker Shurlds wrote:
> 
>> Compared to the ASM-1, the comparator is
>> set up differently: there's an extra diode and capacitor and it's
>> lacking a BJT connected to the base of the FET.
> 
> 
> Are you referring to the NPN implicit in the output stage of the  
> LM311?  Sometimes it is shown explicitly, even though it is actually  
> part of the comparator -  compare the original v1.0 ASM-1 VCO  schematic 
> with the later V1.1 version.  I seem to recall Magnus  pointing out this 
> little issue on the list a while back.
> 
>> Also the integrator is weird, uses the virtual 5V from the opamp in  the
>> upper left instead of using a virtual ground from somewhere.  Because
>> of this (I'm guessing), it lacks an inverter that the ASM-1 has before
>> the waveshaping.
> 
> 
> Quite a few VCO designs do this - for example the Oberheim SEM has a  
> similar arrangement.  It has the advantage of keeping an opamp out of  
> the integrator with correspondingly faster reset time - you're left  
> with a voltage buffer and a comparator (all discrete in the SEM).
> 
> Cheers,
> Neil
> -- 
> http://www.njohnson.co.uk
> 
> 

The Moog 921 VCO was also another interesting example.
The implementation of a high-speed/high impedance buffer for the timing 
cap using a CA3080 with a matched J-FET pair on the input was a clever 
work-around for the time when the 741 reigned supreme.

- Oren