[sdiy] Yusynth VCO

[Neil Johnson]

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