vco8 (Axxe VCO)

[Joachim Verghese]

Tom wrote:

> I was thinking about using a 3096 (2 pnp + 3 npn). That would put
> everything but the fets on the same chip, I thnk it's pretty stable
> using discrete trannys.

Sounds good.

> If I wanted to do temp-comp I think I would use the npn expo-tranny
> instead of the schmitt trigger pair. Am I wrong?

No, but there are *several* temperature sensitive points in most
VCO designs, not just the expo converter. The schmitt comparator
is one of them, although it isn't nearly as sensitive as the
expo converter.

> Now a question for you...Do you have an example of this vco that
> has a sync and/or fm option?

ARP offered a sync upgrade for the 2600, utilising two NPN transistors
and some resistors. This same circuit was included in the Solus
-- see the schematic at Synthfool:

http://www.synthfool.com/schm.html

BTW, another very stable discrete VCO design is the one in the
Oberheim SEM (schematic at Synthfool). This design uses a +18.5V
supply in addition to the standard +/-15V. Perhaps there is a
way to simplify the design -- a thing you seem to be good at
(meant as a compliment!). :-)

cheers,

-joachim