[sdiy] Aries 317 VCO

[Tom Wiltshire]

On 30 Mar 2011, at 07:58, Donald Tillman wrote:

> On Mar 29, 2011, at 11:01 PM, <lanterma at ece.gatech.edu> wrote:
> 
>> See here: http://synthfool.com/docs/Aries/AR317s.gif
>> 
>> It's pretty interesting! It looks like it has a triangle core, with a 3080 OTA dumping current into a cap - but it gets kind of weird from there. There's a second OTA and a weird NPN/PNP (Q5/Q6) pair that I, uhm, think is acting as a comparator, but I'm not sure. 
>> 
>> Has anyone dug into this? Does it have pros and cons?
> 
> 
> Hey Aaron,
> 
> It's arguably a dual core, a triangle main core and a second sawtooth core sync'd to the triangle. 
> 
> U2 is the bipolar current drive for the triangle core, U4/Q5/Q6 is the triangle core comparator, which also triggers Q7/Q8, the reset circuit for the sawtooth.
> 
> It has the advantages of a triangle core (better tuning because there's no retrace, and you don't have to derive the triangle waveform from a sawtooth) and at the same time he doesn't have to derive a sawtooth from a triangle and deal with a glitch at the zero crossing.

Hohoho! What a great circuit! Let's build *two* VCOs instead of one, because each of them only does one waveform well!
Nice find, Aaron - one of those "you won't believe it till you see it" ones.
T.