[sdiy] SAW core VCO flyback time

[Tim Ressel]

When talking about the traditional ASM1 style saw core there are several 
areas of concern. The one under discussion here is the tracking.  I'm 
not sure one needs 10+ octaves of tracking when most keyboards are 5 
octaves or less, but there you go. The next is temperature stability. 
Most designs I see use a temco resistor to compensate. I prefer the 
AN299 solution. Then there is the op amp wig out issue. This happens 
when the cap across the op amp gets shorted to ground and the op amp 
loses its mind. This is usually fixed with a series resistor.

An alternate topology is the one used in the Elektor Formant synth. 
Instead of a cap across an op amp, it uses a constant current source to 
charge the cap. I think this would work better for minimizing retrace time.

Of course, Ian's triangle core design has got me thinking about going 
that direction. Eliminates (?) the retrace issue.

One thing I want to do is characterize the two parts of the VCO 
separately: the ramp generator and the expo convertor. This will take a 
bit of work, but with some accurate measuring gear it should be doable.

--TimR


On 8/30/2016 2:41 PM, Donald Tillman wrote:
>> On Aug 30, 2016, at 10:40 AM, mskala at ansuz.sooke.bc.ca wrote:
>>
>> On Tue, 30 Aug 2016, Richie Burnett wrote:
>>> BTW, I think it's fascinating how the analogue designers strive to achieve
>>> perfection in the form of "instantaneous" sawtooth resets, low-distortion
>>> sinewaves, and symmetrical triangle waveforms without the little nipple at the
>> I was holding off on saying it, but if you're serious about analog design,
>> do you even *want* a super-fast reset?
> The high C on a traditional piano is 4186 Hz.
>
> A 2uSec reset delay would pull that down to 4152 Hz, for a 35 Hz beat.
>
> A 7uSec reset delay would pull it down to 4066 Hz, halfway down to the next lowest note.  That would be unplayable.
>
> The effect is worse when you're playing along with stringed instruments with stretched turning.
>
> Besides the capacitor discharge, the delay would include the comparator sensing the limits.
>
> So the concern about reset time is not for the timbre.  I mean, hell, sawtooth waves are so boring already that any timbral variation would be welcome.  Amiright?
>
> The ARP/Rhodes Chroma VCO had a unique charge pump VCO core that had a long reset time, but the reset didn't introduce a tuning delay in the cycle.
>
>    -- Don
>
> --
> Don Tillman
> Palo Alto, California
> don at till.com
> http://www.till.com
>
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-- 
--Tim Ressel
Circuit Abbey
timr at circuitabbey.com