[sdiy] Saw core JFET mystery

[Magnus Danielson]

Dave Manley wrote:
> On 3/1/2010 12:41 PM, Eric Brombaugh wrote:
>>
>> Perhaps David meant the "De Franco resistor", a technique discovered by
>> Sergio De Franco to improve the response time of JFET reset transistors
>> in VCOs? Googling turns up these documents with some details:
>>
>> http://www.oakleysound.com/vco4-doc.pdf
>> http://www.oakleysound.com/vco5-bg.pdf
>>
>> Eric
> 
> More here with schematic:
> 
> See R17: http://home.swipnet.se/cfmd/synths/friends/stopp/asm1vco-1.1.pdf
> 
> Text: http://home.swipnet.se/cfmd/synths/friends/stopp/#3.1
> 
> -Dave
> 
> P.S.  It is definitely "De Franco".  Not sure about the "Sergio".  There 
> is a UCSF professor Sergio Franco, that has published a number of times 
> on synth related circuits.  He also has a relatively good text book on 
> opamp circuit design.

That has the typical CCO-core in it... LM311 and all.

Sergio De Franco is correct.

The trick is to insert a series-resistor which will offset the voltage 
by R*I such that the reset-time equivalent voltage is pre-charged and 
then it can integrate the remaining time as intended and be spot-on 
(with relation to reset-time).

I covered it briefly on the ASM-1 homepage:
http://rubidium.dyndns.org/~magnus/synths/friends/stopp/

Cheers,
Magnus