JFETs in ASM-1 VCO

[Caloroso, Michael E]

> Dear All,
> 
> 	Thanks for the individual responses (Paul, Magnus, Paul, Romeo,
> 	Jorgen).
> 
> 	Tried an experiment last night by increasing the comparator the 
> 	comparator ON time by changing the recommended 18pF capacitor
> 	for a 33pF - and yes the sawtooth amplitude increased !
> 
> 	Conclusions:-
> 
> 	1.	Use a JFET with a lower Rds (ON) - will try the J112 from
> 		Siliconix
> 
> 	2.	Could increase the comparator time constant (will probably
> 		need to recalculate the series resistor value in the integrator
> 		feedback to compensate)
> 
> 	3.	Use paralleled 2N3819 JFETs - perhaps as costly as an
> 		alternative with a lower Rds (ON)
> 
> 	Is this why Moog used BJTs for discharging the capacitor in his
> 	VCOs ?
> 
There are two approaches to discharging a cap, 1) short the cap terminals ala the FET switch, or 2) apply a voltage to the charged side ala the BJT, IE if the cap charges from ground to a positive voltage, then reset the cap by applying ground to the charging end.

Some Moog synths use the E112 FET (Prodigy/Liberation), but more often they use a garden variety BJT.

MC
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Opinions (and mistakes) expressed herein are my own and not those of my employer.