[sdiy] using a vactrol for a voltage divider?

[nicolas]

In a standard triangle core LFO there are three main ways the freq is controlled. 

1.  The output of the schmitt trigger goes to the input of the integrator via a variable resistance (ie the R of the RC can be variable).  This variable resistance is normally a pot, but could be easily replaced with a vactrol.

2.  The output of the schmitt trigger goes through a voltage divider to ground and this variably reduced voltage goes to the input of the integrator via a fixed resistance.  This voltage divider is normally a pot, but may be what you are talking about replacing with a vactrol+resistor.  Probably easier to do 1 above instead.

3.  The freq control is as per a VCO, where the output of the schmitt trigger is used as a +/- multiplier to alternately flip the sign of the incoming control voltage which then goes through a fixed resistance to the input of the integrator.  This allows voltage control which is more instantaneous and more linear than any vactrol based method.

So unless you really want the slow response and non-linearities of a vactrol, method 3 is probably the best way to do it.  The VCLFO in the ASM-2 is an example which can be conveniently built from a TL074 if you ignore the offset trimmer on the integrator and leave out the LED driver stuff.

http://www.elby-designs.com/asm-2/lfo/lfo2-asm2-cct.pdf

Cheers,
Nicolas



--- On Mon, 21/7/08, Antti Huovilainen <ajhuovil at cc.hut.fi> wrote:

> From: Antti Huovilainen <ajhuovil at cc.hut.fi>
> Subject: Re: [sdiy] using a vactrol for a voltage divider?
> To: "Dan Snazelle" <subjectivity at hotmail.com>
> Cc: synth-diy at dropmix.xs4all.nl
> Received: Monday, 21 July, 2008, 3:00 AM
> On Sun, 20 Jul 2008, Dan Snazelle wrote:
> 
> > or is there a better way to approach changing the
> pitch of an lfo.
> 
> Yes. A normal triangle core VCO should work fine as an LFO.
> Chances are that the voltage divider even simulates a
> current source, so 
> replacing it with a voltage controlled current source would
> be rather 
> trivial.
> 
> Antti
> 
> "No boom today. Boom tomorrow. There's always a
> boom tomorrow"
>    -- Lt. Cmdr. Ivanova
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