[sdiy] using a vactrol for a voltage divider?

Tue Jul 22 02:58:14 CEST 2008

Hi Dan

The LFO you linked to uses a fourth method which is that the pot changes the switching voltage of the schmitt trigger.  This works fine if you are mostly interested in the square output, but isn't so good if you want the triangle output as the amplitude changes with frequency.  Not the usual way to make an LFO.

Your questions:

1.  Look at the labels - u552 is the integrator, u551c is the schmitt trigger.  Changing r560 will affect the amplitude of the triangle output and affect the frequency, but is not the best way to do it.  If you want to stick in a vactrol replace r558.

2.  Yes pot p553 is the voltage divider I mentioned originally.

3.  Multiplication of arbitrary voltages and waveforms involves OTAs, but square wave multiplication can be done with a couple of diodes - d551 and d552.

If you want to simplify the asm2-lfo, these bits are not necessary and can be left out:
u551d (the four remaining op-amps can be in a single chip)
j552, d553, r564, p555, r563
p554, r556, r557, r559

Depending on how much control you want over the frequency, you may wish to leave out either p551 or p552 or p553.  You only need r562 and r565 if you want the square amplitude to match the triangle.

That leaves a bom of 1 IC, a couple of caps, a couple of diodes, a couple of pots and about 10 resistors.

OR ..................

If you want to go the vactrol route, leave out:
u551a, u551b, u551d (the two remaining op-amps can be in a single chip)
j552, d553, r564, p555, r563
p554, r556, r557, r559
p551, p552
r551, r552, r553, r554, r555
d551, d552

And replace r558 with the vactrol LDR.

r556 and p553 are optional, if you leave them out, then just connect the vactrol LDR as though it were replacing both r558 and r556.

Now you are down to something as simple as your current circuit, but which will work much better for triangle purposes.  Check out the MFOS soundlab LFO.  We have just simplified the ASM2 LFO to the point where it is now much the same as the soundlab one.

Cheers,
Nicolas

--- On Tue, 22/7/08, Dan Snazelle <subjectivity at hotmail.com> wrote:

> From: Dan Snazelle <subjectivity at hotmail.com>
> Subject: RE: [sdiy] using a vactrol for a voltage divider?
> To: nicolas3141 at yahoo.com.au, "sdiy list" <synth-diy at dropmix.xs4all.nl>
> Received: Tuesday, 22 July, 2008, 1:08 AM
> this is some great information!
> 
> i just might switch to the ASM2 lfo. it does look a little
> bigger than what i am using though.
> 
> i have a few questions: ( i will use the ASM-2 schematic as
> reference)
> 
> 1. you said " The output of the schmitt trigger goes
> to the input of the integrator via a variable
> resistance"
> is the schmitt trigger U552a and the integrator u551c? so i
> could use r560 as a pot?
> 
> 2. is this p553?
> 
> 3. how does 3 work? i dont see any OTA's here so i am
> interested in understanding this method.
> 
> the LFO design i was working with didnt have number 3 so i
> figured a vactrol was the only way.
> 
> HERE IS A LINK to what i have on my board. any ideas for
> adding VC?
> 
> http://www.diystompboxes.com/smfforum/index.php?topic=68582.msg548699#msg548699
> 
> (its a redraw of a roland lfo from their modular system i
> think)
> 
> i had to mess with the resistor values a lot to try to get
> the triangle louder.
> 
> 
> THANKS
> 
> 
> 
> --------------------------------------------
> check out various dan music at:
> 
> http://www.myspace.com/lossnyc
> 
> 
> http://www.soundclick.com/lossnyc.htm
> 
> 
> http://www.indie911.com/dan-snazelle
> (or for techno) http://www.myspace.com/snazelle
> 
> 
> > Date: Sun, 20 Jul 2008 17:17:07 -0700
> > From: nicolas3141 at yahoo.com.au
> > Subject: Re: [sdiy] using a vactrol for a voltage
> divider?
> > To: synth-diy at dropmix.xs4all.nl
> >
> > 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  wrote:
> >
> >> From: Antti Huovilainen 
> >> Subject: Re: [sdiy] using a vactrol for a voltage
> divider?
> >> To: "Dan Snazelle" 
> >> 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
> >> _______________________________________________
> >> Synth-diy mailing list
> >> Synth-diy at dropmix.xs4all.nl
> >>
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
> >
> >
> > Start at the new Yahoo!7 for a better online
> experience. www.yahoo7.com.au
> >
> > _______________________________________________
> > Synth-diy mailing list
> > Synth-diy at dropmix.xs4all.nl
> > http://dropmix.xs4all.nl/mailman/listinfo/synth-diy

      Start at the new Yahoo!7 for a better online experience. www.yahoo7.com.au