[sdiy] using a vactrol for a voltage divider?
nicolas
nicolas3141 at yahoo.com.au
Tue Jul 22 03:56:40 CEST 2008
Its more of an anti-rectifier. That arrangement of diodes and op-amps takes the DC CV and makes it alternately +ve then -ve, ie it changes it to AC.
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, 1:15 PM
> thanks a million!!!! this all helps so much. NOW i know why
> that LFO had a weak triangle. I tried a lot of things to
> fix it but now i know i should start over with this asm
> method.
>
>
> i had never seen a method where you could change freq by
> feeding a rectified CV into an integrator before.
> interesting.
>
>
> i am printing out your responses and i am sure i will learn
> a lot from them. i already went through the ASM2 MAnual for
> the lfo with a highlighter.
>
> this has been a good week!!!
>
>
>
>
>
>
>
> --------------------------------------------
> 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: Mon, 21 Jul 2008 18:05:43 -0700
> > From: nicolas3141 at yahoo.com.au
> > Subject: RE: [sdiy] using a vactrol for a voltage
> divider?
> > To: synth-diy at dropmix.xs4all.nl;
> subjectivity at hotmail.com
> >
> > Just to clarify, if you leave out r557 and r559 they
> should be replaced with a direct connection to ground. If
> you leave out r565 it should be left with no connection
> (common sense?). R561 and r562 are there just to protect
> against short circuits on the output, necessary in a
> modular, but not necessary if you are hardwiring the output
> to a known destination.
> >
> > Cheers,
> > Nicolas
> >
> >
> > --- On Tue, 22/7/08, nicolas wrote:
> >
> >> From: nicolas
> >> Subject: RE: [sdiy] using a vactrol for a voltage
> divider?
> >> To: "sdiy list" , "Dan
> Snazelle"
> >> Received: Tuesday, 22 July, 2008, 12:58 PM
> >> 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
> >> wrote:
> >>
> >>> From: Dan Snazelle
> >>> Subject: RE: [sdiy] using a vactrol for a
> voltage
> >> divider?
> >>> To: nicolas3141 at yahoo.com.au, "sdiy
> list"
> >>
> >>> 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
> >>>>>
> >> _______________________________________________
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> >>>>> Synth-diy at dropmix.xs4all.nl
> >>>>>
> >>>
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
> >>>>
> >>>>
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> >>>>
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