[sdiy] using a vactrol for a voltage divider?

Tue Jul 22 03:15:10 CEST 2008

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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>>>>>
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>>>>
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