[sdiy] LFO VC Skew?

Wed Feb 19 12:59:54 CET 2020

I just checked it out. Sadly, it has a skew control, but not voltage control.
Cute balanced modulator made from a 3360 tho.

Mark

> On Feb 19, 2020, at 6:25 AM, Michael E Caloroso <mec.forumreader at gmail.com> wrote:
> 
> The Crumar Spirit has a VC'd skew LFO.  Schematics are online somewhere.
> 
> MC
> 
> On 2/18/20, Quincas Moreira <quincas at gmail.com> wrote:
>> The Multiwaves has one digital LFO/VCO and two analog LFOs, all of which
>> have the VC tilt function. However, the analog ones do NOT preserve the
>> frequency. I still find them useful anyways. The digital one does preserve
>> the frequency, and in VCO mode that makes for some beautiful faux filtering
>> sounds, when you tilt that sine, cosine or triangle wave...  I'l ask him to
>> come pitch in here ....
>> 
>> On Tue, Feb 18, 2020 at 5:54 PM David G Dixon <dixon at mail.ubc.ca> wrote:
>> 
>>> Well, the circuit is pretty simple.  However, there is one major snag,
>>> which
>>> I will only discover upon trying it...
>>> 
>>> The maximum stated (amplification) gain of a 2164 VCA is 22dB.  My
>>> circuit
>>> requires 40dB of gain.  Even at that gain, the VCA only has to process
>>> about
>>> 133uA of current, so it should be within the realm of possibility if the
>>> gain limitation is really a limitation on output current (which I was led
>>> to
>>> believe was closer to 300uA) and not on actual multiplication factor.  If
>>> the gain is truly limited to 22dB, then the saw/ramp flybacks are going
>>> to
>>> be fairly slowish -- possibly still useful, but...  If I can actually
>>> squeeze 40dB out of the beast, then we're in business.  Either way, I'll
>>> have a test circuit finished before I go to bed.
>>> 
>>> If it works well, then yes, I will give Danjel van Tijn first dibs on
>>> taking
>>> the design for Intellijel (which he has already suggested he may want to
>>> do)
>>> and will not be sharing the tricks with y'all (sorry -- my loyalties are
>>> clear).  I won't discourage you from figuring it out yourselves though,
>>> based on the hints I've given.
>>> 
>>> Of course, if the 2164 only gives me 20dB and I need 40dB, it's tempting
>>> to
>>> just string two 2164s together sharing the same VC voltage.  However,
>>> this
>>> won't work.  It will increase the frequency towards the middle of the
>>> morph
>>> (where the triangle lives).  Because I'm working with logarithmic math,
>>> the
>>> arrangement is pretty darn unforgiving.
>>> 
>>> 
>>> -----Original Message-----
>>> From: Jason Proctor [mailto:jason at redfish.net]
>>> Sent: Tuesday, February 18, 2020 1:48 PM
>>> To: David G Dixon
>>> Cc: Tom Wiltshire; SDIY List
>>> Subject: Re: [sdiy] LFO VC Skew?
>>> 
>>> Will this mean a fancy new Intellijel LFO? :-)
>>> 
>>> fwiw, always liked the waveshaping on the venerable MOTM-320 LFO. Verrr
>>> nice.
>>> 
>>> 
>>> On Tue, Feb 18, 2020 at 12:54 PM David G Dixon <dixon at mail.ubc.ca> wrote:
>>>> 
>>>> Ugh, gawd...   Mine's gonna rock.  I'm gonna put the finishing touches
>>>> on
>>> the design in about 15 minutes, once the fentanyl wears off just a little
>>> bit more (although it didn't prevent me from practicing a Haydn sonata
>>> just
>>> now).
>>>> 
>>>> 
>>>> 
>>>> ________________________________
>>>> 
>>>> From: Tom Wiltshire [mailto:tom at electricdruid.net]
>>>> Sent: Tuesday, February 18, 2020 4:24 AM
>>>> To: David G Dixon
>>>> Cc: ackolonges fds; SDIY List
>>>> Subject: Re: [sdiy] LFO VC Skew?
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> On 18 Feb 2020, at 02:37, David G Dixon <dixon at mail.ubc.ca> wrote:
>>>> 
>>>> 
>>>> 
>>>> It's easy to get saw-to-tri-to-ramp with VCAs, but the trick is to
>>> maintain the same frequency when you do so.  That's the impossible part.
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> Many of the same problems you have when doing it analoguely (that's the
>>> equivalent of "digitally", right?!) turn up in the digital version.
>>>> 
>>>> 
>>>> 
>>>> I used phase distortion to produce the modified waveforms. Essentially,
>>> you have a ramp core (an NCO) and you apply a wave shaper. To get the
>>> phase
>>> distortion, you run the first half of the NCO's cycle at a different
>>> frequency to the second half. To avoid frequency wobble when altering the
>>> shape, the two sets of frequency increments have give the same overall
>>> frequency as one steady increment. This means they needs to be pretty
>>> accurate (how accurate depends on audio versus LFO and how much you
>>> care).
>>>> 
>>>> 
>>>> 
>>>> This accuracy requirement also gets worse as you get to the extreme
>>>> ends
>>> (because you finish up with a division that approaches division by zero).
>>> Using integer math, you reach a point where the results overflow the
>>> available accuracy. For this reason, I limited the Distort CV on my chips
>>> to
>>> 4% to 96% duty cycle. This means you *can't* get a really vertical saw
>>> waveform edge if you start by distorting a triangle wave.
>>>> 
>>>> 
>>>> 
>>>> It's funny how the difficulties remain the same, even using a
>>>> completely
>>> different method and technology.
>>>> 
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>> 
>> 
>> --
>> [image: QMA]
>> 
>> Quincas Moreira
>> Director | QMA
>> mobile:  5534988825
>> site:  quincasmoreira.com
>> email:  quincas at gmail.com
>> 
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