[sdiy] LFO VC Skew?

[Michael E Caloroso]

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.
>> >
>> > _______________________________________________
>> > Synth-diy mailing list
>> > Synth-diy at synth-diy.org
>> > http://synth-diy.org/mailman/listinfo/synth-diy
>>
>> _______________________________________________
>> Synth-diy mailing list
>> Synth-diy at synth-diy.org
>> http://synth-diy.org/mailman/listinfo/synth-diy
>>
>
>
> --
> [image: QMA]
>
> Quincas Moreira
> Director | QMA
> mobile:  5534988825
> site:  quincasmoreira.com
> email:  quincas at gmail.com
>