[sdiy] LFO VC Skew?

Pete Hartman pete.hartman at gmail.com
Thu Feb 20 04:14:37 CET 2020

With the pitch shift, and doubling of the LFO frequency, and the shape,
suddenly I'm strongly reminded of Jaws :-D

Nice work!


On Wed, Feb 19, 2020 at 8:27 PM David G Dixon <dixon at mail.ubc.ca> wrote:

> As promised, here is a link to a YouTube video I made an hour or so ago,
> showing the voltage controlled morphing LFO I made over the last two days.
> https://youtu.be/TCQsk-6ARPM
> It's a really crappy video, but it shows the essentials.
> The basic architecture of this thing is as follows:  It is essentially the
> KORG LFO as written up on Music from Outer Space, so it's a tri-square
> oscillator with an integrator (which generates the triangle) and a
> comparator (which generates the square wave).  The output of the comparator
> is sent through an attenuator and an op-amp buffer, then through a fixed
> resistor, and then is split into two diodes pointing in different
> directions
> to separate into the upper and lower lobes.
> In the original circuit, these are connected to the tabs of a
> potentiometer,
> and the wiper of the pot is connected to the inverting input of the
> integrator.  Hence, the pot determines the waveform shape.
> It is important to note that the fixed resistor (1k if a 100k pot is used)
> ensures that the saw/ramp flyback doesn't get too fast or disappear
> entirely.  It is important that it be there.
> In my circuit, the pot is replaced by two opposing 2164 VCAs, each one
> mimicking the pot's resistive elements on either side of the wiper.  These
> are controlled by circuits which are very similar to the Irwin
> linearization
> circuit.  However (and this is the key point), where the Irwin circuit
> gives
> linear conductance VCAs, what is needed to mimic the pot are linear
> resistance VCAs.  Not only that, but the resistance of the two opposing
> VCAs
> must add up to a constant value, just like the potentiometer's resistive
> element on either side of the wiper.
> So, how does one convert an Irwin Linear-Conductance VCA to a Doc Sketchy
> Linear-Resistance VCA (AKA, a voltage-controlled pot)?  Maybe I'll tell
> y'all later.
> I think this thing is basically good to go, but I do need to add a little
> something more to limit the behavior to the points where the pot would be
> either fully CW or fully CCW.  In the video, it is obvious that if the CV
> signal is too large or Morph pot isn't well centred, then the morphing is
> allowed to go past the extreme points, and the waveform disappears.  I'll
> be
> messing around with this tonight.
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