[sdiy] LFO VC Skew?

David G Dixon dixon at mail.ubc.ca
Thu Feb 20 03:25:42 CET 2020

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.


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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