[sdiy] LFO VC Skew?

David G Dixon dixon at mail.ubc.ca
Sat Feb 22 21:58:56 CET 2020


Hey SDIY Team!

So, after a night of hospitality at Vancouver General Hospital's Emergency
Department to treat my extreme nausea and dehydration, I was able today to
make all the planned changes to my stupid morphing LFO.  Here's what I did:

1) I rearranged, and kludged a couple of diodes to, each of the control
opamps to limit the output of those opamps to 5V.  This prevented loss of
the signal at the extremes of CV.

2) I changed resistors on the VCA linearizing opamps to mimic a 3k fixed
resistor into a 100k pot (5V into 1M), instead of the 1k resistor I was
mimicking (5V into 3M).  This softened the transition to pure saw or ramp.

3) I decreased the integrator capacitor from 332 to 681 to speed up the LFO.

After making all these changes, I think I'm done, and here is my official
assessment:

While this might be great for morphing an LFO, it is probably not terribly
useful for morphing a VCO.  The problem is that, when the waveform
approaches the saw or ramp, it "snaps in" to that, and in the process gets
slightly bigger and droops by about a whole tone.  This has nothing to do
with the VCA circuit per se, I don't believe.  It is just the nature of the
beast.  I looked at my existing morphing LFOs which just have pots, and they
did a bit of the same thing -- not as extreme, but enough to render them
out-of-tune if they were VCOs.  I could probably play with all the values
and the gains and the cap and make it a little better, but I don't believe
that this circuit will ever be able to transition from saw to tri to ramp
without sudden changes in frequency at the extremes.  Which is really too
bad, because every other aspect of its performance is perfect.  The saw and
ramp are really clean and sharp.

One way around this (partly) would be to put in an even larger faux fixed
resistor (decrease the 1M resistors to something even smaller, like 499k).
That would make the transition to saw or ramp less hard, but eventually it
won't reach saw or ramp, and if it doesn't get far enough (i.e., if the
flyback part of the saw doesn't get vertical enough) then there is little
timbral change (not enough overtones) and it becomes a bit useless.

I guess the question is, is there a compromise where the endpoints are
"sawey" or "rampey" enough, but not so hard that they pull the waveform into
a lower frequency?  Maybe I'll have one more go, just to see where that
critical transition is.  Maybe I'll be scientific about it and compare the
endpoint frequencies with the triangle (midpoint) frequency as a function of
the fixed-resistor-mimicking resistor value.



-----Original Message-----
From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of David
G Dixon
Sent: Thursday, February 20, 2020 8:11 PM
To: 'Jason Proctor'
Cc: 'SDIY List'
Subject: Re: [sdiy] LFO VC Skew?

So, after I made that video, my gastritis/duodenitis/pancreatitis flared up,
and I got extremely nauseous, so all I could do was sit in my easy chair and
stare at the TV.

I felt a little bit better today, but still crappy.  In any case, I have
added four diodes to the control circuit in my simulation, and can state
with confidence that this will eliminate the loss of signal at the extremes,
even if the CV signal is too big.

My plan is to kludge these diodes onto my board (which will be a challenge,
because I made the layout rather tight) and confirm, and make a new video.
Hopefully, I'll feel well enough to do that tomorrow.



-----Original Message-----
From: David G Dixon [mailto:dixon at mail.ubc.ca] 
Sent: Wednesday, February 19, 2020 6:26 PM
To: 'David G Dixon'; 'Jason Proctor'
Cc: 'SDIY List'
Subject: RE: [sdiy] LFO VC Skew?


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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