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<p>Those look like fun!</p>
<p>I recall an Electronotes article about a simple circuit that
takes in a +- 5V sawtooth and outputs a voltage controllable (-180
to +180 I think) degree phase shifted sawtooth.</p>
<p>I had breadboarded it one time. It uses a comparator/level
shifter/mixer combo to split the original signal and recombine it
as a phase shifted version. Works well for its simplicity. The
only limitations are a small glitch in the output, and that the
input has to be a sawtooth at a set amplitude.</p>
<p>It does also become an interesting "timbre modulator" with other
signals fed into it.</p>
<p>- Oren<br>
</p>
<div class="moz-cite-prefix">On 5/22/24 8:58 AM, jslee via Synth-diy
wrote:<br>
</div>
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<div>Devin Weston has been been selling his “PA0” VCO for a few
years now. they are quite fun.<br>
</div>
<div><br>
</div>
<div><a href="https://www.westonaudio.com/pa0.html"
moz-do-not-send="true" class="moz-txt-link-freetext">https://www.westonaudio.com/pa0.html</a><br>
</div>
<div><br>
</div>
<div>My pair of these were the first “serious” VCOs in my modular,
and are still my favourites. Wish they’d appeared several years
earlier!<br>
</div>
<div><br>
</div>
<div>John<br>
</div>
<div><br>
</div>
<div><br>
</div>
<div>On Mon, 20 May 2024, at 09:09, Tim Parkhurst via Synth-diy
wrote:<br>
</div>
<blockquote type="cite" id="qt" style="">
<div dir="ltr">
<div>Hi All, <br>
</div>
<div>Here's a weird little something I came up with several
years ago that I think might be fun. It uses two VCO cores
and a special synchronization circuit that allows voltage
control of the phase between the two cores while they track
at the same frequency. It's something I haven't seen before,
so I'm thinking it's a super-special invention I came up
with. In any case, that's my story, and I'm sticking to it.
The theory below is a bit of a read, but I've got a little
more documentation to go with it if anyone is really
interested. <br>
</div>
<div>============================<br>
</div>
<div>
<div>
<div>
<div>
<div>
<div>
<div>
<div>
<div>
<div>
<div>
<div>The VCPO features two triangle core
oscillators in a setup that allows one
core to drive the second. The output
from the second core is variable in
phase relative to the first. Let's see
if I can give it a decent
explanation...<br>
</div>
<div>===<br>
</div>
</div>
<div>The circuit uses two triangle core
VCOs. It derives a signal from the first
core (call it A) and uses that to drive
the second core (B). In the schemo here,
I'm using two VERY simple 2164 VCOs, but
the same sync / drive scheme could be
applied to other triangle cores. This
particular setup doesn't quite track 1
v/oct, but that is just because I used
the simplest setup I could come up with
for this first shot. With some
adjustments and component tuning, I
think it could be made to track just
fine. As an LFO though, it works nicely.
You could probably build one of these
that tracked pretty well with a
13700-based core, or a slightly more
elaborate 2164 circuit.<br>
</div>
<div><br>
</div>
</div>
<div>Core A is based around U5, U6, and U7.
U1 and U2 form the CV summer. The CV
drives U6 (Core A) and U6 (Core B). Core A
is just a basic, plain vanilla tri core,
with U5 acting as the comparator, and U7
acting as the integrator.<br>
</div>
<div><br>
</div>
</div>
<div>Core B is based around U8, U6, and U9.
Core B is also fairly standard, BUT NOTE
that there is no hysterisis feedback around
the comparator, U8 (Core A uses R10). This
is important, because the comparator gets an
input signal that is a combination of the
Core B integrator (through R21) and the Core
A integrator (through R20). This is a big
part of what keeps the two cores locked to
the same frequency.<br>
</div>
<div><br>
</div>
</div>
<div>Now here's the "secret sauce:" U4 and Q1
change the polarity of the reference voltage
going to the comparator of Core B. Normally,
if you have a triangle going to a comparator
input and you vary the reference voltage,
you'll get pulse-width modulation. However,
what we do here is use Q1 and U4 as a sign
flipper, so that the reference voltage to the
comparator SWITCHES POLARITY depending on
whether the triangle at the comparator input
is on the up slope or the down slope. The
result is that we get a variable PHASE square
wave coming out of the comparator.<br>
</div>
<div><br>
</div>
</div>
<div>Basically, we take the triangle from Core A,
and compare it in U4 to a reference voltage that
changes polarity with the slope of the triangle.
This gives us a variable phase square, and we
use THAT to drive the Core B integrator. R21
provides a 'weak feedback' that helps keep the
Core B triangle levels correct. The sign flipper
(Q1, U4) is driven by the square wave from the
Core A comparator (U5).<br>
</div>
<div><br>
</div>
</div>
<div>In the Phase CV Summer / Sign Flipper (U3, U4,
Q1), I use R37 to compensate for a slight offset
that was introduced into the Core B outputs
(B1=Square, B2=Tri). A better method might be to
use R33 instead. I have not tested the R33 method,
but you would adjust this until the Phase CV
coming out of U4 was equal in magnitude when it
flipped sign. Again, if you use R33, you might not
need R37. D3 and D4 are also untested, but are
meant to avoid having the Phase CV from U4 exceed
the +/-5V limits of the triangle outputs. If this
CV goes above +5 or below -5, a DC offset will be
introduced into the Core B outputs.<br>
</div>
<div><br>
</div>
</div>
<div>So basically, that's it. Two triangle cores,
remove the hysterisis feedback from the B
comparator, and use the sign flipper to derive a
variable phase square wave from Triangle A to drive
Comparator B. Again, my guess is that the U3, Q1, U4
circuitry could be applied to sync just about any
two triangle core designs. Ideally, you want the two
tri cores to track together fairly well to avoid
introducing more level differences between the A and
B outputs. This might work well with the two gain
cells available in an LM13700. Also, the output
current from the CV Summer would need to be doubled
from that of a typical application, since it will be
driving two VCO cores.<br>
</div>
<div><br>
</div>
</div>
<div>I've tested this circuit on a breadboard and it's a
lot of fun. The sign flipper scheme allows you to vary
the phase of the Wave B outputs, but it avoids the
waveform discontinuities you'd get with a traditional
capacitor reset "sync" circuit.<br>
</div>
<div><br>
</div>
</div>
<div>One limitation of this circuit is that it 'only'
allows the B output to vary up to 180 degrees from the A
output. This means that you can't quite get 'through
zero' phase cancellation between the two outputs. To
overcome this, I've thought of using the output of Core
B to drive a third VCO core (C) utilizing another Phase
CV Summer / Flipper. In theory, this would allow the C
outputs to vary by up to 360 degrees from the A outputs.<br>
</div>
<div>=============================<br>
</div>
<div>Okay, there it is. Whaddaya think? <br>
</div>
</div>
</div>
<div>
<div><br>
</div>
<div>
<div dir="ltr" class="qt-gmail_signature">
<div dir="ltr">
<div>
<div dir="ltr">
<div>Tim (going through a phase) Servo<br>
</div>
<div>---<br>
</div>
<div>"Imagination is more important than
knowledge." - Albert Einstein<br>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
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<div><b>Attachments:</b><br>
</div>
<ul>
<li>VCPO Schem_TParkhurst_2024-05-16A1.PNG<br>
</li>
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