Erica Synths EDU DIY Wavefolder Eurorack Module Kit - Manual (Page 16)

PROCESSED & BUFFERED TRIANGLE
Got a good-looking triangle out of this? Great! Next, try in- and decreasing the input
signal’s frequency. You’ll probably notice that your triangle suddenly doesn’t look so good
anymore. That’s because of slight shifts in the sawtooth wave’s vertical o
ff set across the
full frequency range. These are a typical byproduct of AC coupling, which the mki x
es.edu VCO uses to center its output around the 0 V line.
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Unfortunately, there’s not much we can do to prevent
this from happening. So instead, we’ll rely on a simple,
but e
ff ective fix: applying a low-pass filter to our saw-
to-tri converter’s output . This will cause the gap
between the triangle’s sides to be smoothed, resulting
in less noticeable buzz in the output signal. In my
experiments, combining a 1k series resistor with a 2.2
nF capacitor to ground gave me decent enough results.
Next, we’ll need to make sure that our triangle is
centered around the 0 V-line. For that, we’ll follow the
lead of our VCO and use AC coupling again. So after
the low pass filter, we’ll set up a 1
μ F capacitor, followed
by a 100k resistor to ground.
Then, we want to bu ff er & boost the signal to avoid
loading e ff ects and get the output up to the standard 10
V peak-to-peak. Since folding the input has slashed its
volume in half, we’ll use a non-inverting op amp-based
amplifier with a gain of 2 here.
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Finally, we should try and increase the converter’s input
impedance a bit. Right now, we’re drawing up to 1 mA
from our VCO – which is quite a lot of current. To get
that number down to more reasonable levels, we’ll use a
nifty little trick:
set up a second NPN transistor as a
voltage bu ff er . If we connect its emitter to the main
transistor’s base, its collector to the positive rail, and its
base to our input sawtooth wave, the bu
ff er transistor
will replicate that sawtooth at its emitter, while pulling
the necessary current directly from the power rail. This
way, we reduce our peak current draw from the VCO to
just 10
μ A.
Read more about AC coupling in the components & concepts appendix (page 33). 11
Read more about op amps in the components & concepts appendix (page 34). 12
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