Processed & buffered triangle – Erica Synths EDU DIY Wavefolder Eurorack Module Kit User Manual

Page 16

Advertising

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.

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

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.

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

. 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