Erica Synths EDU DIY Wavefolder Eurorack Module Kit - Manual (Page 8)

THE SIMPLEST WAVEFOLDER
Okay, but how do we make this happen? The circuit will probably be quite complex,
right? Yeah .. not really. All we need are three components: two same-value resistors and
an NPN transistor. If we set them up like this and send an oscillation into the transistor’s
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base, we can pick up a folded version at the collector.
Now, if you’ve worked with NPN transistors before, you might say „hold on – how is this a
wavefolder? It’s just a bog-standard amplifier!“ And you would be right. What’s worse: it
doesn’t really amplify anything, since it has a gain of -1. So all it does is invert the input
signal while keeping the volume the same.
Then why do I call this a wavefolder? Because it is if you abuse it! To understand why and
how, let’s dissect this circuit step by step. Functionally, we can split it into two sections:
the resistor up top and the transistor and resistor below. Let’s start with the lower section.
These two components in conjunction serve one simple
purpose.
They’re meant to convert voltage into
current . More precisely: the voltage we apply to the
base into a current flowing through transistor and
resistor. Okay, but what is the relation between those
two variables?
Well, for the transistor on its own, the relation between
base voltage and collector current is exponential.
Conversely for the resistor, the relation between the
voltage across it and the current flowing through it is
strictly linear.
Read more about resistors and transistors in the components & concepts appendix (page 5
28/37).
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