Erica Synths EDU DIY Wavefolder Eurorack Module Kit User Manual
Page 14
Interestingly, this doesn’t result in a voltage relation of 1:(-)2 across the board. Instead, we
get a 1:-2 relation in the non-saturated region, while the saturated region sticks with the
1:1 relation. But that’s not all. As you can see in the graph, the folding threshold also
drops noticeably, now sitting at about -3.5 V. Which is really interesting for us.
Because instead of adding an o
ff
set to our input sawtooth, we could simply lower
the folding threshold to make both line up perfectly
. Of course this would come with
an unintended side-e
ff
ect: if the voltage relations in the saturated and non-saturated
regions di
ff
er, then our output triangle will be asymmetrical – since one side is going to be
more slanted than the other.
Not ideal – but since we care about the small footprint here more than getting a perfect
triangle output, it’s a concession we can reasonably make. So next, let’s think about the
resistance value we’re going to add at the transistor’s collector. You might be tempted to
just try a few di
ff
erent values here until you’ve found the correct one.
There’s a problem with that approach, though.
Every
transistor is di
ff
erent from the next when it comes to
its “natural” folding threshold (i.e. the one we’re
getting with equal resistor values)
. The 450 mV we’ve
been assuming so far are just a rough average.
This means that the added resistance working for your
specific transistor will not work for most others. So we’ll
go with an adjustable solution instead and use a
precision trimmer set up as a variable resistor. Since
the correct resistance value will be somewhere between
10k and 20k, a 10k trimmer should work just fine.
Read more about trimmer potentiometers in the components & concepts appendix (page 32).
You can try this chapter’s circuit in a simulator. I’ve already set it up for you right here:
you can change all values by double clicking on components.
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