Erica Synths EDU DIY EG Eurorack Module Kit User Manual

Page 21

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Turns out it’s doable quite easily with another op amp,
which we’ll set up as an inverting bu

ﬀ

er. But instead of

connecting the non-inverting input to ground –

as you

would normally do –, we give it a constant 6 V from a
simple 50

% voltage divider.

To understand how this works, I find it very helpful
to imagine that the voltage we apply to the non-
inverting input is the mid-axis against which we are
inverting (or mirroring) the signal

. If that mid-axis is

V, then an input of 12 V is going to give us an output

of –12 V. But if the mid-axis is 6 V, then the same input
will give us an output of 0 V instead. And conversely, a 0
V input results in a 12 V output.

Since our envelope is operating squarely in the range
between 0 and 12 V, the output curve will simply be
flipped on its head while staying within that range.
Which is exactly what we said we’re after. So let’s add
this to our breadboard and see how we fare.

Try driving your VCF with the newly added inverted output. You should be able to verify
that it now starts o

ﬀ

open and then closes down as the envelope cycles through its

phases.