Erica Synths EDU DIY Dual VCA Eurorack Module Kit User Manual
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Because this is what op amps do: they subtract the voltage at their inverting input from
the voltage at their non-inverting input. And after that, they multiply the result by a fixed,
internal gain. Because an op amp’s gain is huge, we have to take some measures to calm
it down though. If we really just want to subtract our two signals without changing the
resulting signal’s amplitude, we can set up our op amp as shown above. Here’s how it
works. Let’s first imagine that both input voltages are the same: 6 V.
Since the two resistors at the non-inverting input form a 50% voltage divider, we’ll
measure 3 V between them. Now for the op amp to enter a state of balance in this
scenario, the voltage at its inverting input would need to be 3 V as well. And the only way
that can really happen is if the op amp’s output sits at a steady 0 V. Because then, both
paths are exactly the same: two 50% voltage dividers with 6 V on one end and 0 V on the
other.
Okay, but what happens if the two input voltages start to diverge? To answer that, let’s
assume that the bottom voltage now sits at 7 V, while the one on top has dropped to 5 V.
This means that the op amp’s non-inverting input reads 3.5 V. Now, since the op amp
wants to get the other input up to the same value, the output really can’t stay at 0 V.
Instead, it will have to make up for the di
ff
erence between the two input signals.
And
to do that, it’ll push out a steady 2 V. Because the midpoint between 5 V and 2 V
is
exactly 3.5 V. Cool, so let’s add this to our circuit and see how we fare.
And that’s it – we should now be able to pick up a properly centered signal from the
subtractor op amp’s output (which is the second pin from the left on the TL072’s upper
pin row). Let’s hook up the oscilloscope to check that out. Make sure the AC coupling is
turned o
ff
, though! If your signal still isn’t properly centered, this means that your
transistors are not tightly matched. But not to worry – we can compensate for a bit of
mismatch with a very simple but e
ff
ective addition to the circuit.
Again, you can watch me do this if you don’
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