Erica Synths EDU DIY Mixer Eurorack Module Kit - Manual (Page 16)

DIODE DISTORTION
With this, our mixer is now perfectly usable. I did promise an additional goodie in the
beginning, though – so let’s go the extra mile! There’s a really simple & nifty trick if you
want to add some distortion and warmth to our mixer’s output. All we need for that are
two diodes, a trimmer potentiometer, four resistors, and another op amp.
Here’s how it works. After our mixer’s regular output, we add in a 100k resistor, followed
by a diode going to and another one coming from a 20k trimmer going straight to ground.
That trimmer is set up as a variable resistor – so we can change its resistance on the fly.
Then, we amplify the result with another op amp. What does this do?
To understand that, let’s first assume that the trimmer is set to 0 ohms – so it’s as if it isn’t
even there. Next, we’ll say that the voltage at our mixer’s output is slowly rising above the
0 V-line. In the beginning, the voltage after our 100k resistor will rise just the same.
This is
because diodes don’t actually conduct below a certain threshold input voltage . 14
I like to think of it like this. Diodes are essentially one-way valves for electricity. But to
actually open those valves, we need to push against them with enough force. So as long
as the pushing force from the op amp’s output is relatively small, it won’t be enough to
open the diode pointing towards ground. Meaning that we’ll see that force build up above
it.
But as it keeps building up, the diode will start to open, allowing current to flow from the
op amp’s output through the 100k resistor and to ground. Which, in turn, will relieve some
of the pushing force. This e
ff ect will only get more pronounced as the op amp pushes
harder and the diode opens up wider. So while the voltage at the op amp’s output
keeps rising, the voltage above our diode will begin to stagnate .
Read more about diodes in the components & concepts appendix (page 23). 14
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