Erica Synths EDU DIY Mixer Eurorack Module Kit - Manual (Page 17)

This is why we need the 100k resistor, by the way: because there is no limit to how far the
diode can open up as the voltage increases. Without the resistor, we’d see the current
ramp up exponentially, until we’ve basically created a short circuit. Since this isn’t
particularly healthy for our components and power supply, we use the 100k to restrict the
amount of current flowing. This way, as the op amp pushes past a certain threshold, the
voltage above our diode will simply stay constant – without anything going up in flames.
In audio terms, we call this e ff ect soft clipping. We’re essentially cutting o ff any part of our
waveform that’s above a certain threshold line. And because the diode opens up
somewhat gradually, the resulting edge will be slightly rounded . (Hence the soft in
soft clipping.)
As you might have guessed, the other diode pointing upwards from the trimmer serves
exactly the same purpose, but for negative voltages. It opens up once the op amp pulls
hard enough, clipping the lower half of our output waveform. Finally, in order to not mess
with this delicate mechanism, we then have to bu
ff er the output voltage with another op
amp. Since the clipped signal is going to be quite low in volume, I decided to give that op
amp a fair bit of gain.
To do that, we replace the straight connection between output and inverting input with a
voltage divider. This will make it harder for the op amp to equalize its two input voltages,
causing it to push and pull with more force. As a result, our clipped output should be
brought back up to the regular output’s volume levels.
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As an input signal, try a sawtooth- or sine-oscillation first. If you check the result on an
oscilloscope, you should see that the top and bottom parts of the waveform indeed get
shaved o
ff . But while it does sound di ff erent, it’s hard to really hear the e ff ect with a static
tone. So go ahead and try a more complex input signal – like a drum beat or a sequence.
Now, you might even find the e ff ect to be too pronounced. If you do, no worries: that’s
what the trimmer potentiometer is for. Because by increasing the trimmer’s resistance
value, you can influence the e ff ect’s intensity .
You can try this chapter’s circuit in a circuit simulator. I’ve already set it up for you right here: 15
https://tinyurl.com/29grj2ps – you can change all values by double clicking on components.
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