Erica Synths EDU DIY Mixer Eurorack Module Kit - Manual (Page 9)

SIMPLE ACTIVE MIXER
To fix that, we’ll simply set up an op amp bu ff er between our two resistors and the output
socket. A bu ff er, if you don’t know, basically just provides a copy of the voltage you 8
apply to it without pulling any current into its input. This way, we reduce the output
impedance drastically, because the bu ff er is able to drive our headphones directly .
Still, we don’t want that output impedance to be zero – simply because it would make 9
our op amp vulnerable to short circuits.
So we’ll put a 1k resistor between our bu ff er and the output socket. This is low enough a
resistance to drive our headphones properly – but high enough to limit the current flowing
to safe amounts in case of a short. For the op-amp, we’ll be using a TL072 IC, which is
two op-amps in a single chip.
Make sure that you set it up exactly as shown here – if
you reverse the power connections, it will heat up and die!
If you send in the same two input signals and check on the output through headphones,
the mix should be boosted back up. But: there is another volume-related issue with our
design. Remember how we said that our voltage divider is creating an average of its two
Read more about op amps and bu ff ers in the components & concepts appendix (page 27/28). 8
You can try this chapter’s circuit in a circuit simulator. I’ve already set it up for you right here: 9
https://tinyurl.com/22pdlvek – you can change all values by double clicking on components.
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