Op-amp basics

A friend of mine was asking about op-amps the other day.
They are used extensively in the synth world.
I found this great little video and thought I'd share.
It's very basic but a great introduction to this very important
building block of electronic music.
https://www.youtube.com/watch?v=K03Rom3Cs28

Around 7.15 into the video is a integrator circuit.
It's a classic way to convert a square wave to a triangle wave.