I wonder if there's a blog post in the three levels of complexity of calculating Sallen-Key filters?

https://upload.wikimedia.org/wikipedia/commons/3/3f/Sallen-K...

Super simple, really stable, and very easy to calculate. The cutoff frequency and Q can be set independently, and while making the cutoff variable can be tricky (as seen in various analogue synthesizers) you can vary the Q by varying the gain of the amp. If you assume it's just a unity gain buffer (in some designs it's just an emitter follower) getting the component values is easy.