Documentation about circuit simulation and “analogue” filter design:

**The Art of VA Filter Design**A very good book with all you need to know to create analogue-like filters. Warning: you’ll have to somehow unlearn things about classical filter design (s-plane prototypes and BLT)

**Andrew Simper’s technical papers**About filters designed with trapezoidal integration (description and pseudo-code)

**Qucs technical papers here and here**Resources about Modified Nodal Analysis (MNA), numerical integration, convergence and everything you need to know to build a basic circuit simulator. This is for more complex circuits, when manually solving tons of equations is impractical.

**A paper and a thesis** with long titles

Once you’re comfortable with the MNA, you can switch to the Nodal DK method and its derivatives, which are more suitable for real-time use.

**KVR 1, KVR 2**Some interesting tips and techniques in these threads.

And of course the

DAFX archive, a mine of information on various related topics.