The Analog Thing: an open source, educational, low-cost modern analog computer
A very popular analog computer patchable with 4mm banana jacks was the Comdyna GP-6.  I went down a rabbit hole of reverse engineering it from photos and schematics years ago in order to build a version that would fit in a 3U panel for Eurorack modular synth interfacing.  Never finished it, but the total BOM was... well, not much.  The panel was going to cost more than the board+opamps+passives. I wanted to buy a real Comdyna but I remember the cheapest I found one for sale was ~$700 and I thought that was expensive.  Not so much when you compare it to this unit which is housed in just open PCBs and a much smaller format.<p>If you are a student and want to learn a bit more about how operational amplifiers, resistors, and capacitors alone can solve differential equations... you can get an educational license for Matlab for nothing and Simulink can emulate all the analog circuitry you want and more.<p>Another approach might be using the Eurorack simulation software VCV Rack - If there isn't an set of modules you can install that do basic gain, integrators,  multipliers, comparators, etc then it would be very easy for someone to write one.