When I started with synth DIY, I knew little about how transistors work, and the famous Moog transistor ladder filter just looked like magic. Intrigued, I stared long at the schematics posted by Yusynth, and worked my way to the very detailed and mathematical analysis by Tim Stinchcombe.
Eventually the transistor ladder filter was the very first synth module I built, initially in a cardboard box and later transplanted to fit behind a eurorack-format panel. My implementation was a kludge on prototyping board, with later modifications bodged in and not-quite-quality components. While the circuit still sounds fine, it now has intermittent issues which are simply not worth tracking down and fixing anymore. Instead, I designed a proper PCB for the circuit and built a few new ones. The PCB fits behind an 8HP eurorack panel with all through-hole components and no wiring required. I sometimes have some of these PCBs available for sale, and the build documentation can be found at the bottom of this page.
My version of the filter is not intended to be an exact replica of the Moog filter. While it uses the traditional transistor ladder core and exponential converter, I used an opamp instrumentation amplifier (inspired by Rick Jansen) as output stage replacing the original discrete outputs. I added a CV input with attenuverter and a Drive control, allowing to adjust the input saturation that is a significant part of the filters characteristic sound.