Transistor Ladder Filter

4-pole transistor ladder filter in 8 HP

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 on prototyping board and later as a proper PCB design. The PCB fits behind an 8HP eurorack panel with all through-hole components and no wiring required.

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.

https://store.kassutronics.net/eurorack-diy-c-1/transistor-ladder-filter-p-7