Ladder VCF
A Eurorack transistor ladder filter
A 12HP voltage-controlled filter built on the Moog transistor ladder: four poles, 1 V/oct cutoff and resonance up to self-oscillation, checked by circuit sims.
The top schematic joins four sheets: the Eurorack power header with reverse protection, the expo converter that turns CV into tail current at 1 V/oct, the ladder of twelve BC550C transistors, and JFET buffers into a difference amplifier. The pots and jacks mount on the back and come through the panel.
Three AC sims at 0, 3 and 6 V of CV land within 0.07 dB of each other at their cutoff, so the expo tracks over six octaves. vcf-res shows the resonance peak and the bass loss that goes with it, and vcf-osc kicks the filter at full resonance and checks it oscillates. The transistors and op-amps are modelled from their datasheets, and the pots are potentiometer blocks so their wipers load the circuit as the real parts do.
Open it
git clone https://github.com/v0l/agentee
cd agentee/examples/ladder-vcf
agentee check
agentee view
The ladder sheet: four rungs of BC550C pairs with a 68 nF capacitor across each, and the bias string beside them. 
Gain and phase of the filter with resonance at 0.9, peaking at the cutoff. 
The filter self-oscillating at full resonance after a kick, with the spectrum of the output.