# 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.

- Board: 2 layers, 58 x 108 mm, 12HP
- Filter: 4 poles, 24 dB per octave
- Tracking: 1 V/oct within 0.07 dB over 6 octaves
- Sims: AC at three CVs, resonance, oscillation

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.

Files: https://github.com/v0l/agentee/tree/master/examples/ladder-vcf