Discrete Ladder Low Pass Filter

The RS-100N is a vintage-style 24dB per octave four-pole low pass filter based on the traditional transistor ladder architecture.

This updated module updates the original RS-100 core with hand-selected, noise-matched transistor pairs and modern thermal compensation. These design enhancements widen the total frequency response while eliminating frequency drift and signal distortion across multiple octaves.

In operation, the filter provides the signature dark, rounded resonance characteristic of classic American ladder topologies. Cranking the resonance pushes the circuit into clean self-oscillation across a range of 10 Hz to 35 kHz. Thanks to the tight transistor matching, the self-oscillating cutoff tracks external 1V/Oct pitch CV like an accurate, warm sine wave oscillator following a short one-minute warm-up period.

The front panel includes a dual-input audio mixer that lets you sum and balance two distinct audio sources directly at the filter stage. Because ladder filters inherently drop low-frequency bass response as resonance increases, dialing in heavy self-oscillation thins out the bottom end of incoming audio signals.