Filters

VCF

A two-knob lowpass: cutoff, resonance. The simplest filter for the simplest voice.

What is a VCF?

VCF is the simplest voltage-controlled filter: two knobs, three jacks. A 24 dB/oct lowpass ladder removes high frequencies above the CUT knob, with RES adding emphasis at the cutoff and, at maximum, a ringing self-oscillation. The MOD CV input lets an envelope or LFO sweep the cutoff over time - the second-most-important patch in modular synthesis after osc -> VCA. The first filter in the Basic series, designed to teach subtractive synthesis with as little surface area as possible. When you outgrow it, Ladder adds drive, simultaneous 6/12/24 dB outputs, and CV attenuverters; Filter (SVF) adds highpass, bandpass, and notch.

In a patch

Patch an oscillator (Basic SAW or Basic SQR are richest in harmonics) into IN, then OUT to a VCA or directly to Output. Turn CUT down until the sound darkens; turn RES up to taste. Patch an AD envelope into MOD to make each note open the filter and close again - the canonical "wow" or "pluck" sound. With MOD scaled at +/-2 octaves per +/-5V, a 0-10V AD envelope sweeps cutoff up by ~4 octaves, which is exactly the musical range you want from an envelope. Patch an LFO into MOD instead for "wah-wah" or wobble effects. Crank RES to 1.0 with no audio input and sweep CUT - the filter rings on its own (a sine-like tone) and becomes an oscillator.

Inputs

IN (audio) — Audio input. Eurorack-standard +/-5V audio level. Patch a Basic SAW or Basic SQR here for a harmonically rich source the filter has something to chew on - sine waves go in mostly unchanged because they have no harmonics for the filter to remove. The input is internally pre-attenuated before the ladder's nonlinear stages, so signals up to +/-5V stay clean and don't saturate the filter (no built-in drive knob - that's a Ladder feature).
MOD (cv) — Cutoff modulation CV. Fixed sensitivity: +/-5V CV swings cutoff +/-2 octaves. A typical 0-10V AD envelope opens the filter by 4 octaves at peak; a +/-5V LFO swings cutoff over a 4-octave range for wobbles. The scaling is intentionally sub-V/Oct so envelopes and LFOs work musically out of the box - if you need wider sweeps, modulate CUT with multiple sources via a CV-Mix, or step up to Ladder/Filter which expose attenuverters. CV adds to (not replaces) the CUT knob value.

Outputs

OUT (audio) — Filtered audio output. 24 dB/oct lowpass tap from the 4th ladder pole - the classic thick, warm filter sound. Output level matches input level (+/-5V) at low resonance; at high resonance the filter loses some bass (authentic transistor-ladder behavior) and adds a peak at the cutoff frequency. Patch into a VCA, an Output module, or another effect.

Controls

CUT — Cutoff frequency. Range 20 Hz to 20 kHz, exponential scale (the knob feels musical because it tracks octaves linearly). Frequencies above the cutoff are attenuated at 24 dB/oct (an octave above cutoff is ~16x quieter). 200-500 Hz: dark and muffled, only fundamentals through. 800-2000 Hz: classic "warm" tone with body and presence. 4-8 kHz: most harmonics through, just shaving the brightest content. 20 kHz: filter is open, no audible effect. Default 1 kHz is a good starting middle ground.
RES — Resonance amount. Range 0 to 1, linear. 0: clean lowpass, no peak at cutoff. 0.3-0.6: subtle emphasis, fattens up the filter sweep. 0.7-0.9: pronounced peak at cutoff, the classic resonant filter sound (loved on basses and acid lines). 1.0: the filter rings on the edge of self-oscillation. Sweep CUT at high RES with no input and you hear a sine-like tone - the filter becomes an oscillator. RES interacts with CUT: at high resonance the filter loses some bass below cutoff (authentic ladder character).

Inspired by

A simpler sibling of Ladder. Same Huovilainen-style 4-pole transistor ladder topology, trimmed to the two essentials a beginner needs: cutoff and resonance. One CV input with a fixed, tame sensitivity so envelopes and LFOs sweep usefully out of the box. The classic 24 dB/oct sound, packaged for first lessons.

Moog 904A
Minimoog
Bob Moog 1965 transistor ladder patent

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