Filters

Ladder

The classic 24 dB/oct transistor ladder - the filter that defined the sound of analog synthesis.

What is a Ladder?

The ladder filter is the most famous filter in synthesizer history. Bob Moog patented it in 1965, named it for its visual resemblance to a ladder of transistors stacked in pairs, and built it into the Moog Modular system, the Minimoog, and every Moog synth that came after. Almost every analog synth you've heard - the Minimoog, the MiniMoog Voyager, the Polymoog, countless Moog-inspired clones - sounds the way it does because of this filter.

Architecturally, a ladder is four cascaded one-pole lowpass sections wired in series, with a global feedback path from the output back to the input. Each one-pole stage rolls off 6 dB/oct; four in series add up to the signature 24 dB/oct slope. The feedback loop is what creates resonance - send some of the output back to the input, the filter starts emphasizing frequencies near the cutoff, and at high feedback it self-oscillates as a pure tone.

The unmistakable Moog character comes from two things. First, the transistor saturation at each stage - the ladder is a nonlinear circuit, and its tanh-shaped distortion adds even harmonics and warmth that no clean digital filter has by default. Second, the bass loss at high resonance: as you crank the resonance, the feedback subtraction at the input attenuates low frequencies. The bottom drops out, the filter starts to sing, and you get that hollow, vocal, almost-self-oscillating quality every Moog bass and acid line is famous for. Some people call it a flaw. Bob Moog never fixed it.

Moog's patent expired in 1982 and the topology became foundational. ARP, Roland, Korg, Sequential, and basically every analog synth manufacturer since has built ladders or close cousins. Every modern modeling synth, every plugin, every Eurorack lowpass that aims for 'that classic warmth' is paying homage to one circuit Bob Moog drew on a napkin in 1965.

Our Ladder

Our ladder uses a careful nonlinear four-pole model - each of the four stages applies tanh saturation in series with a one-pole lowpass, in the spirit of the analog transistor pair it pays homage to. The feedback path is 2x oversampled for stability and harmonic clarity at high cutoff frequencies. Self-oscillates cleanly at maximum resonance as a 1V/Oct-tracking sine.

All three lowpass tap points (24 dB, 12 dB, 6 dB) are live simultaneously. Patch any of them, mix several together for hybrid slopes, or send each to a different destination. Cutoff has full V/Oct tracking, drive scales the input gain into the saturation stages (1x to 5x), and every parameter has a CV input with an attenuverter. 8 HP, 6 knobs, and a ±5V Eurorack-scaled output.

The character is honest. The bass-loss-at-high-resonance behavior is preserved (we didn't 'fix' it). The tanh saturation gets thicker as drive climbs. Self-oscillation produces a clean sine. We aimed for the sound of 1969, not the sound of a sterile lowpass that happens to roll off at 24 dB/oct.

In a patch

The Moog signal flow is canonical: VCO -> ladder -> VCA. Run a sawtooth into the input, modulate the cutoff with an ADSR, route the same envelope to the VCA - that's a Minimoog bass patch in three modules. Two oscillators slightly detuned plus the ladder is the architecture behind every fat analog lead from 1971 onward.

For acid and 303-style basses, push resonance high (around 0.7-0.85) and use a snappy envelope on cutoff. The bass loss tightens the low end, the resonant peak tracks the envelope, and you get the squelchy, rubbery character those records lived on. Add a touch of drive and you're in the right neighborhood.

For thick analog pads, take the 12 dB output instead. More harmonics make it through, the filter sounds more open, and resonance behaves more politely - perfect for slow-attack pads where you want warmth without the ladder's most extreme character. The 6 dB output is for taming brightness on rich sources without obviously filtering them.

Inputs

Audio In (audio) — Signal input. Feed in audio from an oscillator, noise source, or any signal you want to filter through the classic ladder.
Cutoff CV (cv) — Cutoff frequency CV input. Modulate with an envelope for filter sweeps, or an LFO for wobble effects. Amount scaled by the Cutoff attenuverter.
Resonance CV (cv) — Resonance CV input. Voltage-control the resonance amount. Amount scaled by the Resonance attenuverter.
Drive CV (cv) — Drive amount CV input. Modulate the input saturation. Amount scaled by the Drive attenuverter.

Outputs

24dB (audio) — 4-pole lowpass output (24dB/oct rolloff). The steepest slope and the signature ladder sound. Very dramatic filtering that cleanly separates the passed and rejected frequencies.
12dB (audio) — 2-pole lowpass output (12dB/oct rolloff). A gentler slope that lets more harmonics through. Good for pads and leads where you want warmth without heavy filtering.
6dB (audio) — 1-pole lowpass output (6dB/oct rolloff). The subtlest slope - just a gentle high-frequency rolloff. Good for taming brightness without obviously changing the character.

Controls

Cutoff — Cutoff frequency (20Hz to 20kHz). Sets where the filter starts removing frequencies. Low values create a dark, muffled tone; high values let the full brightness through.
Resonance — Resonance (0-100%). Feeds the output back into the input, emphasizing the cutoff frequency. Higher values thin out the bass (the ladder characteristic). At maximum, the filter self-oscillates.
Drive — Input drive (0-100%). Overdrives the signal into the tanh saturation stages. Adds harmonics and grit, thickening the sound. Works especially well on bass.
Cutoff Atten — Attenuverter for the Cutoff CV input (-1 to +1). Controls how much and in which direction the CV modulates the cutoff frequency.
Resonance Atten — Attenuverter for the Resonance CV input (-1 to +1).
Drive Atten — Attenuverter for the Drive CV input (-1 to +1).

Inspired by

Bob Moog patented the transistor ladder filter in 1965; it became the heart of the Minimoog, the Modular system, and the Voyager. Four cascaded one-pole sections in series with a feedback loop produce a 24 dB/oct lowpass that loses bass as resonance climbs - the signature ladder behavior. Our implementation uses Antti Huovilainen's improved nonlinear ladder model with per-stage tanh saturation and 2x oversampling for stability and warmth.

Moog 904A
Minimoog
Moog Voyager
Bob Moog 1965 transistor ladder patent

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