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Filter Overview

Filter Core

Swell uses a modern voltage-controlled 4-pole low pass filter IC at its heart. This circuit design provides a 4-pole (24dB/Oct) filter, with temperature compensation and output compensation based on resonance level.

The valve stage can be placed either before or after the filter via the Valve switch, fundamentally changing the character of the filtered output. The filter core can be driven into a classic overdriven state to introduce additional distortion. Increasing the filter's input signal overdrives the filter's input transconductance cell, producing asymmetric soft-clipping and introducing even-order harmonic content. The character of this overdrive differs depending on whether the Valve switch is set to Pre or Post.

Filter Controls

There are three filter controls: Cutoff Frequency (fr), Resonance (Res.), and FM Modulation (fm). Their contributions are summed at the filter's cutoff control.

Cutoff Frequency (fr)

The centre knob controls the filter's cutoff frequency.

  • Range: full audio range and beyond — calibrated across 8 octaves (1V/Oct), with response extending above 20kHz.
  • Tracking: 1V/octave via dedicated CV input for precise tracking across 8 octaves
  • CV Input: 1V/Oct jack (bottom panel)

The cutoff frequency knob acts as an offset when used alongside the CV input and/or the fm input — setting a base cutoff that other modulation sources sweep around.

Resonance (Res.)

Controls the filter's resonance (Q factor)—the emphasis at the cutoff frequency.

  • Knob Function:
  • No CV patched: Acts as resonance level control (0% to self-oscillation)

  • CV patched to RES. input: Knob setting is added to the incoming CV — the knob provides a resonance offset that sums with the CV

  • Self-Oscillation: The point of self-oscillation varies considerably depending on the overall module settings, in particular the valve stage.

  • CV Input: RES. jack (bottom panel). Expected 0 to 5V range.

FM Modulation (fm)

Frequency modulation attenuverter for the filter cutoff.

  • Knob Range: -100% to +100% (attenuverter—controls both amount and polarity). Logarithmic response with a centre tap.
  • Normalization: The FM input is normalized to the module's audio output, creating a built-in feedback loop with no patch cable required. (Normalized: the output is internally connected to the FM input by default; inserting a cable disconnects this and routes the patched signal instead.)
  • CV Input: fm jack (bottom panel). 10Vpp CV and audio-rate Eurorack signals expected.

Patching behaviour:

  • No cable in fm jack: FM knob controls the level and polarity of audio feedback (output feeds back to filter cutoff)
  • Cable patched to fm jack: normalization is broken; FM knob attenuates/inverts the incoming CV or audio signal instead

Using fm for pitch/cutoff modulation depth control:

The fm attenuverter provides a built-in way to scale any incoming CV without an external attenuator. This is particularly useful for envelope modulation: patch an envelope directly into the fm jack and use the attenuverter to set the sweep intensity. The fm and 1V/Oct inputs can be used independently or combined — both sum at the cutoff control. A common use is 1V/Oct carrying keyboard or sequencer pitch while the fm input handles the envelope sweep with its own independently scaled depth.

Feedback polarity behaviour (no cable in fm jack):

  • Negative settings (CCW from centre): The output is fed back with inverted polarity. At audio rates this creates a dynamic where signal peaks suppress the cutoff and troughs open it — a form of frequency-domain wavefolding. The result is progressive harmonic enrichment: the waveform grows increasingly complex as the feedback level increases, adding upper harmonics and a folded, dense character without becoming unstable.

  • Positive settings (CW from centre): The output is fed back with the same polarity, creating a regenerative loop. Positive signal excursions raise the cutoff, allowing more signal through and reinforcing themselves. At moderate settings this intensifies resonant peaks. At higher settings the loop destabilises into beating, pulsating behaviour and rapid timbral fluctuations — the filter and signal interact in unpredictable, self-modulating ways.

Pre/Post interaction: In Post mode, the valve sits downstream of the filter and within the FM feedback path (filter → valve → output → fm → cutoff). The valve's saturation becomes part of the self-modulation character, producing different timbral results from the same FM settings compared to Pre mode. See Valve Operating Modes — Chain location for detail.

Compensation (Comp Switch)

The Swell filter experiences natural gain loss at the cutoff frequency when resonance (Q) is increased. The Comp switch enables Q compensation circuitry that counteracts this effect, maintaining consistent signal levels across all resonance settings, allowing aggressive resonance without losing overall volume.

  • OFF: No compensation — natural filter response with gain loss at high resonance
  • ON: Q compensation enabled — maintains more consistent output level regardless of resonance setting, for aggressive resonance sweeps and near self-oscillation timbres.

Filter Connections

Signal Function Normalization
IN Audio input —
1V/Oct Filter cutoff tracking —
fm Frequency modulation Normalized to OUT
RES. Resonance CV —
OUT Filtered audio output Feeds back to fm (norm)