bartola-instruments-swell

Bartola Instruments Swell

Voltage-controlled low-pass filter with an integrated vacuum-tube stage.

14 HP Eurorack module

Swell is a voltage-controlled four-pole low-pass filter with a configurable vacuum-tube stage that adds harmonic generation, compression, distortion, and asymmetric wavefolding. Its valve configuration, bias, gain staging, placement, resonance compensation, and normalized FM feedback provide a wide range from clean filtering to unstable self-modulation.

Features

  • Four-pole 24 dB/octave voltage-controlled low-pass filter with temperature compensation and voltage-controlled resonance.
  • Switchable Triode and Pentode valve configurations provide warm even-order or aggressive odd-order harmonic characters.
  • The valve can operate before or after the filter.
  • The bipolar FM attenuverter controls an external modulation signal or, when unpatched, a normalized output-to-cutoff feedback loop.
  • Switchable Q compensation maintains a more consistent level as resonance rises.
  • The filter self-oscillates at maximum resonance and can serve as a standalone oscillator.
  • The 1V/Oct input provides chromatic control of self-oscillation and filter-ping pitch.
  • The valve-window indicator distinguishes normal operation, output-stage saturation, isolated HOT Pentode grid-driver saturation, and simultaneous saturation.
  • Remember: use clean, stable +/-12 V power; excessive ripple, noise, or voltage spikes can disrupt operation or damage the module.
  • Remember: do not press the valve or flex its leads; mechanical stress can break the hermetic seal and cause failure.

Controls & Jacks (17)

  • Drive— Controls the pre-valve gain stage, pushing the valve grid from subtle saturation into compression, distortion, and—especially in HOT Pentode
  • Cutoff Frequency— Sets the 4-pole low-pass filter cutoff frequency and acts as the base-frequency offset for the 1V/Oct and fm inputs.
  • Input Gain— Sets the main input level into the active signal chain; in Pre mode it feeds the valve first, while in Post mode it drives the filter input
  • Audio Input— Audio input to the filter and valve signal path; it can also accept a short gate or trigger to excite filter pings.
  • Resonance— Sets filter resonance from minimum emphasis through self-oscillation; higher settings also lengthen filter-ping decay.
  • Audio Output— Filtered audio output, limited to approximately 10 Vpp; it is normalized internally to the fm input.
  • 1V/Oct Input— Exponential cutoff-frequency control input calibrated for 1 V/octave tracking; controls pitch when the filter is self-oscillating.
  • Saturation Indicator— Multistate indicator in the circular valve window: off when unsaturated, white with brightness tracking output-stage saturation, red for iso
  • Resonance CV Input— 0–5 V control-voltage input for resonance; the incoming voltage is summed with the Res. knob setting.
  • FM Input— DC and audio-rate cutoff-modulation input processed by the fm attenuverter; accepts approximately 10 Vpp Eurorack signals.
  • Mode— Configures the valve as Triode for warmer, predominantly even-order harmonics or Pentode for more aggressive, predominantly odd-order harmon
  • FM Amount— Bipolar attenuverter controlling the depth and polarity of the signal patched to the fm input.
  • Bias— Sets the valve operating point: COLD provides warmth or asymmetric compression, while HOT is most linear in Triode mode and produces the str
  • Valve Position— Places the valve before the filter in Pre mode so its harmonics are filtered, or after the filter in Post mode so resonance and the filtered
  • Boost— Selects standard valve-stage gain when off or adds approximately 6 dB when on, increasing saturation intensity and helping recover level in
  • Q Compensation— Enables resonance gain compensation, maintaining a more consistent output level at high Q and lowering the self-oscillation threshold while
  • Post-Valve Gain— Selects low or high post-valve gain for level compensation or final-stage clipping; High is particularly useful for restoring level in HOT P

How-To Guides

  • Start with Clean Filtering
  • Use Swell as an Oscillator
  • Create a Filter Ping
  • Modulate Cutoff with an Envelope

Explore the interactive Swell panel on BleepDex — tap any control to learn what it does.

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