Bartola Instruments - Swell

Swell is a 24 dB/oct low-pass filter built around a modern analogue filter core and a Soviet subminiature valve. The valve itself sits directly in the audio path and can be placed either before or after the filter. Put it first and the filter gets a harmonically richer signal to chew on; put it afterwards and resonance peaks hit the valve instead.

There is quite a bit of control over how that saturation happens. The valve can operate in triode or pentode mode, its bias can be shifted, and separate Drive, Boost and gain stages let you decide where the signal starts to bend. That takes Swell from fairly clean filtering and soft compression through denser overdrive to asymmetric folding and decidedly more aggressive distortion. The FM input has another clever trick: with nothing patched, it is normalled from the output, turning the FM attenuverter into a bipolar feedback control.

If you push the resonance far enough Swell will go into self-oscillation, with dedicated 1V/oct tracking and the same valve circuitry available for shaping its waveform. In Pre mode you can keep the self-oscillation comparatively clean; in Post, the valve turns that basic filter sine into something considerably less basic. So although Swell starts with the familiar idea of a resonant four-pole low-pass, most of its character comes from deciding what happens around the filter — and, importantly, what gets fed back into it.

Features:

• 4-pole analogue low-pass filter
• 24 dB/oct slope
• Soviet subminiature valve in the audio signal path
• Switchable valve placement before or after the filter
• Triode and pentode valve modes
• Cold / Hot valve bias settings
• Variable Gain and Drive
• Valve Boost switch with +6 dB gain
• Low / High post-valve gain setting
• Voltage-controlled cutoff
• Dedicated 1V/oct cutoff input
• 1V/oct tracking across seven octaves in self-oscillation
• Voltage-controlled resonance
• Self-oscillation
• Bipolar FM attenuverter
• FM input normalled from the audio output for internal feedback
• External audio-rate or CV frequency modulation
• Switchable resonance/Q compensation
• Filter-core overdrive with asymmetric soft clipping
• Saturation indicator LED