Swell Harmonic Valve VCF

Bartola Swell

Swell from Bartola pairs Soviet valve technology with a rich low pass filter to add everything from subtle filtering to wild feedback oscillations to anything you throw at it. The filter core is a Moog-style, resonant 4-pole 24dB/Oct lowpass filter with switchable Q compensation to preserve the low end at higher resonance settings. It includes unattenuated CV inputs for the resonance amount and a calibrated 1V/Oct input for precise filter tracking over eight octaves. The FM input features an attenuverter to control whether the modulation increases or decreases the cutoff frequency and by how much. Swell includes normalization between the filter output and the FM input. With nothing patched to the FM input, it creates a feedback loop where the filter's output alters the filter's cutoff frequency, altering its output. At high resonance settings, the filter begins to self-oscillate, creating an audible tone on its own. Couple this with the self-feedback and the valve circuitry to create complex and noisy tones and timbres. The Valve switch sets where your signal passes through the valve, before or after the filter. Having the filter after the valve lets you tame the harmonic content it imparts to the input signal. Putting the valve after the filter retains all the harmonic content imparted onto your signal from the valve without smoothing out any of the harsh or rough edges. The Bias and Mode switches set the internal configuration of the valve itself, letting you dial in the perfect amount of harmonic distortion for your sound. The Boost switch works in conjunction with the other valve settings, and adds 6dB of gain to the valve. The Drive knob sets the pre-valve gain, while the Gain knob affects the signal after it passes through the valve. Swell is perfectly suited to removing or adding harmonics to any signal, imparting everything from subtle warming to rich distortion, making it a flexible sound shaping tool that goes above and beyond typical filtering applications.

Swell Features

• Valve-based harmonic VCF
• Soviet-era submarine tube adds distinctive warmth
• 24dB/Oct low pass filter with normalled feedback FM
• Switchable pre or post valve filtering
• Bias and Mode switches set the valve timbre
• Control over pre and post valve gain
• Boost switch adds 2x gain
• Q compensation retains low end