The Slightly Nasty Model 2251 Multiband Filter is a versatile multimode 12dB Oct state-variable filter that provides four simultaneous filter outputs: lowpass, highpass, bandpass, and notch. Both cutoff frequency and resonance are CV-controllable, with attenuverters provided for both. A dedicated audio FM knob allows add extra texture to be added without tying up an external mixer. In addition, one of the unused LM13700 OTA stages is used to implement a bonus utility VCA that operates independently of the filter.
The 2251 was designed to provide very controlled and consistent resonance across the operating range, so that very dynamic CV control of the resonance within a patch would always result in controlled and predictable signal amplitudes without excessive distortion or clipping. The resulting character of the filter is quite smooth, while still providing a satisfyingly big analogue filter sound suitable for basses, leads, and general sound design. Despite this self-limiting resonance, the filter still self-oscillates and can be “pinged” by setting the resonance on the threshold of oscillation and hitting the audio input with a transient signal.
The Audio FM knob allows for some additional dirt and texture to be easily added by modulating the cutoff frequency with the input signal, which results in a sound somewhere between soft distortion and harmonic FM. The extra VCA is ideal as a final envelope/output VCA, meaning that a single oscillator and 2251 filter can provide a complete signal path for basic subtractive synth sounds.
Current Documentation (Rev A for PCB v1.0):