I finished the Slightly Nasty Model 2251 Multiband Filter, the third Slightly Nasty module for my 4U Serge project. Of the three, this module was the easiest to build, due to the smaller number of parts.
This is a 12dB/octave state-variable filter that provides four simultaneous filter outputs: lowpass, highpass, bandpass, and notch. Cutoff frequency and resonance are CV-controllable, with attenuverters provided for both. The audio FM knob (dark red) 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. A separate VCA is included.
The Build
Like the Slighty Nasty Discrete Oscillator, the Multiband Filter is built on two PC boards that stack together, connected with joined headers and secured with standoffs at the corners. (See my post about the Discrete Oscillator for details of the headers and standoff hardware mounting parts.)
At the top of the above picture you’ll see the two silver polystyrene capacitors. I often use this type in critical audio applications such as the integrator capacitors of filter and oscillators. The part number is Mouser 23PS122, 220pf.
The power header, two bypass capacitors and the five trim pots are soldered to the back side of the main PCB. This is also the back of the module.
The banana jacks are first mounted on the front panel. The panel is then placed over the pot board and secured by the pot nuts. Then small wires are soldered from the jacks to lands on the PC board. The downside of this is that in order to reach the top side of the pot board, all the jack wires have to be removed again. Fortunately, on this filter there are not many parts on the top of the board.
Once again, I chose jack and knob colors matching my BugBrand theme. Yellow for signal inputs, blue for CV inputs, and green for outputs. Medium blue Davies 1900 Clone knobs are used for the CV attenuverters and a dark red knob for the audio FM knob, just above the filter input jack.
Note that the audio FM knob is NOT a level control for the input signal. It’s used to mix the input signal into the frequency CV, a novel feature not seen on many modular filters.
A Few Comments on the Design
There is only one frequency CV input, the one with the attenuverter. Interestingly, this filter is capable of tracking 1V/octave, but there’s no 1V/octave CV input. The trimming procedure explains how to get close to 1V/octave tracking, by adjusting the CV Offset and CV Scale trimmers. This is done to get the measured output frequency (when self-oscillating) to match up as close as you can to the frequency markings on the large knob.
This filter oscillates by turning the Resonance knob beyond about 2 o’clock. When properly trimmed, it will keep oscillating into the LFO range, down to 1 Hz.
The VCA
The separate VCA (jacks at top of the module) has one trimmer for CV rejection. After that’s set you will find that unity gain is obtained at about +5.5V on the blue CV jack. And, interestingly, when the CV input exceeds 5.5V the gain remains at unity.
