Sherman Filterbank 2 Compact

The Filterbank's status as an analogue sound mangler is legendary. Can this smaller variant keep it relevant in the modern world?

after spending most of the 1980s making proto-industrial new beat music, Herman Gillis turned his attention to electronic musical instruments. He'd always tinkered with electronics for his various bands, but in the early 1990s he decided to focus on it full time and in 1995 early prototypes of the Sherman Filterbank appeared. The rest, as they say, is history.

The Filterbank is based around two flexible, 12dB/oct, multimode filters. Throw in a beastly input overdrive circuit, pitch tracking, frequency modulation, amplitude modulation, envelope following, an LFO, flexible routing and a generous number of audio, MIDI and CV inputs/outputs, and you have an effects unit that can turn its hand to many tasks in the studio and on stage.

Compact

The Filterbank 2 Compact is the latest variant, of which there are several (see the 'All The Options' box), and the layout and functionality is almost exactly the same as the Filterbank 2, bar a few very minor tweaks. The smallest Filterbank to date (3700 x 1200 x 440 mm), the biggest change is in its height, with the older Filterbank 2 standing twice as tall. Also, the audio and MIDI ports are on the top of this unit, rather than on the rear, which is great for a tabletop setup: you don't lose space to cables sticking out the rear, and re-patching, which you'll want to do often, is easy.

Audio inputs and outputs are on mono 6.35mm TS unbalanced jacks. As well as the main audio input and output there are a number of audio and CV connectors, including FM input, ADSR trigger, ADSR output, amplitude modulation input, AR trigger and a separate output for filter 1. A pedal input allows you to toggle bypass with a footswitch, and a modified pedal can be used to control filter 1's frequency.

The lack of connectivity on the rear panel is intentional — with all the I/O on the top panel, the Compact lives up to its name.Nestled among the audio sockets are five MIDI ports: an input, an output and three (yes three!) thrus. The input can be used to control various things, like filter cutoff, FM amount, AM depth, and envelope settings. You can trigger the AR and ADSR using specific MIDI notes. The envelopes also send MIDI notes when triggered, though the knobs don't send MIDI control changes. Then there's the three MIDI thru ports. It's not clear why there are so many, but in my years of using a Filterbank 2 in the studio they've come in handy several times.

The front-panel controls are laid out pretty much in signal-flow order. At the left, the input gain allows for attenuation or amplification of the input signal. The 12 o'clock position is roughly unity level. Boosting the signal quickly results in a beautiful overdrive which ranges from subtle warming to total distortion. The input level also adjusts the trigger sensitivity for the ADSR and AR envelopes. A three-way switch allows you to boost or cut high frequencies in the amplifier circuit.

The output, on the right, is controlled by a main dry/wet control (labelled 'Bypass/Effect'), which is ideal for parallel processing. There's no output volume control, though, so the Filterbank is best used with a mixer or other device via which you can set the final signal level before recording.

The Filters

The bottom row of knobs controls the filters, and everything above controls modulation and signal flow. Both filters are identical in operation, with duplicate controls except for the Harmonics section of filter 2, which I'll get to shortly. The requisite cutoff and resonance controls are present, of course, but the second pair of controls is more interesting. The first of them sweeps between low-, band- and high-pass. The second Sherman call the 'Correction' knob — and in combination they can create some complex filter shapes. In the centre position, the Correction knob does nothing. Move it to the left and it subtracts a phase-inverted version of the band-pass signal from whatever setting you have on the main control. So at low- or high-pass settings, subtracting the bandpass will make the filter curve steeper. If you subtract the band-pass filter from the band-pass setting, you'll get no signal at all, essentially turning the filter 'off'. Move the control to the right, towards the '-B+LH' setting and you again subtract band-pass but also add in some low- and high-pass signal. So, with a main setting of band-pass and the correction knob set to '-B+LH', you create a notch filter. The system takes some getting used to, but the manual provides helpful graphs that show what various combinations do. And if it still doesn't make sense, adjusting things by ear is by far the easiest way to learn!

The two filters can be blended between serial or parallel configurations using a continuous control. The frequencies can also be linked using the large Harmonics knob: in its Free position, the two filters are completely independent, but at any other setting, filter 2 follows filter 1's frequency at an interval dictated by the knob's position. (A blue LED indicates that the filters are linked.) The intervals are based around mathematical divisions of filter 1's frequency, and the options are 1, 1.5, 2, 3, 4 etc., all the way up to 16. These represent intervals like a quint down (1.5), an octave down (2), all the way to four octaves down (16). The musical relationship is not always obvious, so some maths (or at least referencing of the relevant table in the manual) is needed to figure out what number represents what musical interval. For example, dividing the filter frequency by 7 results in a frequency that is 2 semitones above 3 octaves down. It would have been nice to have musically relevant options, such as +3rd, +5th, +7th, but there's still a lot of fun to be had from linking two filters at sympathetic frequencies, especially when you crank up the resonance and introduce modulation or FM.

Modulation

Speaking of which, Sherman have added enough modulation sources to keep the filters bouncing for days. An FM input allows for frequency modulation of both filters using an external signal; if no cable is plugged in, the main input signal is used. The FM knob controls the amount.

Each filter has a dedicated ADSR amount control, which applies positive or negative modulation from the ADSR generator. The envelope has four controls, one for each stage. Triggers can be the input signal going above a certain threshold (set by the input gain), an external signal arriving at the Trigger ADSR input, or a MIDI note. The envelope acts like a standard ADSR, with the sustain holding for as long as the input signal remains above the threshold. Two useful LEDs show the input trigger and the output ADSR signal. The sustain level is bipolar (it can go into positive or negative values), allowing the filter cutoff to modulate above and below its current setting.

The Filterbank is an extremely flexible processor capable of anything from subtlety to destruction — and the best bit is that it always sounds good.

A flick of a switch and the ADSR becomes an envelope follower which tracks the amplitude of the input signal. In this mode, the Trigger ADSR input becomes the envelope follower input, so you can follow an external signal instead of the one you're processing. The sustain knob becomes a sensitivity control, while attack and release remain the same and the decay knob is unused. The envelope follower is perfect for all kinds of signal-dependent filter effects, since it allows you to open or close the filter depending on the amplitude of another instrument.

Moving along, we find the LFO controls. Sine and sawtooth waveforms are possible, and the sine wave's phase can be reset by the AR trigger signal. (Oddly, the sawtooth's cannot.) A master depth control applies the LFO to both filters' frequency cutoff: at its centre position, this applies no LFO; turn to the right and the LFO will act equally on both filters; turning to the left applies opposite modulation to filter 1, so the filters sweep 180 degrees out of phase to one another. The LFO speed ranges from 2-3 minutes per cycle up into the audio range.

Amplitude modulation is possible via the AM input jack, and a knob controls the amount. If nothing is plugged into the jack, filter 2's output is used as a source. The second amplitude modulation source is the AR envelope — and this is your final amplitude envelope. It can be triggered externally using the Trigger AR jack or, if unpatched, from the same audio input as the ADSR. With attack set to minimum and release to maximum, the Filterbank is effectively always on, but at other settings it can be used for gating, pumping or pulsing effects. A pseudo side-chaining effect can be achieved by sending a kick drum or trigger signal to the trigger input, and setting the attack so the level rises up with each trigger.

The final modulation source is a pitch tracker. Activated by a switch, this attempts to track the pitch of a monophonic input source. Filter 2 will be tuned to that pitch, while filter 1 follows according to the Harmonics setting. A Track Low option caters specifically for tracking bass frequencies. I must confess that I found the pitch tracker difficult to use. The manual suggests putting the signal you want to follow into the FM input and raising the FM amount slightly, but FM is still active when the tracking is switched on, so the signal quickly becomes complex. Despite trying different sources, I was unable to get any discernible pitch-following from the filters.

In Use

Describing the sound of a filter is always a difficult task. At first, the Sherman Filterbanks can seem unruly beasts. Indeed they have something of a reputation for that, but there's more to them and it's all dependent on the input gain. At settings below 12 o'clock, the filters have plenty of headroom, and even at high resonance settings a clean, but slightly warm-sounding signal passes through. The resulting sound is one you might associate with 12dB/oct filters: mellow and round, a filter that ices the cake. But once you crank the input gain, the sound becomes saturated, before moving into more obvious distortion territory. The filters have to fight for headroom, which means unstable resonances are pummelled into the sound. The truth is that the Filterbank is an extremely flexible processor that's capable of anything from subtlety to destruction — and the best bit is that it always sounds good doing it!

The profusion of hands-on controls is divided into two broad groups: the bottom row controls the filters, and the rest govern modulation and signal flow.

I've had a Filterbank 2 in my studio for the best part of a decade, and in that time it has been used in many ways. I adore it as a parallel drum-bus processor; adding grit and analogue mojo to any drum bus, the input saturation is quite delicious. Another common application is to use the LFO or envelope follower to add some movement to synths, guitars, bass... in fact, to anything. Again, I like to set this up as a parallel process and add further effects, such as delay or reverb. And if you bring MIDI to the party, you can turn almost any input signal into something rhythmic. Judicious use of high-pass filter, resonance, envelope and a noise source (see the 'Bring The Noise' box) can result in some excellent analogue hi-hats. Use the separate filter 1 and main outputs and pan them in stereo, turning any mono input signal into two distinct and separately processed analogue signal paths. The Filterbank's modular approach keeps things flexible and means you'll be finding new uses for it for many years to come.

Conclusion

The Filterbank is a studio Swiss Army knife. Figuring out exactly how to fit it into your setup might take some time, and some of its features can be difficult to master, even to a time-served owner. Like any real instrument, it takes time to learn, but will reward you with bucket-loads of analogue charm. The Filterbank has more character than a Marvel franchise, and offers something that software still struggles to deliver — unmistakeable analogue charisma.

As to the original question of whether the Sherman Filterbank remains relevant to modern music production, the answer is a resounding yes. The fact that it hasn't really changed in two decades is testament to its good design and durability. This new Compact version is capable of the same sounds and effects, and delivers that with the same build quality. The new form factor makes it ideal for modern tabletop setups, and it plays perfectly with modular synthesizers too. The Sherman Filterbank 2 Compact will undoubtedly take the Filterbank into its 25th anniversary and well beyond.