Sherman Filterbank

Analogue filter banks are not exactly a new concept, but this one combines an unusually flexible specification with MIDI control and an affordable price. Chris Carter passes some filter tips...

Filters — don't you just love 'em? Sharp, soft, bright, dark, they each have their admirers and their uses. The current retro renaissance shows no signs of slowing down, with all the major (and many minor) players rolling out 'new retro' products monthly. With a massive international dance music scene as the current vanguard of ever‑more exotic filter sounds, it's not hard to see why manufacturers are keen to tap into this lucrative vein.

The Sherman Tank

The Sherman Filterbank under review here consists of a lot more than the name implies and is actually quite a package of goodies. There's a preamp (with overdrive), two multi‑mode VCFs (Voltage Controlled Filters) with selectable harmonics, an ADSR (Attack/Decay/Sustain/Release) and an AR generator, an Envelope Follower, an LFO (Low Frequency Oscillator) and two VCAs (Voltage Controlled Amplifiers). It is, in essence, a synth module without an oscillator.

If first impressions are anything to go by, this sturdy‑looking, rackmountable steel unit, with colour‑coded, rubberised knobs and bright, multi‑coloured LEDs scores points as soon as you unpack it. It's also compact enough, at 17 x 4.5 x 3.2 (inches), to drop into a gig bag or holdall.

That sloping front panel hosts 25 knobs and switches for controlling the various filter functions and modes. The rear of the unit features 10 jack sockets, for audio and control connections, plus a MIDI In, Out and three MIDI Thru sockets.

At first glance, the profusion of knobs and LEDs can appear confusing, but spend a little while familiarising yourself with the layout and functions and things become clear. Working from left to right along the top row of controls, the first one we come to is a single audio input and level control. This input knob also acts as a sensitivity control for triggering the ADSR/Envelope Follower and the AR generator. The input can take almost anything you care to throw at it — sampler, drum machine, Bassline, synth or guitar. An Overdrive feature is introduced by turning the input knob past its centre detent, and this can be adjusted between soft harmonic and grungy industrial distortion. Achieving a balance between having enough level to trigger the ADSR/AR and adding too much Overdrive can be a little tricky, though.

Next along is the FM knob, which modulates the filter. The FM depth is fed internally by the audio input, but if a second source (audio signal, CV/Gate or another LFO) is plugged into the external FM input, that can be used to modulate the filter instead.

The ADSR generator, next in line, is similar to the type found in most synths and is very versatile, handling short, long and inverted envelopes with ease. An LED indicates triggering, and a bi‑colour LED indicates whether the Sustain signal is negative (red) or positive (yellow). A small switch toggles between ADSR and Envelope Follower modes. In Envelope Follower mode, the ADSR control signal follows the level of the audio signal rather than just being triggered by it, with most of the controls working as in ADSR mode and the bi‑colour LED acting as a level indicator by turning red when the level is too low. The ADSR/Envelope Follower can also be triggered from a socket on the rear which will accept audio or Gate signals such as percussion samples, a Bassline Gate output, and so on. There's also an ADSR CV/Gate output socket for triggering external drum pads, sequencers and keyboards from the Filterbank.

The LFO has two control knobs, Speed and Depth, and produces a basic sine wave. The speed is indicated by a bi‑coloured LED and is adjustable from an extreme one cycle per minute up to a rate of 3kHz. This extends well into the audio range and is great for making bubbly, ring modulator effects. The LFO depth control modulates the VCFs in anti‑phase (great for stereo) and produces a positive waveform when turned to the right and negative to the left, with a centre indent for no output.

The AM input modulates the VCAs and is driven by the output of VCF2. The result of this is a type of modulation that also varies proportionally to the amount of VCF2 resonance. If an additional source, audio or CV/Gate, is plugged into the external AM input, a kind of rhythmic distortion can be superimposed on the sound.

Next is the AR generator, which controls the VCAs. While not as versatile as the ADSR, this is capable of producing some very usable pulsing and gating effects. It is also triggered by the audio input but has an external trigger input socket that can accept audio and CV/Gate signals.

The last two knobs (before we move onto the filters) are the Parallel/Series and the Bypass/Effect controls. The first allows a variable mix of the filters in either parallel (2 x 12dB) or series (1 x 24dB), or a 50% mix of both modes. The variable Bypass/Effect control is a welcome change from the usual in/out switch and allows smooth changes between heavily filtered effects, through more subtle effects, to a completely straight sound.

Filtered Or Plain?

The two VCFs are identical 12dB types, and each can function as a low‑pass, high‑pass or band‑pass filter, with various combinations of all three modes also available. Each VCF has a control knob for Frequency, ADSR depth, Resonance, low‑/band‑/high‑pass Balance, and Mode Correction. These last two controls need some explanation. It's pretty obvious what the Balance control does, but note that the filter mode is fully variable and not a switched type, so some nice tonal changes can be achieved by sweeping the knob back and forth. The Mode Correction control is marked ‑B (extreme left) to ‑B+L(extreme right) and has a centre indent for no correction. Let me give you an example of its function: if you set the LP/BP/HP Balance control to band pass and turn the Correction control right, toward ‑B+LH, this configures the VCF as a notch filter and produces a subtle phasing effect. VCF1 has a separate audio output and some very extreme swirling stereo effects can be achieved by setting the filters to different modes, in parallel.

Unlike some other filters of this type, the Filterbank refuses to go into full self‑oscillation. It tries, but only manages the mid/upper kHz range, so you can't produce the sub‑bass sine waves that some VCFs can. But it does turn out some pretty good high‑pitched sweeps and warbles, very techno (and 1950s sci‑fi). Just be careful you don't blow your tweeters.

Common to both VCFs is the large Harmonics/Sync 12‑way rotary switch, with blue on/off LED. Its first position is labelled 'Free' and allows both filters to run independently, while the subsequent positions are labelled with numbers. At setting 1, both filters are in sync and act as a single 24dB VCF. In this mode, the frequency knob and ADSR depth knob of VCF1 control both filters. Turning the switch to the next positions (1.5 to 16) introduces progressively lower harmonics until, at position 16, the harmonic content is four octaves lower than the original.

Retro MIDI

MIDI is included as standard, and the designer of the Filterbank has expanded the unit's usefulness tenfold by doing this. Although MIDI is implemented here in a fairly basic fashion, it does open up all manner of automated control options to the user. MIDI‑controllable functions include cutoff frequency and resonance (for each VCF), adjustable FM depth, VCA level (volume), AM depth, attack, decay, sustain and release times. MIDI can be used to trigger the ADSR and AR generators independently and the internal, audio input trigger can be turned on or off via MIDI. The Filterbank can also convert incoming audio and CV/Gate triggers to MIDI notes, from the external inputs of both the ADSR and the AR — a very useful feature indeed.

The only minus point is that the MIDI output only transmits MIDI trigger notes from the ADSR and AR, and nothing else — it doesn't transmit System Exclusive messages or Controller information. For Cubase users, a mixer map can be supplied on request, configured for the above MIDI Controllers. The current map is a little basic but does give an idea of the potential for using the Filterbank as part of a sophisticated MIDI system. Apparently, an updated version of the mixer map will be available from the Sherman web site soon.

Punching And Kicking

While this isn't the warmest analogue filter I've ever heard, it certainly has a lot of character. In the upper ranges, it has a pleasant 'grainy' sound, while in the middle ranges, particularly in band‑pass mode and with resonance, it can cut through a mix like a knife. But it's down at the bottom end that things get really serious. The effect that the Harmonics switch and the Overdrive feature have on a sound can be extraordinary, particularly if a lot of bottom end has been filtered out by the VCF. A thin and lifeless rhythm or bass line can take on a whole new life, with added bass punch and kick. And the Filterbank shouldn't be restricted to processing electronic instruments exclusively. I tried feeding a guitar through the overdrive input while triggering the ADSR/VCF from a drum‑machine snare and triggering the AR/VCA from a hi‑hat pattern, with the LFO adding some modulation to the VCF. The phrase 'polyrhythmic acid funk' could sum up that sound. And, of course, with MIDI control you can achieve unbelievably complex filter sweeps, ADSR triggering and VCA modulations. The Filterbank works particularly well for breathing new life into samples, drum machines, digital synths, guitars, and even the ubiquitous TB303.

But beware! This unit could seriously damage your speakers, and there are dire warnings concerning low frequencies printed in the manual and on the Filterbank unit. If you're really worried about the potential for damage, though, you could use the unit through a compressor/limiter.

There were no noise specifications available for the review model Filterbank, but I found the audio quality to be clean and noise‑free, even at extreme settings. However, I did notice a couple of idiosyncrasies: one was a slight, but not unpleasant, instability when the filters were at maximum resonance. Another was the tendency for the volume to disappear, or suddenly jump to maximum when sweeping the ADSR depth control — although this could have been to do with the nature of envelope generators, rather than anything wrong with the unit.

Pandora's Box

What really sets the Sherman Filterbank apart from others in this crowded market is the additional combination of overdrive, harmonics, envelope generators, LFO, VCAs and MIDI control. While it is probably better suited to studio use than live work it will undoubtedly become required kit for many remixers, dance acts and producers (see 'Famous Names' box for some of the artists already using it). In this price range there is nothing quite like it, and whether you're into retro‑sounding gear or not I can wholeheartedly recommend it. This is a hell of a processor. If a Moog is a Rolls Royce, the Sherman Filterbank has to be a Porsche.