Ask anyone who knows anything about classic analogue synths what gave them such a special sound and they'll tell you it was the filters. Can that quality ever be recaptured in a contemporary design? Ask Peter Forrest...

In case you didn't know, there's more to Cornwall than ice cream and pixies. Not far from the Aphex Twin's old haunts lives a man with a mission - to keep the flame of analogue burning strong into the next century. Bob Williams, featured in MT May '93, has turned from collector/dealer into manufacturer. His company, Analogue Systems, have set out their stall as producers of modern classic analogue devices. In the pipeline are a sequencer and a synthesiser module, but available now, as the first of the line, is the FB3 voltage-controlled filter. I must come clean and declare my interest in the unit - it's fallen to me to write the manual for it - however, with the best interests of MT readers at heart, I can promise you an honest review - warts and all.

OK, so first of all, what exactly is it? The name 'filter bank' conjures up images of simple multi-band EQ device - the sort of thing that EMS (another Cornish legend) are associated with. In reality, the FB3 turns out to be a 1U rackmount module containing three voltage-controlled filters connected in parallel, and capable of treating any audio input. Each filter has individual controls for cutoff frequency and resonance, and there's a master frequency control as well.

If you whack up the resonance, each filter will break into oscillation and produce a sine wave of its own, whose pitch depends on the frequency setting. As you might imagine, this is great if you want it but potentially embarrassing if you don't: the switching system only routes the input to the filters - it doesn't stop the filter's output from proceeding to your amplifier.

There's a quaint little switch for choosing one, two, or all three of the filters, and as with the three oscillators on the Minimoog, there are times when less is more; when adding in the third filter actually detracts from the sound the first two are making. The way the switch is set up means you can't go from filter 2 to 3 without passing through 1, which means that the change isn't seamless; but in practise this doesn't prove to be any real problem.

The filters themselves, incidentally, aren't simply off-the shelf chips, but have been custom designed for the FB3, offering a choice of four outputs from which signals may be derived. Two (the highpass and lowpass filter outs) give a 24dB/octave slope (like early Moogs), the other two (notch and band-pass) offer 12dB/octave (like the 2-pole filters on early Oberheims). The great thing is that you can use any or all of these outputs simultaneously, opening up an excellent range of possibilities. For instance, you could connect up 'opposite' outputs (like high and lowpass) to left and right of a stereo amp, or put all four into a mixer and use the faders for subtle timbral variations.

A built-in LFO is included for those wicked periodic filter sweeps, featuring variable speed and depth controls, an LED indicator and also its own output so that it may be used either to control a second FB3, or an external synth. All inputs and outputs, incidentally, are on the front panel for easy access; the only thing on the back is the on/off switch. To round things off, there's an external CV (control voltage) knob on the front panel, and this can go from full-on positive to full-on inverted, so that incoming voltages can control the filters in a variety of ways. Having just one CV input does seem rather limiting, but any more would presumably have had considerable cost implications.

A good number of old monosynths (the SH101 and the ARP Axxe, for example) have a keyboard CV output socket and by using this you could make the frequency of the FB3's filters track the notes you're playing. However, the FB3 really comes into its own when used alongside a modular or semi-modular synth. This needn't be anything rare and expensive like a Moog, Serge or E-mu. A much more modest instrument like a Korg MS20 or, better still, an old Digisound, will also do the job - which is to feed control voltages to the FB3.

Obvious connection points would be the outputs from the envelope or sample-and-hold circuits. It would be best to add together several control voltages in your modular system, so that you had, say, an envelope controlling the filter cut-off, but also include a keyboard tracking CV to keep the filter response equal across the whole keyboard. One of the beauties of the old CV/gate system is the ability to add any number of control voltages together, from any number of sources, without having to worry about the addition of the analogue equivalent of a MIDI merge box.

Given that the classic, dramatic filter effects always employ a degree of envelope control, one has to ask if the FB3 would still be worthwhile for someone who doesn't have a synth system with this kind of output facility. On first acquaintance with the unit my answer would have been a definite 'no'. But as I became more familiar with it I have to say I began to change my mind. Even without any control voltage input, the FB3, with all three filters set to similar (but not identical) frequencies, produces some delicious timbres, thickening up an MS20, humanising JX10 strings, and even doing the impossible - warming up further a Minimoog sound! This, of course, can only happen because of the introduction of small variations in pitch response and phase correlation, and in that sense it may not suit the purist. But as far as I'm concerned if it sounds good, use it. And the FB3 sounds very good indeed.

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