Mutator reviewed by Paul Nagle

The predictability of modern digital sound sources, while convenient, may have you longing for some of the chaotic complexity of analogue. PAUL NAGLE looks at an analogue filter which could put the surprise value back into your music..

 With the thirst for analogue warmth in a cold digital age seemingly unquenchable, the number of devices aimed at the retro market shows no sign of slowing. While some companies are busy packaging old sounds in new boxes, others are concentrating on devices through which you can force your squeaky-clean sounds, to have them emerge suitably endowed with retro-credibility. New boys to this market are Mutronics, and their first offspring, the Mutator, aims to bring the warmth of the analogue filter to our warm analogue ears.

 The Mutator is basically a pair of 24dB-per-octave analogue filters with envelope follower capabilities. This latter point means that the input volume level of a sound can determine the cut-off frequency of the filter – in other words, the filter 'follows' the 'envelope' of the input signal. An external audio source can also be used as the control signal, allowing the Mutator to superimpose the envelope of one sound onto the filter contour of another – in much the same way as an external key input on a compressor might enable one sound to control the compression characteristics of another.


 Since the Mutator is a two - channel device, the controls for each channel are largely duplicated, although there are some important differences that I'll point out as we pass along. Over on the far left of the front panel is the Bypass/Effect switch, enabling quick and convenient wet/dry comparisons. How often this simple control is forgotten by manufacturers who should know better! Next up is the Internal/External control source switch, which determines whether the treated signal or an external audio feed will be used as the control source to drive the envelope follower. The Gate/Envelope switch tells the Mutator how its envelope will react to the control source. In 'Gate' mode, the control source becomes a trigger, turning the envelope follower fully on as soon as the signal reaches the threshold set by the Sensitivity knob, and fully off again as soon as the signal falls back below the threshold. In 'Envelope' mode, the control source is continuously tracked by the envelope follower, to provide a variab control voltage for the VCF (Voltage Controlled Filter) and VCA (Voltage Controlled Amplifier), with the Sensitivity knob taking on the task of controlling the envelope drive level.

 The envelope controls themselves are quite basic for anyone used to the multi-stage variants found in most of today's digital synthesisers, providing adjustment of attack and release times only. This is probably the weakest area of the Mutator's spec. It would be nice to have a full implementation of an ADSR envelope, which would give a much more interesting opportunity to provide filter modulation over an otherwise static signal. One way around this is to use an external control source to provide the envelope follower with the ADSR characteristics required – when used with a synth this could be achieved by programming a second sound to play in unison with the first, and feed this from a separate output into the Mutator's External input. Usefully, the envelope follower is equipped with an LED level indicator that shows the current output level. This LED makes setting drive and threshold levels relatively simple..

 The onboard LFO (Low Frequency Oscillator) is capable of running from approximately 0.1Hz (one cycle every 10 seconds) to 100Hz (100 cycles per second). This is a good, healthy range, allowing subtle changes in timbre over time, or clangorous, pseudo-ring modulation effects. Four waveforms are available for the LFO, these being the familiar triangle, square, ramp and inverted ramp. A single LFO depth control is provided, doing duty for both the VCF and VCA. A 3-way switch determines whether the LFO will modulate VCF only, VCA only, or both. In keeping with the Mutator's suitability for stereo signal processing, Channel 2 will happily make use of Channel 1's LFO. The LFO waveform sent from Channel 1 may be inverted, for dramatic stereo auto-panning or filter frequency-dependent sweep effects..

 The voltage from the envelope follower arrives at the VCF via an Envelope Sweep control knob. This knob is capable of generating both negative (turn to the left) and positive (turn to the right) control voltages to govern the filter's cut-off frequency – it's good to see the possibility for negative envelope sweeps. Apart from the more obvious uses, such as those delightful squelch synth 'twangs', the Mutator can also perform frequency 'ducking' – perhaps to drop the high frequencies from a heavy guitar backing during vocal passages, for instance?.

 VCF cut-off and resonance both have their own dedicated controls. With resonance turned way up, the filter can be driven into good old fashioned self-oscillation. A switch on the far right toggles the VCA in and out of circuit.

 The review machine came equipped with the optional MIDI board. This option allows the Mutator to respond to various MIDI message types. At its most basic, the MIDI board might be put to work triggering the Mutator's envelopes from MIDI Note On commands. The filter cut-off tracks the incoming MIDI note number and may also be swept across its entire range by use of pitch-bend commands. Resonance is controlled from a modulation wheel. The Mutator's response to MIDI controller numbers is hard-wired at the factory, so if you don't like a particular controller assignment you'll have to use some form of external MIDI processor to do some re-mapping. I found the assignment of mod wheel to resonance the most problematic, since it precluded the use of mod wheel vibrato. I got around this by programming my MIDI patchbay to filter out mod wheel messages to the Mutator. The VCA level will respond to standard MIDI controller 7 volume messages, but an option to apply key velocity to VCF cut-off and VCA level would have be welcome. MIDI channels are selected with a rotary switch on the rear of the machine, which is a tad inconvenient! While Channel 1 responds on the chosen MIDI channel, Channel 2 makes use of this channel + 1 (or channel 1, if the selector is set to 16). Unfortunately, there is no way to allow both of the Mutator's audio channels to share the same MIDI channel. Also on the rear panel (although I am assured by Mutronics that they will make it to the front in production models!) are two switches responsible for toggling the Mutator's use of MIDI Note On/Off commands. We've already visited 'Gate' mode, which merely triggers the envelopes. When the Mutator is in 'Envelope' mode, MIDI Note On commands can be used to reset the LFO back to the beginning of its waveform cycle. This voltage will be held until a MIDI Note Off command is received, at which point the LFO will continue to produce its normal waveform. This simple feature is an extremely inspiring tool for creating all kinds of subtle – or not so subtle – rthmic variations. You could use this facility to keep a repeating synth pulse in time, or just make random stabs to add a little anarchy to the proceedings.


 With my mouth watering like a vampire in a blood bank, I plugged in, looped a couple of favourite sequences and began to play around with the filter controls. The Mutator began to burble and squeak in a most appealingly analogue manner. I found the envelope follower extremely responsive and was able to achieve excellent results with a variety of recorded sources, such as bass or rhythm guitar tracks, synth pads and backing vocals. By triggering the VCF and VCA from a drum track and feeding the bass guitar track through the Mutator, I was able to generate a whole new bass part! With more subtle settings, I found that I could add rhythmic emphasis to synth pads without making the effect too obvious, the drum hits just lifting out the higher frequencies in the sound. The LFO retrigger option had simple bass lines squawking and zipping across the speakers like a demented parrot! Even pedestrian sustained chord parts took on a whole new meaning with a little stereo LFO filter sweep and some externally triggered abs. At a pinch, the Mutator also does a reasonable job as a pair of single-ended noise reduction devices or as basic noise gates, although the results are not as refined or controllable as one would expect from a dedicated processor.


 The next job was to plug in my trusty MiniMoog and see just how good the Mutator's filter really is. I was pleasantly surprised to discover that the answer is "pretty damned good, actually!" I had to check that I'd opened up the Moog's own filter because it was occasionally difficult to tell the difference – now that is a good filter! The more I attempted to overdrive the Mutator, the warmer it sounded. A sampled sawtooth waveform from an Akai S1100 sampler emerged from the Mutator with an altogether different character, thicker and much more worthy of attention in a busy mix. This is without doubt the best-sounding retro filter I have come across. Hear it and weep. In fact, the only real drawback is that lack of a full ADSR envelope, which would really be the icing on the cake. I seemed to spend far too long setting up separate feeds from my sampler to provide envelope templates, when a simple twist of a control would have been much less traumatic. The Mutator's inability to have both channels trigger fr the same MIDI note message made setting up for auto-panning a bit awkward too – a 'MIDI link' feature would certainly be on my wish list for future developments.

 As a production tool, the Mutator is pretty much in a field of one. I can think of no other device that would achieve quite the same results – and certainly none that would be as quick to operate. Into one side goes a relatively mild-mannered musical part and out the other climbs a party animal with a 12-pack and an attitude! When in use as a synth retro-filter, the Mutator is a little clumsy due to its limited envelope, its basic and inflexible MIDI specification and the lack of any programmable memories. For those who are prepared to put in the effort, the results are the best that I have heard from the new breed of analogue filters, but the Mutator is arguably more at home in a production environment than hanging off the back of a sampler. This machine has its little idiosyncrasies, but then so do all the best things in life. I have a strong suspicion that the Mutator will carve out a niche for itself and will invoke the kind of fanatical devotion reserved for similar niche products – the Roland DimensioD and dbx Boom Box immediately spring to mind. This device scores highly as a production tool, and should certainly be checked out by remixers everywhere. In the right hands, it is a highly creative device that could just give your productions that elusive edge.


 Each of the Mutator's audio channels has four associated sockets on the rear. The audio in and out are pretty much self - explanatory. The external input is where a second audio source is plugged in to act as modulation source. The final socket is a simple CV (Control Voltage) input, which will allow modulation of the filter cut-off, perhaps from a modular synth system or a MIDI/CV converter. The audio in, external in and CV In of channel 1 are normalised to channel 2, which helps with setting up for stereo operation (i.e. 1 in, 2 out). Power is supplied to the Mutator via a standard Euro - plug connector with easily accessible fuse – none of those nasty wall - warts or line lumps here, thank you very much.


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