Something A Bit Special

by Paul White
Sound on Sound (United Kingdom) – August 2008

German company elysia clearly feel that compressors should be able to do far more than simply smooth out level fluctuations, and their mpressor stereo compressor, reviewed here, aptly demonstrates this. On the face of it, the 2U-high mpressor looks much like other boutique compressors, with independent channel controls that enable it to be used as two mono compressors or, in linked mode, as a stereo compressor. The inputs and outputs are on the usual balanced XLRs and there are side-chain inputs (also on XLRs) that can be switched in and out from the front panel. But while it can certainly handle routine compression tasks, the mpressor is also intended for significantly more creative applications – and elysia use words such as ‘fat’ and ‘freaky’ to describe its capabilities.

Inside Information
With its massively thick front panel, custom metal knobs and annular red lights around the buttons, the mpressor certainly looks built to impress – but is it equally impressive inside? The circuitry inside this compressor is all analogue and uses high-quality, discrete components. The signal path is based on transistors operating in Class A, and using as little negative feedback as is practical. The side-chain and power-supply components are also fully discrete circuits. The power transformer and capacitor bank is very generously rated, and all the controls use conductive plastic pots for smooth and reliable operation.

Switching is handled by encapsulated relays, while certain parts of the circuitry (presumably the gain control transistors) are mounted beneath temperature-controlled metal blocks, to ensure consistency of operation. The mpressor doesn’t use an input transformer: instead, the circuitry is designed to behave as much like one as possible, with good common-mode rejection and low capacitance, to prevent loading the input at high frequencies. So, as with a transformer, if the unit is used unbalanced, the unused pin must be grounded and not left floating. The class-A output stage is a low-impedance, high-current design, able to drive long cables without loss.

Innovations
Before I go through specific controls, it is interesting to see what innovations set the mpressor apart from more conventional compressors. The design includes what elysia call their Auto Fast function (taken from their alpha compressor), that allows the compressor to respond very quickly without introducing distortion or other unwanted artifacts when using fast attack and release times — though if you like your compression to be obvious there’s also an ‘Anti Log’ release curve circuit that can be used to emphasise effects such as pumping and gain breathing. Because the circuitry uses a feed forward design (based on monitoring the compressor’s input rather than its output), it is also possible to set negative compression ratios (expansion), which may be used to create seriously over-the-top compression effects. For example, you could take a drum track mix and use negative ratios, with Auto Fast and a fast release time, to dramatically reduce the levels of the loudest drums (typically kick and snare) without affecting the beats in between.

With Auto Fast switched on, the attack time automatically adapts to the characteristics of the incoming audio, so that it is at its fastest on steeply rising transients. Essentially, Anti Log adds an anti-logarithmic characteristic to the release time curve, linking the release time to the amount of compression taking place. As the input signal level falls, the release time is shortened automatically, making the compression more obvious-sounding. According to the manual, this feature may be used to create BPM-dependent dynamic effects, to reshape the envelope of reverb or to process single sources such as vocals or bass in a more obvious way.

Yet another novel control feature is the Gain Reduction Limiter, which allows the user to set a value for the maximum amount of gain reduction that can be applied. This limits the level of the gain-reduction control voltage, so that beyond a certain point no further gain reduction is applied. This allows mid-level signals to be compressed quite heavily without reducing the dynamic range of very quiet or very loud signals. The Gain Reduction Limiter can also be used in conjunction with the side-chain to achieve more precise ducking, and has enough flexibility to apply only upward compression, where low-level signals are effectively raised in level but higher-level signals are left untouched (a characteristic sometimes associated with parallel compression).

To further extend the creative potential of the device, a Niveau Filter is available after the compressor section. This filter is a type of ‘tilt’ equaliser that’s used to balance the two halves of the frequency range of a signal about a fixed rotation point (frequency). In other words, it boosts the high frequencies beyond the (user-adjustable) centre frequency and cuts the low frequencies at the same time, or vice versa. The designers claim to have made this equaliser sound more extreme than [on the alpha compressor], yet only two knobs are needed to operate it. This can be used very gently in conventional applications, but more extreme settings work well for some styles of dance music.

It would appear, then, that the design aim was to build a top-quality compressor that could handle all the expected requirements of mastering and mixing with appropriate finesse. However, on top of this they added a handful of off-the-wall features, for those occasions when something more brutal is called for. So the range of compression that can be coaxed out of the mpressor goes from the mild but precise dynamic-range control needed during mastering to extreme dynamic envelope modification and deliberate pumping effects. Despite this incredible flexibility, though, the designers have obviously worked hard to produce a design that means the engineer doesn’t have to work too hard to achieve the desired results.

In Control
Both the left and right channel control sections are identical, with the two bypass buttons and the Link button (the left channel acts as master in link mode) located between the channels, beneath the two gain-reduction meters. The top four knobs in each section address the familiar Threshold, Attack, Release and Ratio functions, with make-up gain provided by the last knob on the lower row. The first two knobs on the lower row are used to operate the gain and frequency of the Niveau filter EQ, while the third is dedicated to the GR Limiter (with a range of 0 to 21 dB).

Between the knobs are six buttons that give you access to the additional features – namely the side-chain input, Auto Fast and Anti Log, as well as the filter bypass, EQ range x10 and GR Limit ‘on’. Normally the filter range is adjustable from 26 Hz to 2.2 kHz, but using the x10 switch it is able to cover the full audio spectrum. As you’d expect from a compressor of this calibre, the bypass switch links the input directly to the output for a true hard bypass. The button I had expected to find, but didn’t, was one that switched the meters from output level to gain reduction. It seems odd that on a compressor of this sophistication there’s no way to check on the levels passing through it – although it has so much headroom it is unlikely to overload before anything else in the chain.

Sounds Mpressive?
Used as a ‘standard’ compressor, the mpressor behaves very predictably and smoothly, reducing the dynamic range without significantly changing the character of the sound, unless you deliberately get very heavy-handed. You can really squash down mixes without them sounding obviously processed and without losing any of the important transient detail or clarity. Increasing the attack time makes the same ‘normal’ mode very effective for beefing up drums or bass, again without losing definition. Indeed, you often don’t notice a change in the basic drum sound: you just notice that the room ambience and decay level have come up. Where more brutality is appropriate, the Auto Fast button jumps on the fastest of transients, and in combination with high ratios this can be quite dramatic.

Bringing in the Anti Log function moves the effect up a gear, making the compression release far more obvious, while in conjunction with negative ratios you can subdue loud transients almost to the point of extinction, or create those ’60s-style compressed cymbals that seem to get louder after they’ve been hit. To enhance transients without squashing everything else, the GR Limit function is a real bonus, and I found myself using this a lot to tame the effects of hard compression while still nailing the level. It’s also a useful function for compressing stereo mixes: more compressors should have this facility! To get the best from the less obvious features requires a little experimentation, but the manual provides some clearly illustrated examples of different settings and goes into some detail to explain how and why they work. Using the external side-chain input, the mpressor also makes a very predictable and well-behaved ducker, as the GR Limit can be used to set exactly the amount of maximum gain reduction that’s required.

If you prefer to venture further into the creative abuse area, you can get some great, nasty, crunchy sounds by using very fast attack and release times with Auto Fast switched off – so the compressor attempts to respond to individual cycles of sound – and that Niveau filter is very handy for doing dance-style filtering, either thinning things out or making them sound as though they’re coming from next door. Some pretty aggressive mix bus settings are suggested, which, again, would suit dance music or electronica – but don’t worry if that isn’t your cup of tea, because all of the more usual ‘safe and sweet’ settings also work extremely well.

Weapon Of Mass Reduction
The mpressor’s designers are clearly enthusiasts who’ve built what they consider to be the ultimate compressor – that is, the one they’d like for themselves! Like any other product in this quality category, you have to weigh the price against the benefits – and for the same money you could buy a leading DAW package and a fast computer to run it on, still leaving change for an audio interface and a pair of monitors – so you either need to be wealthy enough not to care or have enough work coming in that the mpressor can earn its keep. Of course, you won’t have to budget for separate mastering, mixing and sound-mangling compressors, as this one does a first-class job in each area. While the mpressor may appear to be an expensive luxury, it is one of the most versatile compressors I’ve ever come across. Its low distortion, ultra-transparent circuitry makes it perfectly suited to mastering and mixing in the traditional sense, while those weapons-grade extras make it invaluable for beefing up drums or bass – or just about anything else you want to energise. The range of sounds goes way beyond the natural, but then that’s the whole point!

Alternatives
I can’t think of any other single unit that does so much with so little effort – although you could probably use a good all-round compressor along with a Transient Designer and a separate filter unit to recreate some of the mpressor’s effects.

Pros
- Extremely flexible
- Pristine audio signal path
- Well thought-out controls

Cons
- Fairly costly
- No level metering

Summary
This compressor really does cover all bases, from transparent dynamic control to creative abuse.