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Alesis CLX440

Compressor/Expander Limiter
By Paul White

Alesis CLX440 comp/lim.Photo: Mike Cameron

Although it features a familiar analogue-style control interface, the CLX440 has a digital interior that allows it to offer advantages such as a 'Look Ahead' facility more commonly found in plug-in processors.

The fact that an increasing proportion of home recording work is now being done via software plug-ins rather than hardware processing doesn't seem to have stemmed the tide of new hardware compressors vying for space in your rack. Alesis have taken a slightly unusual approach for their new dual-channel CLX440, in that either of the two channels can be used in mono (single channel) or stereo, and that — despite the analogue-looking user interface — the internal circuitry is based on purpose-built 24-bit/48kHz Alesis processing ICs. Only analogue I/O is provided (balanced, on TRS jacks), with further jacks for the expander Key inputs, the compressor side-chain inputs and the Direct Out (L plus R). This last connector is a ground-compensated (pseudo-balanced) output carrying a pre-compression mono mix of the left and right signals, and is used for connecting an equaliser or other processor into the side-chain, for the setting up of frequency-conscious compression. A rear-panel switch configures the CLX for use in either -10dBv or +4dBu systems.

Controls & Functions

Each of the CLX's two compressors is furnished with the familiar Ratio, Attack and Release controls, as well as Input and Output gain controls, a variable Knee control (progressively adjustable from hard to soft) and a 'Detect' control. Detect varies the side-chain between Peak and RMS detection, so instead of having to select from one or the other, you can choose something in the middle where necessary. As a rule, Peak detection is most useful for percussive sounds, while RMS is appropriate for vocals, complex mixes, or other non-percussive sounds.

The output from the compressor section then passes through a separate limiter stage (post-output gain control) governed by a single Threshold knob. Having a separate limiter is a good idea, especially when feeding digital inputs, as it acts as a safeguard against significant periods of clipping. A separate (and extremely bright) 'limit' LED shows when the limiter comes into play, while input levels, output levels and gain-reduction amounts (left and right) are shown by four 12-segment meters on each channel. This means that the compressor and expander gain reduction can be viewed independently, rather than being combined into a single display.

The next control gives away the fact that this compressor is digital — it's the 'Look Ahead' button. Look Ahead applies a 2ms delay to the signal so that the side-chain can get a 2ms head start on detecting and correcting peaks. This is a feature often found in plug-ins, but very rarely in hardware compressors, due to the need for digital delay circuitry. Illuminated Bypass, Side-chain and Key buttons are provided for both channels, whereas Look Ahead operates on both channels, presumably to ensure that there is no timing offset between them.

Most compressor/expanders feature a fairly simple expander section, but here the expander has exactly the same controls as the compressor, with the exception of the Detect control, which is replaced on the expander by a Hold knob. Hold forces the expander to stay open for a period of time after the signal falls below the expander threshold, either to deliberately extend the sound (as in the case of gated drums), or to prevent the expander from chattering when the decay of the sound is fluctuating in level. Only signals below the threshold are processed by the expander, whereas a compressor processes only those signals that rise above the threshold. In most instances, expanders are used as a gentler alternative to gating, but they may also be used to increase the dynamic range of over-compressed material, providing sensibly low ratio values are chosen. Both the compressor and expander share the same Bypass button, which means that you need to set the expander's threshold to its minimum value if you don't want to use it.

Using The CLX440

Like the all-analogue Alesis 3630 that came before it, I found the CLX440 a very capable compressor, able to cope with a wide range of sound sources. Unless it's used very heavy-handedly, the compression is very transparent, so if you need something to sound like it's had a good smacking, you also need to goad the limiter a little more than you might normally do.

If anything, the lack of an obvious character is one of the weak points of the CLX440 — though transparency is obviously desirable in some applications. Given that the compressor is all-digital, I feel Alesis may have missed a trick by not including an element of modelling, to emulate popular types of compressor. For example, a selector knob for tube, opto, FET, and so forth, would have been commercially appealing. An auto release setting may also have been useful for processing mixes or sounds with inconsistent dynamics.

When 'Look Ahead', which affects both compressor channels, is engaged, noticeably more compression is applied in Peak mode, no doubt due to peaks being caught that were otherwise sneaking past. In RMS mode the difference is less obvious, but Look Ahead still increases the amount of gain reduction slightly. Expanders, on the other hand, are at their best when they are at their least obtrusive, and this one is extremely good, partly because of the amount of control the user has over the settings. You can make it work just like a gate, or it can sneak in very gently to quieten pauses without making its presence obvious — very nice indeed.


The use of custom chips has made it possible for Alesis to offer two stereo channels of compression where you would normally only expect a single channel. Furthermore, the limiter stage is available at the same time as the compressor, and the expander is as flexible as you could hope for. Sound quality, in terms of noise and distortion, is extremely good, while the gain control is transparent and clean and there is no obvious dulling of transients. For some uses the compressor may be too transparent, but as most studios need more than one compressor, that's easily remedied by choosing something more 'coloured' as your other model. The CLX440 is best at invisible gain control and is particularly good for treating finished stereo mixes, though it can tackle pretty much anything without breaking into a sweat. Alesis seem to have combined the best elements of analogue (the control interface) and digital (low cost and high audio quality) into what is really a very impressive piece of kit.


  • Attractively priced.
  • Sonically very clean.
  • Separate limiter and expander.
  • Two stereo channels.


  • Lacks character for some applications.


A novel approach to digital compressor design that compenates for its lack of character by offering good value and audio transparency.


£329.99 including VAT.

Numark +44 (0)1252 341400.

Published September 2002