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Lexicon MPX550

Dual-channel Effects Processor By Paul White
Published January 2003

Lexicon MPX550 dual effects processor (angled view).

Lexicon update their MPX500 with more editing parameters, extra mastering processes, and more user patches.

While many studio processors have been replaced by software plug-ins, host-powered reverbs tend to be either a little unrefined or inordinately greedy when it comes to CPU power. The very best reverbs are way out of the price range of most UK project studios, while entry-level models often sound somewhat disappointing, but in the £300-750 range are to be found a number of excellent-sounding units that, although they don't equal their high-priced cousins, can come surprisingly close. One such was the Lexicon MPX500, which has just been replaced by the MPX550, offering more user memories, more effect algorithms and new dynamics processing.

At the heart of the MPX550 is the same Lexichip III reverb engine that powers some of Lexicon's high-end machines, though the algorithms have been simplified to some extent, as has the degree of user editability. The I/O is nothing if not comprehensive, featuring both XLR and jack analogue I/O (balanced) plus S/PDIF digital I/O on phono connectors. Sample rates of 44.1kHz and 48kHz are supported. Power comes from the mains, not an annoying adaptor, and the 24-bit converters deliver more dynamic range than most home studios could hope to do justice to. In fact the only pro feature that's missing is a word clock input.

Lexicon MPX550 £399
  • Good-sounding reverb algorithms, including ambience.
  • Friendly user interface.
  • Non-reverb effects are also good.
  • Mastering programs have slightly too much delay to be used for individual track processing via insert points.
A worthwhile upgrade to the already excellent MPX500.


Like the MPX500 before it, the MPX550 offers true stereo operation with a number of routing options including stereo-in/ stereo-out, dual mono-in/stereo-out and dual mono-in/mono-out. There are also several dual effect algorithms based around both serial and parallel configurations. MIDI In and Out/Thru connectors are provided for patch dumping (the Out/Thru function is software configurable from within the System menu), real-time parameter control or tempo control, while a TRS footswitch jack enables two footswitches to be connected to operate the bypass and tap tempo functions.

So far, then, the description is very close to that of the MPX500 and, indeed, aside from new front-panel artwork, the controls and display are identical. The real differences happen inside the box, specifically the new effect algorithms and the increase in memory space. Now there are 255 preset effects and 64 user memories, whereas the MPX500 had only 30 user memories. Compression can now be used in conjunction with EQ, and there are new dynamics algorithms offering full-featured compressors within the reverb algorithms, as well as dedicated mastering dynamics algorithms. For those unfamiliar with the original MPX500, I'll quickly go through the operating system, as that's one part of the machine I really like.

Driving The MPX550

The controls start out predictably enough with an Input trim control, but, instead of LED meters, stereo bar-graphs are available via the display. Patches are selected by turning the Program dial to move through patches one at a time or by pressing and turning it at the same time to step through banks. Pressing Load loads up the selected program, though a system preference option provides for automatic program loading 0.75 seconds after the Program knob stops turning. If you select a new patch without loading it, the display switches back to the original patch after a few seconds, but the number of the new one is 'remembered' and may be loaded by pressing Load at any time after selecting it.

In edit mode, a custom LCD window shows a page of up to four parameters at once, where each can be adjusted directly using one of four dedicated rotary Edit knobs to the left of the LCD. Many functions also now include graphic elements, which makes the effect of adjusting the control more obvious — for example, changing the reverb decay shows a typical exponential decay envelope that changes in length as adjustments are made.

Lexicon MPX550 front panel controls.

The first parameter in the first page is always Adjust — a control that changes multiple parameters at once to help speed up editing of programs. For example, Adjust control of room liveness may change decay time, early reflections and tonality all at once. Each effect algorithm has up to 20 adjustable parameters (four more than the MPX500) which means up to five pages of four settings to adjust. The Edit Pages button makes light work of skipping through these and the display always tells you which page you are on. As a rule, the most commonly accessed parameters are on the top page, while the compression parameters within reverb algorithms tend to be found on the last page.

System setup, accessed using the System button, allows various utility settings to be made relating to the operation of the bypass control, the digital I/O settings, MIDI settings, patch loading mode and so on. You can also choose whether the wet/dry mix settings apply to every program or whether they are saved separately for each program. Normally, if you're using the MPX550 via a mixer's aux send/return, you'll want to set the patches to output wet signal only.

A Tap button quickly sets tempo-related delays and other effects, and some tempo-related parameters may also be sync'ed to MTC or MIDI Clock. Delay and modulation parameters can be set to tempo over the range 40-400bpm, and it's possible to choose a global tempo value for all patches or save each program with its own tempo setting. There's also an audio tap feature that uses the time measures between two audio events — holding down the Tap button activates this mode until Tap is released. Tempo can also be locked to MTC or MIDI Clock by activating this feature in the System menu. Tap and Store may be pressed together to initiate the MIDI learn mode, where any parameter can be accessed using continuous MIDI controllers, aftertouch or pitch-bend. Turning the edit knob associated with the parameter identifies it as the one to be mapped, after which you only need send the unit the desired continuous controller message and the relationship is formed.

The comprehensive I/O features both XLR and jack analogue connections (balanced) plus S/PDIF digital I/O on phono connectors.The comprehensive I/O features both XLR and jack analogue connections (balanced) plus S/PDIF digital I/O on phono connectors.

Using The MPX550

Unlike some really sophisticated effects boxes where you can set up everything from scratch, the MPX550 adopts the now familiar format whereby you pick a preset that uses the algorithm you want (including the routing option you need), then adjust the parameters to suit. The dual mono-in/stereo-out algorithms enable the unit to behave as two separate effects processors, where the stereo outputs are mixed, so you can feed it from two aux sends and return it through a single stereo aux return. There are also dual mono-in, mono-out modes for where that is appropriate, but for anything involving reverb I find it's best to work with stereo outputs.

Alongside the now very familiar reverb, modulation, delay and pitch effects are versions of the famous Lexicon Ambience algorithm, rotary speaker (complete with separate high and low rotors and proper acceleration/deceleration dynamics), and of course the new Dynamics section. Compression is included in many of the reverb algorithms and dual-effect programs, but the separate Dynamics section provides dedicated stereo processing intended for mastering applications. The manual rightly points out that, because such complex processing introduces a perceptible delay, it should not be used via individual channel insert points while mixing. This section includes a Peak expander, which makes all sounds above a user-defined threshold louder, A conventional compressor with variable ratio, threshold, attack and release, and a tape saturation emulator that attempts to bring back some of that analogue warmth by recreating the non-linearities of hard-driven tape.

The MPX550 is joyously simple to use, and the fact that it is filled with so many good-sounding reverb algorithms means that you generally only need to tweak a couple of parameters to get exactly the sound you want. The room and ambience programs exude a sense of real space, and even the longer reverbs can be used at quite a high level without drowning the original signal or fogging up the mix.

The non-reverb programs are also very strong, and though few offer anything truly new, they are clean and professional sounding. The pitch-shifter isn't great, but then it seems impossible to make a good one at a project studio price, but for detuning and theatrical effects, it's perfectly fine. I was also favourably impressed by the inverse reverb algorithms, which lend a convincing backwards character to the sound, and the addition of compression to the reverb algorithms adds a nice sense of density and space while keeping the reverb level firmly under control.

As to the mastering dynamics algorithms, I wouldn't dream of suggesting that they're a suitable replacement for a proper multi-band dynamics processor, but they do a good job without inflicting too much of their own character onto the sound being processed. Even the tape saturation algorithm is quite subtle, but it definitely adds warmth and smooths off those abrasive peaks. Used with care, demos can be made to gel better using these facilities and, as with most Lexicon boxes, you have to work quite hard to get a bad sound.

As a replacement for the already fine MPX500, the MPX550 has a lot going for it, not least its extremely good reverb algorithms and its friendly user interface. Good reverb makes a lot of difference when mixing, so choosing to use something like the MPX550 in place of a plug-in or budget unit could make more difference to your music than you might think. The sound of the MPX550 is significantly better than the budget MPX100 and its direct offspring. Although it won't fool a 480L user, it comes closer to the sound of the PCM80s and PCM90s than you might expect, and its reverb quality is every bit as good as the MPX1. If you already have an MPX500, I don't think there's any pressing need to rush out and upgrade, as the basic reverb quality seems to be exactly the same, but given that the MPX550 costs no more in the UK than the MPX500 used to, the extra memories and dynamics algorithms are worth having if you're in the market for a new effects unit that does serious reverb.

 Test Spec 
  • MPX550 OS v1.02
infop.gif £399 including VAT.