The MPX 550 Multi-effects processor combines the famous Lexicon reverbs with time- and frequency-based effects and dynamics processing.

Lexicon reverbs have a distinctive sound that has made them a staple in the studio. Dense, smooth, and full of character, the reverb just seems to stick to whatever you run through it. To many musicians and engineers, mixing without a Lexicon reverb in their palette is like painting without one of the primary colors.

Lexicon's multi-effects processor, the MPX 550 Dual Channel Processor, offers a lot more than the company's famous reverb algorithms. The unit's 255 factory programs include a variety of time- and frequency-based effects as well as dynamics processing. This means the MPX 550 can be used as an insert, across the stereo mix bus, or between an instrument and an amp.


Lexicon reverbs rely on a custom processor called the Lexichip. Although the MPX 550 uses only one Lexichip, its features and sound quality bring it well above the entry level.

The MPX 550 offers two discrete channels. You can use it as a stereo or dual mono processor, or run two mono signals through different effects and keep them separate. Some of the programs throw all the available horsepower at one high-quality effect; the reverbs are a good example of this.

The MPX 550 includes S/PDIF digital I/O, balanced analog ¼-inch and XLR I/O, and MIDI I/O (see Fig. 1). The processor offers extensive real-time MIDI control, including the ability to sync delay times and modulation rates to MIDI Clock. It can even get its tempo from a suitable audio source.


At the center of this 1U device is a backlit LCD that displays the current program, a graphical representation of the DSP building blocks and how they're patched together, and level meters. Even though it's possible to go as far as five screens deep when editing, the MPX 550's interface is very easy to navigate and understand.

The MPX 550 includes four rotary encoders to the left of the LCD. Each knob's function is labeled at the bottom of the screen, rather than directly over the knob itself. The large rotary knob on the other side of the screen scrolls through presets and user programs, and you can push it to get to Banks. Normally, the current program remains active until you push the Load button, but there is a preference setting that causes the device to load programs automatically when you park on them for a short time. Once chosen, the programs load quickly.

The Bypass button can mute the entire unit, mute the input only (so you hear only the effects at the output), or pass the input directly to the output through the unit's 24-bit digital converters. This last feature allows you to use the MPX 550 merely as a 24-bit A/D converter. You can also send the dry signal through the digital outs while the analog outs carry the wet signal, which is useful in situations such as live recording.


The preset programs are arranged in five categories: Single, where the entire processor is used for one effect; Dual, which gives you two effects simultaneously; Special Effects; Mastering Dynamics; and Live-FOH (front of house) for live applications. Within each category, programs are grouped into Banks of similar types of effects, such as plate reverbs, hall reverbs, and gated reverbs.

Remarkably, the MPX 550 provides the same level of editability that you'd get from more expensive Lexicon processors, which is something I didn't expect. But it's not always necessary to dig deep into the menu layers when editing. The first soft knob is assigned to a control labeled Adjust. This gives you instant access to predetermined parameters that Lexicon thinks you would reach for first when tweaking a particular program. In the hall reverbs, for example, Adjust moves the midfrequency reverb time and the decay slope simultaneously. On plate reverb programs, the Adjust parameter controls the reverb decay and high-frequency rolloff simultaneously. In other programs, Adjust controls only a single parameter, such as feedback in a delay program.

The MPX 550 includes a wide variety of delay programs in various rhythmic configurations. You can select a standard delay that merely repeats the original sound, or a filtered tape-echo simulation where the sound gets darker with each repetition. Or you can program the delay so that each of the six independent delay channels changes relative to the current tempo setting.

Single programs include tremolo, chorus, flange, pitch-shifting, and delay/echo, which offers 5½ seconds of mono delay with up to six taps and feedback. The MPX 550 includes a lovely rotating speaker program, Rotary, that is based on the Leslie Model 122, which simulates the woofer and tweeter spinning in opposite directions.

The dual programs are designed to get more mileage out of the unit. These programs consist of flange, chorus, or pitch-shifting, with reverb or delay. They're arranged in various configurations, such as in series or in dual stereo. In general, the dual programs don't sound quite as rich as the single programs, but they're completely usable.

As you would expect with a Lexicon device, the MIDI implementation is well thought out. MIDI controller numbers are preassigned to editing parameters, and you can easily change the assignment from the front panel. Alternatively, you can set the assignment by having a knob learn what MIDI controller you're moving; the distance you move the controller scales the distance necessary to move the parameter through its range. MIDI is also used for program changes, and you can dump and load the entire user bank, the current program, or the system setup. All of the MPX 550's program parameters can be controlled using MIDI System Exclusive data.


The final edit page of every program (except the mastering-oriented dynamics programs) accesses a compressor. Signals going to an effects processor often benefit from extra dynamics control, so it's nice to be able to apply that right inside the box. You can set the compression parameters globally or use the ones stored with each program.

The MPX 550 also includes dynamics processors for mastering, including peak limiting, expansion, and compression. The expansion and compression programs have a fairly subtle saturation algorithm, available in “modern” and “vintage” variations. The result is an increase in density with a slight loss of clarity. My preference would be to use it on individual tracks or possibly groups of tracks rather than an entire mix, in order to keep detailed instruments, such as cymbals, out of it.

One thing that surprised me about using the S/PDIF digital I/O with the MPX 550's Mastering Dynamics programs is that the effect reduces the input level. You have to raise the gain to 7 in order to match the level of the bypassed signal, even when all the parameters are set flat.


While there are differences in the complexity of the reverb sound when you compare the MPX 550 with the high-end Lexicon processors — I used one of my NuVerb cards for comparison — the differences are subtle. The MPX 550's reverbs are great, and the other effects are equally outstanding.

The factory programs cover a wide range of applications very well: the vast majority of the programs are useful, bread-and-butter effects. Overall, the MPX 550 is a sophisticated, versatile processor that sounds better than what you'd expect from a multi-effects unit in this price range.

Nick Batzdorfis a writer, composer, and general audio nerd who hails from Los Angeles.

MPX 550 Specifications

Analog Inputs(2) XLR; (2) ¼" TRSAnalog Outputs(2) XLR; (2) ¼" TRSDigital I/O(1 pr.) S/PDIF (RCA coaxial)Other ConnectionsMIDI In, Out/Through; ¼" TRS footswitch jackPreset Programs255User Locations64A/D Converters24-bitD/A Converters24-bitA/D Dynamic Range105 dBD/A Dynamic Range101 dBInternal Processing24-bitFrequency Response20 Hz-20 kHz (±1 dB)Sampling Rates44.1 and 48 kHz (selectable)Dimensions1U × 4" (D)Weight3 lb.

MPX 110

Lexicon's entry-level MPX 110 ($299) updates the MPX 100 with higher-resolution converters (24-bit versus 20-bit) and internal processing (24-bit instead of 16-bit). The MPX 110 also offers some of the Banks of single and dual programs found in the MPX 550, but without the LCD screen of its larger sibling. Instead, you use a 16-position rotary encoder to select programs (see Fig. A).

The MPX 110 comes with 240 presets and 16 user slots. The processor has unbalanced, high-impedance analog inputs designed to accept instrument or line-level signals, and unbalanced outputs (although the left output can power a pair of headphones). There's a S/PDIF output on the MPX 110 with 24-bit, 44.1 kHz resolution, but no digital input. You can use the MPX 110 as a standalone 24-bit A/D converter by engaging the effect bypass switch. Overall, the MPX 110 is a simpler, less programmable unit than the MPX 550. However, it includes the Adjust control, which behaves the same way as the one on the MPX 550, and many parameters can be controlled with MIDI. Priced at $299, the MPX 110 is an unbeatable entry-level effects unit that offers a wonderful collection of programs.


MPX 550
multi-effects processor


PROS: Good sound quality. Excellent selection of programs. Easy interface. Quick access to programming parameters. Hassle-free digital I/O.

CONS: Mastering effects reduce input level.

Lexicon, Inc.
tel. (781) 280-0300
e-mail info@lexicon.com
Web www.lexicon.com