Lexicon MPX-500
A 24-bit dual-channel processor that Lexicon pitched as the most powerful digital engine it had ever built. The service manual documents it down to the test voltages.
Overview
The Lexicon MPX-500 service manual tells the technician to feed the left input 3.083Vrms at 1kHz and sweep from 20Hz to 20kHz, expecting the output to hold within ±1.0dBr the whole way. It doesn't promise the world. It assumes you'll measure it. The analog-in-to-analog-out frequency test, the crosstalk verification, the THD+N check and the dynamic range sweep are all laid out step by step in Chapter 4, "Performance Verification." The manual describes the chapter as a quick check of the unit's operation and the integrity of its analog and digital signal paths.
The headline claim, at least as Lexicon's own advertising framed it, was the engine. Period promotion billed the MPX-500 as carrying Lexicon's most powerful digital engine ever built, the proprietary LexiChip, paired with 24-bit converters and true stereo processing. EQ (January 2000, p.15) introduced it as a dual-channel processor running the proprietary Lexichip with 24-bit conversion, with dual-channel processing and an easy Learn mode for dynamic MIDI patching of front-panel controls. The pitch was a studio-grade reverb and effects brain at a price that put Lexicon's algorithms within reach of project studios that couldn't justify the big rack units.
The analog side is more interesting than the marketing copy. The dynamic range test asks the technician to apply a 1kHz sinewave at 7.746Vrms, set a 0dB reference, pull the input cable, engage the 30kHz and 20kHz low-pass filters on the analyzer, and verify the output noise floor is better than -97dBr. The typical figure is -101dBr. That number is measured through the actual analog input and output path, not lifted from a converter datasheet. For a processor in this class, a noise floor that low means the effects stay clean even when you stack them.
The test equipment list shows what Lexicon expected of the people servicing these. You need a low-distortion oscillator with under 100 ohms output impedance and under 0.005% THD, an analog distortion analyzer with switchable 30Hz high-pass or 20-20kHz bandpass filtering, and a 100MHz oscilloscope. The cables are specified as balanced with shield, XLR or ¼-inch on one end and the appropriate connector on the other. This is bench-grade verification for a unit meant to live in a rack.
The MPX-500 sits in a family, and it helps to sort out which unit does what. The MPX-550, which Sound on Sound (January 2003, p.40) described as an update to the MPX500, moved to 24-bit converters with more dynamic range than most home studios could do justice to. The review also noted the MPX550 shipped with a proper mains lead rather than an adaptor. The MPX200, covered in the same publication (Sound on Sound, August 2001, p.40), is a different animal. It's a smaller, more knob-per-function box with compressor, plate, hall, and dual routing laid out on the front panel. The MPX-500 sits between those two, with more processing power and I/O than the 200, and it's the direct ancestor of the 550 rather than a stripped-down version of the bigger rack processors.
The manual is specific about maintenance. Clean the exterior with a soft, lint-free cloth slightly dampened with warm water and mild detergent. Alcohol, benzene, and acetone-based cleaners are out, and so are abrasives like steel wool or metal polish. In a dusty environment, a vacuum or low-pressure blower is the approved method for clearing the exterior.
The I/O is described through the test procedures rather than a marketing block. The unit has left and right analog inputs and outputs, balanced, with the manual specifying XLR or ¼-inch connectors depending on the cable. The power cord list covers North America, Europe, the UK, Italy, Switzerland, Australia, Japan, and a universal option, with Lexicon part numbers for each. So the MPX-500 was sold into every major market with proper regional power. The service-return instructions also mention a footswitch as an accessory not to ship back with the unit unless requested, which confirms footswitch control was part of the ecosystem.
The service and warranty language is standard Lexicon. Consult customer service before returning anything, get a Return Authorization number, and put it on both the inside and outside of the package. Insure and prepay the shipment, and don't include manuals, cables, or footswitches unless specifically asked. Lexicon assumes no responsibility for the unit in shipment from the customer, in or out of warranty.
The MPX-500 is the model that established the template the MPX-550 refined, and it carries Lexicon's proprietary processing engine. Its analog path was designed to be verified to a standard most budget processors never documented. The 24-bit conversion and true stereo processing are the headline specs. The -101dBr typical noise floor and the ±1.0dBr frequency response tolerance are the numbers that describe what the box does when you plug something into it. It's not the flagship, and the manual treats it like a piece of test equipment.
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