Lexicon MPX-1

A 1U rack processor with two independent microprocessors, 18-bit A/D conversion, and a parametric EQ deep enough to notch out feedback. It sold for the price of a decent stompbox collection.

Overview

The MPX-1 is a single-rack-space multi-effects processor built around what Lexicon called Multiple Processor FX technology. Two independent microprocessors sit inside - the proprietary Lexichip handles reverb and effects, and a second processor works alongside it. That let the MPX-1 run dual-engine effects chains without the compromises of a single-chip design. The unit measures 19 inches wide, 1.75 inches high, and 9 inches deep, and weighs just over six pounds. It's a standard 1U rack mount, and the rear panel is more serious than the price point suggests: balanced XLR inputs and outputs, 1/4-inch TRS phone jacks, coaxial RCA digital I/O conforming to S/PDIF CP-340 Type II, and full MIDI connectivity via 7-pin DIN for MIDI IN and powered bidirectional remote, plus standard 5-pin DIN for THRU and OUT.

The converter stage tells you where this box sat in the market. The MPX-1 uses an 18-bit A/D converter and a 20-bit D/A converter, with 32-bit internal DSP paths and 24-bit S/PDIF I/O. Frequency response is quoted at 20 Hz to 20 kHz, ±1 dB across the analog-to-analog path. Signal-to-noise ratio is 95 dB minimum on the D/A side and 95 dB typical, A-weighted, on the analog-to-analog path, with total harmonic distortion plus noise rated at 0.01% maximum from 20 Hz to 20 kHz. Crosstalk is held to -60 dB maximum. Those numbers won't impress anyone comparing spec sheets against modern interfaces, but in the late 1990s they were solidly professional. The A/A S/N figure of 95 dB typical is the one that matters most, since it describes the full signal path from analog input through the converters and DSP back out to analog.

The digital section runs at a fixed 44.1 kHz sample rate. The MPX-1 can clock to its own internal oscillator or lock to an external digital source, selectable via the Input/Clck menu as Analog/Int, Analog/Ext, or Dig/Ext. It only accepts external signals at 44.1 kHz. There's no 48 kHz support, which matters if you're trying to fit one into a modern setup. In Analog/Ext or Dig/Ext mode, the unit repeatedly tries to establish lock with the external source, and if it can't, an error message appears on the display. LEDs in the front-panel numeric display show when digital signals are present and when lock is achieved. The Soft Sat control inserts an analog soft-knee limiter ahead of the A/D converter. It trades the harshness of digital clipping above -3 dBfs for a gentler analog-style distortion. Increased analog distortion occurs in the -3 dBfs to 0 dBfs range, but that's far less ugly than a hard digital overload.

The parametric EQ section is where the MPX-1 gets useful for problem-solving. Gain is adjustable in discrete steps from +24 dB down to -72 dB, with sweepable center frequencies and Q adjustable from a broad 0.1 up to a sharp 10.0. That Q range is the key spec. At Q=10, you can carve out a narrow notch to kill a feedback frequency or remove a specific noise artifact without gutting the surrounding material. At Q=0.1, the same EQ becomes a gentle shelf for broad tonal shaping. The manual is explicit that this precision is the point: minimizing the amount of EQ applied to program material reduces the adverse side effects that always come with equalization. A shelf mode is also available, with Q settings from 0.1 to 10.0 and gain up to +10 dB.

The effects are built from a modular architecture. The Chorus algorithm is a true stereo, multi-voice design with dual 2-tap modulators and cross-resonance. The Pulse Width controls let you reshape the modulation waveform independently - at PW=0, the sine wave becomes a sawtooth with a fast rise and slow fall. Rate runs from 0 to 50 Hz or can be set in cycles-per-beat from 1:24 to 24:1, so you can sync modulation to tempo rather than dialing in Hz and hoping. Depth is adjustable from 0 to 100%, and the resonance controls (Res 1 and Res 2) range from -100 to +100 for left-to-right and right-to-left cross-coupling. The Flanger effects are two-tap delays where the first tap is fixed and the second sweeps past it, driven by an internal LFO. That's the classic comb-filter effect. The Centrifuge effects take a pair of modulated left and right delays and route them into a single auto panner, with mod and pan rate and depth modulated by a second set of rate and depth controls. The result is undulating rhythms that are hard to get from a standard chorus. The Comb effects combine the input with a micro-delayed copy, causing specific frequencies to cancel or reinforce. Comb 1 is a mono flanger with single-pole low and high cut filters, while Comb 2 is a dual mono flanger with a second tap controlled by a single LFO with adjustable phase from 0 to 270 degrees.

The pitch-shifting section includes Shift (S) - a stereo version of Shift (M) with synchronized crossfades - and Shift (D), which provides mono input to two independent pitch shifters controlled by Tune 1 and Tune 2. The first shifter goes to the left output and the second to the right. The manual notes that Shift (S) and Shift (D) are among the largest effects in the MPX-1, meaning they use the most processing resources. The Volume effects (Mono, Stereo, and Dual) can be placed in front of or behind other effect blocks to provide smooth level control through a 95 dB range. They're useful for dynamic input or output control, EQ input trim, stereo-to-mono mixing, crossfade control, or as a volume pedal. The Test Tone generator is an audio sine wave with output quantized to a chromatic scale (A=440 Hz) over a nine-and-a-half octave range. It's mainly a tuning reference, but it's playable from a MIDI keyboard by patching Last Note to Note and an ADR to Level. Pitch accuracy is better than 1/4 cent.

Editing on the MPX-1 is where the "more editing power in Play mode than most processors do at their deepest editing levels" claim - attributed to Jim Aikin in Keyboard (March 1997) - starts to make sense. Parameters displayed in milliseconds or Hz have coarse and fine editing: delay times adjust in 100 ms coarse increments or 1 ms fine increments, LFO rates in 1 Hz coarse or 0.01 Hz fine increments, and EQ center frequencies in 100 Hz coarse or 1 Hz fine increments. When you select a parameter for editing, either the left or right portion of the value can be selected - the left for coarse adjustments, the right for fine. An edit indicator appears in front of the parameter name if the value has been changed since the last store operation. The Value button toggles between assigning "select" and "adjust" functions to either the knob or the < and > buttons, whichever is more convenient.

The patching system is deeper than the front panel suggests. Multiple patches can be routed to the same destination, and when they are, the destination value becomes the sum of the individual patch values. The manual's example: if a footpedal and the knob are both assigned to Mstr Mix, the Mix value will be the sum of both patch destinations. To avoid hitting the ceiling, you set individual destinations so their sum stays within the parameter's maximum - 50% each, or 25% and 75%, and so on. When the sum exceeds the maximum, the parameter stays pinned at max until the sum drops back below it.

The System menus cover the operational details. Input Mode can be set to Stereo (left and right processed separately), Mono (L only), or Mono (R only). Output Mode selects between Analog and Digital output. Dig In Lvl is a master control for digital inputs, adjustable from Off or -89 dB to +6 dB to optimize the level of the incoming digital signal. ChanStatus determines whether Channel Status information received via the digital input is passed through unchanged or whether the MPX-1 generates its own - the options are Pass thru and Generate CS. Pgm Load can be set to Manual, where programs are scrolled through with unloaded programs identified by an asterisk and a flashing Program LED, or Auto (the default), where programs load automatically after a brief timeout. The Mix parameter selects between Program (each program loads with its own master mix setting) and Global (any program's master mix setting becomes global for all subsequently loaded programs).

The input section handles both balanced and unbalanced sources. The rear panel input level switch offers +4 dB/BAL with 100 kΩ impedance and a minimum level of -2 dBu for full scale up to +20 dBu maximum, or -10 dB/UNBAL with 50 kΩ impedance, -14 dBu minimum for full scale, and +8 dBu maximum. Common-mode rejection ratio is 40 dB minimum from 10 Hz to 20 kHz in the balanced position. The outputs are 600 Ω unbalanced with +18 dBu maximum at full scale. Getting levels right takes attention at three stages: the rear panel input switch conditions the analog front end, the front panel Input knob sets the level into the A/D converter, and the Lvl parameter in each effect block optimizes signal level within the DSP. The manual is clear that proper setting of both the switch and the knob is important for best performance of the A/D converter. You can't just plug in and ignore gain staging.

The control interface is comprehensive. MIDI uses a 7-pin DIN connector for MIDI IN and powered bidirectional remote, plus 5-pin DIN connectors for MIDI THRU and OUT. A 1/4-inch TRS phone jack accommodates three independent footswitches, and a separate 1/4-inch TRS jack handles a foot pedal with a minimum impedance of 10 kΩ and maximum of 100 kΩ. A 2.5mm barrel connector accepts 9VAC remote power. The power supply is universal: 100-240VAC, 50-60Hz, 25W, via a 3-pin IEC connector. Operating temperature range is 32° to 104°F (0° to 40°C), storage is -22° to 160°F (-30° to 70°C), and humidity is up to 95% without condensation. The unit carries UL1419 and CSA 22.2 No. 1-94 safety compliance, EN60065 (TUV-GS and CE marking), FCC Class B, and EN55022 Class B with EN50082-1 EMC compliance.

The MPX-1 got strong press in its day. Sound on Sound (February 1997, p.26) grouped it under a "Price/Quality Breakthrough" heading, positioning it for users who needed top-quality reverb and multi-effects but couldn't stretch to the expense of the flagship units. Lexicon's own advertising in EQ (November 1999, p.17) billed it as offering "professional studio quality effects at [an accessible price]" with 200 superbly-crafted presets, and the same publication (April 1997, p.33) highlighted the connectivity - balanced XLRs, 1/4-inch analog, digital S/PDIF, and full MIDI - as central to its appeal. The most quoted line came from Jim Aikin's Keyboard review (March 1997), which EQ reproduced in its own advertising: "they'll have to pry it out of my cold, stiff fingers." That's a marketing use of a review quote, not an independent verdict. EQ (July 1997, p.47) also noted that the MPX-1 "boasts more editing power in its Play mode than most processors do at their deepest editing levels."

The MPX-1 is not a PCM 70 or a PCM 60. Those are the classics, the boxes with the reputations. The MPX-1 is the practical tool: more flexible, more editable, with digital I/O and MIDI implementation that the older units can't match. The 44.1 kHz-only external clocking is a limitation, and the 18-bit A/D is a generation behind what came later. But the effects algorithms are pure Lexicon, the parametric EQ is genuinely useful, and the patching system rewards the time you put into it. The dual-processor design means you can run complex effect chains without the processing compromises of single-chip boxes.

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Lexicon MPX-1

Overview

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