Lexicon MPX100

A Lexichip III processor that asks for 9 volts of AC, a double footswitch, and roughly the price of a decent stompbox. It gives back a studio reverb engine in a half-rack box.

Overview

The MPX100 is the first in a range of processors built around Lexicon's Lexichip III reverb engine, a custom device the company was clearly proud of. Sound on Sound (August 1998, p.44) framed it as the entry point to that family, noting that the unit cost around £250 at the time of review.

The service documentation makes clear this is not a stripped-down toy wearing a Lexicon badge. The digital heart is a 10 MHz Z80 running at 9.0316MHz, paired with a 128k EPROM (27C010, 70ns) and an 8K × 8 SRAM. The Lexichip III sits alongside its own audio memory - a single 60ns, 4Mbit DRAM (1M × 4), with four 4-bit accesses assembled into a 16-bit word. Non-volatile storage comes from a 2-wire serial 24C04 EEPROM, a 4K-bit part holding user data across power cycles. Z80 housekeeping, Lexichip III doing the audio work, DRAM for delay memory: that is the same basic recipe Lexicon used across its processor line, scaled to a budget chassis.

The clocking scheme is fixed. For a 44.1kHz sample rate the Lexichip III takes an 11.2896 MHz crystal input, and its internal PLL multiplies that by four to a 45.1584 MHz master clock. The conversion clocks - FS, 64FS, and 256FS - derive from the same 11.2896 MHz crystal. So the MPX100 is a 44.1kHz device, and it does not pretend otherwise. The Sound on Sound review (August 1998, p.44) described the Lexichip III as an extremely powerful custom engine capable of running Lexicon's famous reverb algorithms, and the clocking documentation backs up the "fixed 44.1kHz, done properly" reading rather than "flexible, done cheaply."

The A/D is a Crystal CS5335, and the D/A is a Crystal CS4327 delta-sigma part. The CS4327 runs in I2S serial mode, clocked from the 256FS master clock generated by the Lexichip III, with data clocked on the rising edge of the bit clock and aligned to the second bit clock following each LRCLK transition. Coming out of the DAC, capacitors C48 and C49 strip a 2.2V offset before the signal reaches the output stage. That output stage is a Sallen-Key second-order unity-gain low-pass filter (U16 and its surrounding components), followed by a first-order low-pass section. Together they form a third-order anti-imaging filter at 100kHz. DC blockers C53 and C59 sit after the filter to prevent wiper noise in the output level pot. It's a conventional, well-executed analog output chain, not a cost-cut afterthought.

Separate unbalanced 1/4" phone jacks handle left and right. If you only plug into the right input, the unit detects the mono input automatically and routes it to both channels. Input impedance is 1MΩ per channel, dropping to 500KΩ when summed to mono. That 1MΩ figure is deliberately high, so a guitar or bass can plug straight in without a DI box. The service manual is candid about the tradeoff: with an impedance that high, an empty jack is a 1MΩ antenna for noise, so the left and right inputs short to ground when nothing is plugged in. Once an instrument is connected, its own relatively low output impedance (1k - 10k) provides the path to ground and the noise problem disappears. Protection diodes D24 and D25 clamp large input signals so nothing at the op-amp inputs exceeds 0.7V above the rails, with R111 and R123 limiting current through the diodes during conduction.

The input buffer has a pre-emphasis shelf: 10dB, starting at 3kHz and ending at 9kHz, a 15/50µS curve with matching de-emphasis on the output. It is baked into the analog path and is not a user-adjustable EQ.

The specified input range is -30dBu to +4dBu, and the manual calls out 500K unbalanced impedance for the direct instrument input specifically. The gain structure is documented precisely. Feed a 1kHz signal at 775mV into the input with the front-panel Input and Output knobs fully clockwise and the Mix knob fully counterclockwise (dry), and the left output should measure 1.95V ±0.05V. The right channel gets the same test and the same expected result. That is the reference gain the service techs use, and it shows the MPX100 has meaningful makeup gain available.

Signal-to-noise is specified as better than 90dBr, measured by disconnecting the oscillator and reading the noise floor with the analyzer set to a -50dBu scale. THD+N is specified at less than 0.05%: apply a 1kHz signal at 220mV to either input and the distortion at the corresponding output should stay under that figure. Frequency response is verified at 20Hz, 3kHz, 5kHz, and 20kHz, with output required to stay within ±0.5dB of the 1kHz reference at each point. Those aren't spectacular numbers by modern standards, but the ±0.5dB tolerance across the audio band is tighter than plenty of budget gear manages.

Eight discrete front-panel LEDs are arranged in a 2×4 matrix, driven by an octal D-flop (U11) that clocks the data bus on the rising edge of CTL_REG/. Column select lines are active-low, buffered and inverted by transistors Q3 and Q4. When a selected column is driven high and a row line low, the LED at that crosspoint lights. The same matrix reads the front-panel switches. Row current is limited to 28mA or less by 100Ω resistors, which keeps the driver IC well under its 100mA total.

Power comes from an external 9VAC transformer rated at 1.9A, terminated in a 5mm/2.5mm barrel connector. Inside, the supply generates three regulated rails: +5VD for digital, +5VA and -5VA for analog. A 0.1µF cap across the AC input guards against noise spikes coming in, and a pair of 470pF caps (C1 and C2) keep circuit-generated RFI from radiating back out through the power line. A 9V AC adapter at 1 amp minimum works, but 1.9 amps is preferred. Reset is handled by monitoring the unregulated +5V rail. When the drop across the regulator exceeds 2V, Q2 turns on and charges C7, generating the RESET/ signal that holds the Lexichip III, the CS5335 A/D, and the CNT_REG/ line until the rail stabilizes. On power-down, C7 discharges through D5.

The MPX100 runs a diagnostic suite on every normal power-up, and the sequence is displayed on the front-panel LEDs. Six tests run in order: ROM checksum, SRAM, Lexichip III WCS, Lexichip III interrupt timer, Lexichip III audio data file, and EEPROM checksum. If any of the red Clip LEDs stay lit after power-up, a diagnostic failure has occurred and the unit needs repair before you use it. That's handy for anyone buying used, since you can see whether the box passes its own self-test before you hand over money.

The service documentation also has a dedicated listening test for isolating analog from digital faults. You power up while holding Bypass, wait for the green Level LEDs, set Variation to 13, and press Store. The Edit, Bypass, and Tap LEDs should all light, confirming the test mode. Then you feed a 1kHz sine at 0dBu, turn Input, Mix, Output, Effect Lvl/Bal, and Adjust fully clockwise, and sweep the Input and Output knobs across their full range, listening for pops, clicks, or scratchiness. If the analog path is clean but processed audio is not, you know where the problem lives.

The Sound on Sound review (August 1998, p.46) noted that reverb algorithms on some units can sound samey from one patch to the next, and that the MPX100's patches vary meaningfully. The Tap button sets delay time on echo-type effects or pre-delay on reverb patches, and some patches map pre-delay to MIDI Clock, so that with clock running you get a 32nd-note pre-delay.

The control set is straightforward: Input, Mix, Output, Effect Lvl/Bal, and Adjust knobs, plus buttons for Edit, Bypass, Store, Tap, and Variation. A double footswitch - Lexicon's own or a Marshall FS02 or equivalent - connects via a 6-foot minimum cable with 1/4" to 1/4" stereo phone plugs (Switchcraft 10BK10 or equivalent). The footswitch handles Tap and Bypass.

The environment spec is 32° to 104°F (0° to 40°C) operating temperature and 95% non-condensing relative humidity. The service manual expects a clean, antistatic, well-lit work area, a digital volt meter, a low-distortion sine wave oscillator, a distortion analyzer and level meter with switchable 30kHz high-pass or 20-20kHz bandpass filter, a stereo headphone amplifier, and the appropriate cables.

The documentation leaves a few things out. There's no mention of 48kHz support, no S/PDIF input, no balanced I/O, and no digital output on the standard unit. The service manual's cover page, though, is titled "MPX 100 Dual Channel Processor, Stereo 44.1kHz S/PDIF Digital Output," which suggests a digital output variant existed. The Sound on Sound review (August 1998, p.44) treats the MPX100 as a £250 entry-level processor, and at that price the feature set makes sense: unbalanced analog I/O, 44.1kHz fixed, footswitch control, and the Lexichip III engine.

For anyone considering one today, the self-test on power-up is the first check. Listen for scratchy pots during the listening test sweep. The Input and Output knobs are ganged dual pots (R116 for input, R97 for output), and wiper noise is a known failure point that the DC blocking caps are there to mitigate, not eliminate. The 9VAC power supply is external, so make sure one is included. Replacing it with a generic adapter is possible, but the 1.9A rating matters. And the EEPROM holds user data, so a unit that won't retain patches may need that 24C04 replaced - a cheap part, but a soldering job.

The MPX100 is the kind of box that made sense in 1998 and still makes sense if you want Lexicon reverb algorithms without paying PCM 70 money. It's not a PCM 70. It's not trying to be. It's a £250 processor with a real Lexichip III inside, a documented self-test, and a service manual thorough enough to keep it running. That's more than most budget gear from the era can claim.

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Lexicon MPX100

Overview

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