Lexicon 224XL (1985–1988)

The 224X got the software. The XL got the hardware. Both got the LARC - and that desk slab is what you actually buy.

Overview

The Lexicon 224XL is the second-generation digital reverb from a company that had already built its reputation on the 224, and then on the 224X. The XL carried the line into the late 1980s. It kept the thing that made the earlier units distinctive: a separate remote console does all the talking while the processing lives somewhere else.

That console is the LARC - the Lexicon Alphanumeric Remote Console. It connects to the mainframe over a flexible 50-foot cable terminated in DE-9 connectors, and it is normally powered straight through that cable. The rear panel has two LARC ports. LARC 1 is the primary connection. LARC 2 (Thru) allows a second LARC to be connected to the mainframe. That second port is also the path for linking a 224XL to a 480L, with both units under the control of one LARC. A 10-foot cable is supplied for that application. For runs beyond 100 feet, Lexicon specified an optional remote supply - 10 to 20 Vdc or 10 to 20 Vac at 6.25 W - feeding a miniature jack on the console.

What the LARC gives you is legibility. The console shows which family of programs is active, which specific program is selected, and which variation within that program is running. It spells all of it out on LED displays rather than encoding it in a two-digit window. Setups can be stored to audio cassette, which is how this generation of Lexicon gear handled user memory.

The rear panel is built for serious patching. Inputs are two channels on 3-pin male XLRs, balanced and transformer-isolated, pin 2 high, with a 20-kilohm input impedance. Outputs are four channels - A, B, C, and D - on 3-pin female XLRs, also balanced and transformer-isolated, with a 90-ohm output impedance. The 224XL accepts input levels from +8 to +18 dBm, and output levels run from slightly below +8 dBm to slightly above +18 dBm. Input and output trims are screwdriver pots reached through access holes on the mainframe's front panel, not knobs on the face. Unity gain is the factory setting, with a maximum output level of +12 dBm. Setting those levels is a procedure, not a guess. The manual directs owners into the delay-line diagnostic programs - Zero Delay or 0.5-second Delay - to set them properly.

There is also a digital I/O connector: a DE9 that provides PCM 1610-compatible digital I/O, with 18-bit word length capability, and it can be slaved to a 48 kHz, 44.1 kHz or 44.056 kHz external word clock. MIDI is present but minimal. MIDI In receives information from other MIDI-equipped devices, MIDI Thru retransmits what arrives at MIDI In without change, and MIDI Out is listed in the manual as currently not in use. An Automation Connector is provided for future computer control and automation features. The manual has a blunt warning about it: never connect a LARC to this connector.

The parameter set is where the XL earns its keep. The control parameters include Preecho Delay, which sets the delay time of the early echoes, and Preecho Level, which controls their level. Size sets the apparent size of the acoustical space produced by the 224XL, essentially the longest dimension in meters. Treble Decay sets the frequency above which sounds decay at a progressively faster rate. Slope controls the decay characteristics of the Inverse Room program. Effects parameters include Band Delay, which sets the delay time for each frequency band in the Multiband Delay program, and Band Level, which controls the relative level of each frequency band in that same program. Crossfeed controls the level and bandwidth of signal fed between channels.

The 224XL has three parameter toggles, accessed by successive pushes of the LARC's PARAM key. Once a toggle has been accessed, press 1 on the numeric-select keypad to turn it on, or 0 to turn it off. The state of the toggle is displayed in brackets after the parameter name - DYN DECAY [1] indicates that dynamic decay is turned on. Toggle settings are stored in registers along with other parameter settings, and most variations have toggles preset to on.

The three toggles are Dynamic Decay, Mode Enhancement, and Decay Optimization. Dynamic Decay activates the LF Stop Decay and Mid Stop Decay sliders. Mode Enhancement activates the Chorus slider. Decay Optimization activates a software module that alters certain reverberation parameters in response to changes in input level - changes that make the decay of sound in the 224X less metallic and more natural. This switch is normally on. For certain kinds of source material, such as soft low-frequency tones from a synthesizer, audible clicks may occur during level changes. If that happens, the manual advises turning off the Decay Optimization toggle. For the Constant-Density Plate programs, this toggle has no function.

The manual is also blunt about the 224XL's lineage and its limits. Boards must not be exchanged between 224, 224X, and 224XL units. They are related machines, not interchangeable ones. Power Supply modules should not be exchanged with new modules. There are also known software version 8.2 bugs: no B output on the Dark Hall program; A and C outputs on Rich Split mute when the Mid Decay control is set to infinite (--); and Variation 5 of Dark Hall can cause reverb runaway.

Starting with software version 8.2, the ROM checksum diagnostics were improved. All the serial communications code is in the first SBC ROM, as is the checksum diagnostics program. So if the first SBC ROM is functioning, errors on the other ROMs will be reported. Each ROM has been given a checksum which is identical to its ROM number - SBC ROM 1 has a checksum of 1, as does NVS ROM 1. The 224XL has full diagnostics which take about 15 seconds. If the 224XL does not respond after power up, the manual advises giving it time to complete diagnostics.

Common faults are well documented. A pop audible when changing programs or using the Mute switch is caused by U7 and U8 (LF353 dual opamps) on the AOUT board. The fix is selecting new U7 and U8 for low bias current. Unpredictable operation with the LARC traces to a four-position switch on the NVS board. All switches should be in the OPEN position, but a few units were shipped with switches in the CLOSED position. And if a LARC doesn't work with 1000 feet or more of cable between it and the mainframe, some LARC transition boards were shipped with a 10 kilohm resistor at R4. It should be a 1 kilohm resistor. That one only bites at extremely long cable runs.

The tape interface is mostly software. The processor recognizes and generates 4800 Hz (logic 1) and 2400 Hz (logic 0) FSK data at a 600 baud rate. The tape output circuitry is rather simple: since the uA9637 (U2) buffer has a differential input, the R29/R30 voltage divider is used to set the transition threshold. The output is protected with clamp diodes CR3 and CR4, then AC coupled, current limited, and low-pass filtered with C6, R6, and C2.

For anyone servicing a 224XL today, the manual's guidance is consistent: always check jumpers, always replace logic gates with equivalent parts, and do not substitute any other family type. The product has particular parts in locations for design reasons. If a part is replaced, use the same manufacturer if possible and always the same family — F, LS, HC, HCT, ALS, AHCT, and so on.

By the mid-1980s, the 224XL was sharing shelf space with its own successor. Mix (September 1985, p.82) noted that Lexicon had released new software updates for both the 224X and the Model 200 reverberators, and the same issue carried news of new 224XL and Model 200 software. The hardware stayed put while the programs kept arriving. Recording Engineer (October 1984, p.45) described the latest software additions as inverse reverb, gated reverb, infinite reverb, improved splits, controllable room size, and additional rich programs. The same coverage noted that the two independent in/out reverbs now offered a greater variety of options, that the size of the environment was now continuously variable, and that more programs were available with greater density and smoother decay.

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