Lenco MPA-2100
A Missouri welding-and-electronics conglomerate built a monitor amp so clean that a 750-watt reference got shown up in A/B listening.
Overview
The Lenco MPA-2100 is a professional monitor power amplifier rated at 100 watts per channel into 8 ohms, 200 watts per channel into 4 ohms, and 400 watts mono into 8 ohms (500 watts mono into 4 ohms). It lists for $2850 and carries a full five-year warranty on parts and labor. Those numbers alone would not raise eyebrows. The builder does. The Lenco Corporation of Jackson, Missouri had eight divisions, including welding, plastics, foundry, and electronics, and its client list ran through every major television network, federal agencies, and high-tech corporate accounts. Its electronics division made sync pulse generators, video monitors, studio distribution amplifiers, test gear, and time base correctors. According to Mix (June 1986, p.97), Lenco was not well known to the professional audio recording world, and the trade press greeted the amp with open skepticism. The magazine then spent its review admitting the skepticism was misplaced.
Lenco spent nearly two years on the design, targeting three things: serviceability, thermal behavior, and sonics. The result is 100 percent modular - a mainframe holding the AC supply, with two removable power modules each carrying its own regulated supply. The modules pull out from the front panel, so a tech can extract a channel without unracking the amp. A "crowbar" action from the front door assembly loosens them. There is no wiring harness in the audio path at all. Every audio pathway is circuit-board etch, a deliberate choice to keep inductance out of the signal. Front-side cooling was specified to fight the overheating that plagues stacked and in-wall installations. The temperature-reactive fans and heat sink are backed by three separate protection circuits: DC power and output fusing for current overload, independent thermal breakers for each power module (placed in the feedback loop rather than the signal path), and a DC output detection circuit to protect speakers. Each fault condition returns the amp to the "0" state.
The measured performance is where this thing gets interesting. Power bandwidth is specified at 1 Hz to 100 kHz, +0/-1 dB, a very wide window that lets program material pass essentially unaltered. Minimum slew rate is 700 V/microsecond into a symmetrical 8-ohm load. Lenco's own advertising claimed 700 V/µS "or better" and noted that unloaded slew exceeds 1000 V/microsecond. Damping factor is 600 minimum from 20 Hz to 20 kHz, load impedance runs from 2 ohms to infinity with musical program, and propagation delay is just 100 nanoseconds into 8 ohms. Distortion figures are 0.02% THD at 100 W/ch into 8 ohms from 10 Hz to 10 kHz, 0.035% at 400 W mono into 8 ohms, and 0.006% IMD at 4 ohms, 200 W, 60 Hz/7 kHz mixed 4:1. Lenco's ad copy went further, claiming THD of 0.005% at 1 kHz into 8 ohms and calling TIM distortion "unmeasurable", a claim the company framed as especially relevant for monitoring digitally recorded material. Treat that ad figure as marketing rather than measurement. The manual's 0.02% is the conservative, quotable number.
Mix (June 1986, p.100) put the amp through A/B listening against a familiar-name 750-watt control room amplifier through Auratone T6 subcompacts and reported "no contest." The Lenco's tight low-mids, superior imaging, and better-defined second and third partials in the 20-100 Hz range cleaned up the whole low end. A second comparison against a comparably priced audiophile amplifier through audiophile loudspeakers narrowed the gap, but the reviewers still judged the Lenco superior, singling out a firm low end and smooth upper mid-range. Their characterization of the top end was "bright, but not brittle." They also noted that the amp is rated at only 100 watts per side yet seems to deliver more. They speculated that the accuracy lets listeners hear the full spectrum comfortably without cranking gain, so "punch" can come from dynamic performance rather than raw wattage.
There are two caveats. First, the high damping factor changes how a room sounds. Lenco warned that dropping the MPA-2100 into a room tuned around a low-damping amplifier will make the Lenco seem bass-shy, because low-damping amps let the cone overexcursion and add bass the source signal never carried. Retune the room and the truer low end appears. Second, the protection circuitry does not auto-reset after a fault. It stays in shutdown. Mix found this genuinely annoying in practice: switching between main and alternate speakers put a glitch on the input, the sensitive protection caught it as a fault, and the amp would occasionally shut down. The reviewers acknowledged the design logic (amps that auto-reset can oscillate on and off in a persistent fault) but still counted it a real limitation.
The MPA-2100 was conceived primarily as an audio monitor amp, but Lenco also pitched it for instrumentation analysis of digital audio signals and the transient intermodulation factors that arise in digitally encoded processing. Absolute accuracy was the stated goal. A sibling model, the MPA-2300, was aimed at dedicated monitors and concert sound reinforcement. At the time of the Mix review, Lenco was developing a 250-watt-per-side (8-ohm) amplifier due that summer, and a consumer version of the MPA-2100 was in the works. Power supplies for other countries were available on request. Contact for the audio product line ran through Jim Rhodes in Lenco's Jackson, Missouri offices.
It is a rack-mount professional tool with a modular architecture built for serviceability, and the front-extractable power modules make maintenance easy four decades on. It is not a pretty amplifier. Mix said flatly it would win no "best dressed" awards. Original list was $2850, and the five-year warranty was unusually generous for pro gear of the period. The quirks (no auto-reset, damping-factor sensitivity) are documented rather than speculative. Anyone chasing one should verify both power modules are present and functional, since the regulated supplies live inside them.
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