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High-efficiency interlude: Have you encountered review terms without grasping their true meaning before? 'twas probably 15 years ago when I first met the—to me very mysterious—Mosfet mist in a column of Stereophile's popular Sam Tellig. Then I was far too greenhorn still to understand its significance. Personal experiments with many different amplifiers hadn't yet broken down the generic transistor class into push/pull and single-ended circuits, never mind various Mosfet, bipolar and power Jfet output devices.

My recent review of FirstWatt's M2 Mosfet amplifier with passive autoformer gain involved comparisons to its J2 and F5 cousins. This netted specific observations that are relevant to today's subject. Prior reviews of ModWright's big KWA-150 and Aaron's XX meanwhile had elucidated bipolar transistors in action. I'd finally arrived at a juncture in my audiophile career where I could begin to appreciate discrete sonic flavors which seemingly attach to specific semiconductor choices.

Battle of the Mosfets: Synchronicity again smiled upon such exploits when Nagra Audio marketing manager Matthieu Latour dispatched a fully burned-in MSA from their nearby lair in Lausanne. I suddenly had three quality Mosfet amplifiers on hand. As counterpoint, I had the Nelson Pass J2 with SemiSouth power JFets; and his F5 whose involvement in prior comparisons had pegged it as a virtual stand-in for the bipolar aroma. Fortunes then presented a unique opportunity by way of Inès Adler's Voxativ Ampeggio. Built by Schimmel Pianos, it's a 98dB single-driver speaker with effective 40 - 20.000Hz bandwidth even in my open-backed setup. Vitally, there's no Lowther shout. Exceptionally luminous and resolved, this would make a brief pit stop for the ModWright and Nagra amps. Clearly, their respective 100/60wpc power ratings render real-world associations with such speakers quite unlikely. Nagra's modulometer in fact showed median playback levels at 0.06 watts whose peaks rarely exceeded 0.6 watts. Would the muscular ModWright come on song this deep in first gear? More to the point given the Voxativ's outrageous transparency and crossover-less speed - would Sam Tellig's Mosfet mist materialize?

Yes on both counts. The KWA-100 came on song even at output voltages so ridiculously puny that most prospective buyers should never encounter them. This spoke very highly to the clarity and responsiveness of Dan Wright's simple and direct-coupled two-stage circuit. Where in this admittedly left-field context the €9.350 Nagra beat it was on dynamic range and responsiveness. The MSA had the apparently higher S/N ratio. Presumably because of its very specialized Swiss switch-mode power supply with power-factor correction, the Nagra's microdynamic cresting also was taller and rose more instantaneously. This was clear even on something as pastoral as "Canto del Barco" from Renaud Garcia-Fons' Linea del Sur [Enja]. Over the Nagra, his grand five-string upright was more expressive and, at the same starting volume, had me jump for the remote when peaks rose higher more unexpectedly violent.

This higher speed also manifested as an extra degree of on-string rasp and general bowing action. The Nagra was the ultimately more resolved machine as its rather higher price would have one hope. Far more relevant was the general overlap of gestalt - moderately warm, tonally dense, minorly redolent and slightly on the soft and more voluptuous side of the Tao. Swapping in the FirstWatt M2 confirmed it to be a virtual doppelgänger to the ModWright. They shared these exact qualities and were subjectively somewhat slower and a tad fuzzier than the Swiss.

That nearly subliminal fuzziness—of suede rather than smooth leather—was associated with softened transients, stepped-back separation, smoothness/warmth and a small degree of thickening which became particularly apparent on this type of speaker. That was the mysterious Mosfet mist in action. In flagrante delicto. Conceptually it's quite similar in fact to capacitor-coupled valve amps when those are compared to good IT-coupled variants. It explains why Mosfets so often are the choice of transistor amp designers who mean to emulate certain tube virtues. Speaking in generalities—there are always exceptions—bipolars are more crystallized, brash and sharp. They separate harder, sparkle more brightly and remove a subliminal soft focus for a more warts'n'all perspective. On the audiophile temperature scale, bipolars are cooler, Mosfets warmer.

The more musical material grows complex and multi-layered, the more this Mosfet mist becomes audible as a gentlifier and mild congealing agent. The extent of this depends on the quality of the amplifier and on the overall resolving power of a system. As music gets simpler to eventually become the quintessential girl with guitar solo act, the same qualities become more and more compelling.

Inserting the J2 introduced a small degree of bipolar incisiveness. This was accompanied by greater top-end sophistication or sweetness. The latter is reminiscent of how direct-heated triodes tend to out-finesse transistors if not on raw extension then in textural smoothness. Power Jfets retain that level of suavity but discard bandwidth limitations. Transient incisiveness seems borrowed from the cooler bipolars yet harmonic color temperature feels closer to the Mosfets. Where the latter achieve their effect by tightening up the weave of the musical threads, the Jfets are airier and more open. I'd peg them between Mosfets and bipolars, combining the best attributes of either.

Moving in the FirstWatt F5 increased sharpness and separation power but also lit up the top even more and magnified a very mellow intrinsic presence-region plateau in the widebander to introduce a dose of paprika and edge. Of the three FirstWatt amps, designer Inès Adler had most fancied the M2 when she delivered her loaner pair. Having played quick musical chairs through most the amps I thought copasetic from my inventory, I hadn't played for Inès the KWA-100. As the M2's bigger brother from the same mother, it should have ticked off the same high marks in hindsight. As you'll agree, that's unexpected performance indeed from a high-power amp strapped to speakers that are tailor-made for micro-power SETs.

Running a cool class AB to the FirstWatt M2's very toasty class A; 100+ watts instead of 25; and a lower sticker with snazzier cosmetics - for most buyers the ModWright KWA-100 would be the preferred choice out of these two. To keep the above notes properly weighted, the ICEpower™ Wyred4Sound ST-500 amplifier with proprietary dual-differential direct-coupled input buffers also in for review proved to be significantly warmer and mellower than any of these Mosfet specimens. The catch-all Mosfet mist descriptor as it actually applied here was far from any end-of-the-road extreme. It was nothing more than a fine aroma that becomes applicable when one compares quality implementations of bipolar, Mosfet and Jfet output devices in power amplifiers. By implication, the KWA-150 is indeed the more highly resolved performer in the ModWright stable - and tonally a touch cooler. This excursion proved that for all its obvious muscle, the ModWright is no brute. It embodies the first-watt credo of delivering stable performance already at the very beginning of its considerable power curve. On the next page, we'll get real with the type of speaker the KWA-100 was actually designed for...