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High vs. low-power Mosfets - KWA-100 & M2: The ModWright is a fully differential DC-coupled 2-stage circuit. Its Talema input transformer converts the single-ended signal for fully balanced power processing all the way to the outputs. The amp runs six transistors per side and works in class A/B to get barely warm to the touch. The M2 meanwhile runs so toasty that carrying it around on the heat sinks gets uncomfortable right quick.

Sonically, these amps sounded surprisingly similar. The M2 had a small edge on edge—the transient definition of bow or plectrum on strings, percussive events, the steep rise of a struck triangle, the power pop of a slap bass—the KWA-100 the greater fill in the sustain portion of tones and a tad more sweetness on top. Both were less resolute than particularly the F5 but also J2 and not quite as keen or articulate. In the ModWright catalogue, the KWA-100 is the smaller, less expensive version of the bipolar KWA-150. The flagship is biased richer into class A before shifting into AB. The designer acknowledges that the 100 is less resolved and warmer. Having owned the KWA-150 before trading it for the 100, I concur fully.

With my personal ideal presently deeper into crystallization and separation than either machine delivered, I thought that the M2's minorly crisper handling of attacks gave it a small lead on my easy speaker loads and within an overall system built around reflexes and fine upper harmonic visibility over mass and density. Once 25wpc into 8 ohms become marginal, the KWA-100 takes the obvious lead no matter what. Priced at $3.250 and thermally far less demanding on the immediate environment, the ModWright should appeal to those for whom the M2 runs too hot and/or doesn't make enough power. What I found interesting is that two designers with stated aims to incorporate certain tube virtues achieved their goals by very similar means - minimum amplification stages, Mosfet outputs, input transformers.

Burson Audio P/P 160 & M2
: The Burson is another differential class A/B design with proprietary discrete regulators and 95/180wpc power delivery into 8/4 ohms respectively. Unusually, this dual-mono stereo amplifier provides high-level inputs for boosting valve amps of up to 25wpc. In booster mode, the amp's 29dB of voltage gain are cut back to 6dB and the preceding valve amp sees the Burson as a resistive 8-ohm load. Burson's declared design goal for the 160 was ultimate transparency, hence the booster-mode option. It is claimed to be sonically invisible as to not impact whatever flavor the preceding low-power tube amp contributes.

With the gorgeous 21 Strings album of the American Al-Andalus ensemble as case evidence—the Banzi couple of oud and flamenco guitar here with session ace Charlie Bisharat on violin—the Burson clearly showed its more pungent and lit-up character. Charlie's semi flageolets where lighter-than-usual bow pressure coaxes out higher harmonics without leaving the fundamental to make for a typically Middle-Eastern flair had more brilliance and the performers were more sharply drawn within their richly reverberant recorded acoustic. Microdynamic cresting particularly of Tarik's virile plucked oud felt subjectively higher to make bigger small waves.

Inspecting tonal balance, I didn't hear a real shift. Still, a very quick observer might have initially pegged the Burson's center of gravity slightly upshifted due to its more Platinum top end. As with the F5, the Burson extracted more apparent ambient data primarily because subtle reflections and dying decays were more pronounced. While I don't agree completely, common terminology often measures these effects on a temperature scale. More lit up and crisp becomes cool. Mellower, softer and fuller becomes warm. Regardless of how accurate such terms are, in their scheme the Burson would be called cooler, the M2 warmer. In terms of subjective speed which the ear clocks as rise-time steepness and the relative precedence (amplitude overshoot) of transient to follow-up waveform cycle, the Burson was faster. For the purposes of this discussion, this very basic characterization shall suffice. How about comparing a micro-power valve amp + Burson to the M2?

The MiniWatt N3 is an unusual $378 EL84 SET + passive pot with switch-mode power supply and 3.5 watts of go juice. I ran the APPJ version with upgraded Synergy Hifi glass. Not as preternaturally lit up and energetic in the treble as the original MiniWatt, the successor is still a very fast lean specimen of the breed. It's still commendably free of noise and a bit more refined if less exciting than the predecessor. Straddling the Burson, the sand amp's pungent semiconductor edges remained. But what those outlines surrounded filled out. It became denser and texturally wetter. Because of how the Burson handled transients in general and particularly those from the midbass on down—transistorized, dry and super firm—the M2 was actually the more tube-like in gestalt. Real tubes piggybacking transistors retained tube tonality but lost some of the overall fluidity and roundness. To overdraw, swing turned to marching band.

In hindsight this should probably have been predictable. As the Burson review will cover in proper detail, boosting a quality SET even with a very neutral sand amp does not completely leave its flavor intact. Depending on what you fancy about valves, such a scheme alters specific drive/control issues. Particularly transistor lovers should prefer that. Committed valve devotees might not. In this instance, if one wanted higher bass amplitude and extension than flat-chested SETs can deliver but not the greater transient sharpness of transistors, the M2 without any glowing bits would actually be the better choice than real tubes boosted by transistors - unexpected but true.

Class D vs. Class A: EJ Sarmento's Wyred4Sound ST-500 runs modified ICEpower boards with their integral SMPS but fronts them with custom dual-differential FET input buffers which raise input impedance to nearly 70K while presenting the B&O boards with fully balanced signal. The input boards originally designed by Bascom King but since modified were voiced deliberately on the warm side to counteract the more incisive behavior of the Danish output boards. Two aspects of class D are nearly universally acknowledged. One, at least earlier iterations lacked treble refinement. Two, properly implemented class D provides incredible bass control. It's why most subwoofer plate amps are class D. This highly damped quality can however exceed realism and turn into what I call cyborg bass. As a current variation on the theme, by designers who were cognizant of those issues, how would Wyred's thermally very efficient circuit compare to the very inefficient Pass?

The ST-500—surprise!—was distinctly warmer and thicker than the M2. If your generalization of class D included adjectives like bright, forward, sharp, lean, bleached and hyper fast, you'd have been gobsmacked by this encounter. It showed just how much voicing variability remains within fixed stereotypes; and that the above characterizations of the M2 as mellower and denser than the F5, J2 and PP 160 were very relative. The class D amp was by far deepest into this polarity to give the Pass Mosfet amp the more informative top end, stronger focus and greater presence lock. So much for quick snapshots down transistor amp alley. How about involving pure valve amps like the Yamamoto A-09S and Trafomatic Audio Kaivalya?

I didn't fully see the ingenuity of the M2's circuit until it faced the very handsome Kaivalya monos. The overriding similarities of presentation far outweighed small remaining differences. The Kaivalyas are IT-coupled push/pull jobs with two feedback loops (one local cathode from ECC82  to separate winding of IT at -0.3dB, one local cathode from EL84 to separate winding of output transformer at 4dB). THD is a lot lower than with no-feedback SETs. Power and control are higher. The valve amps were texturally slightly fuller and temporally more nubile - what audio lingo calls flow. The Mosfet amp was a tad more incisive on the attack, a touch grippier on bass beats and slightly more illuminated in the treble. Fundamentally, that was it. A nut-shell assessment would view the white amps as tubes well linearized and exorcised of typical identifiers, the black amp as transistors deliberately tweaked to emulate tubes. The upshot was that whilst not completely interchangeable, these designs really did meet at less than arm's length on that imaginary dividing line between valves and semiconductors.

Locked into the F5/J2-type sound as a personal ideal, I'd initially weighted the M2 relative to that and keyed in on what it didn't do. Once I understood and then appreciated how Nelson Pass had indeed cloned the essence of a resolved and well-controlled direct-coupled valve push/pull sound of Kaivalya caliber, the proverbial bell tolled. The M2 doesn't emulate classic SET flavor. Rather, it approaches the kind of tube sound a gifted SET designer like Sasa Cokic gets when commissioned to employ pentodes in class A push/pull. To confirm that conclusion, I finally leashed up my Japanese 300B SET.

Beyond the expected—plumper looser bass, softer top—the direct-heated triodes demonstrated what's missing on the M2's tube-cloning curriculum. The Mosfets can't do the same color temperature and from-within pressure of the triodes which subjectively create more dimensional relief even though low-level ambiance recovery is actually inferior. The 300Bs were overall looser and fluffier. Simultaneously they reminded me of how my new HP2710m computer monitor differs from the Samsung Syncmaster 245B which recently died. Like the iMac, the HP screen runs highly glossy glass. The Samsung had been flat plastic. The color sheen or wetness of the newer glass screens seem quintessentially SET in action. It's the key thing the M2 doesn't do. If that's what you equate with valves, the M2 fails. Likewise if you want the big, meaty and thick sound of powerful pentodes as Rogue and Octave champion them.

The M2 is quicker and leaner than such muscle amps. It's also more defined and resolved than typical SETs. It is tube in how it combines the softer handling of transients—what some refer to as Mosfet mist—the slightly rose-gold rather than Platinum treble and a denser more voluptuous midband. In the FirstWatt stable, the M2 is a bit of an anachronism. Since the F5, the trend has been for lower and lower distortion semiconductors like SiC JFETs. Concomitant resolution, lucidity or purity have increased. The M2 moves back into the past. It gives up some fine resolution, gets a tad coarser and steps down edge definition. If you idealize the F5 and J2, this will seem like a slight setback. It's also reflected in the designer's admitted hesitation to launch the M2 in the wake of the J2.

It all makes perfect sense however once you compare the M2 to tube amps of equivalent power which operate in a faster, more open-bandwidth EL84 milieu. To get so close to their sound without glowing bits is remarkable. What the M2 throws into the mix as a bonus is a bit of extra overall grip, probably less operational noise and most likely a few degrees more illumination. Fundamental gestalt is perfectly interchangeable. The M2's project thus fully meets the mark of its brief and makes orange juice from lemons - a particular tube gestalt from transistors. In the hands of the right person, it's not so much the ingredients which matter. It's a firm vision and accumulated experience on just how to manifest that vision which carry the day. Nelson Pass clearly was perfectly positioned to make that point. For today, it's called the M2.
Quality of packing: Very good.
Reusability of packing: A few times.
Ease of unpacking/repacking: Effective.
Condition of component received: Flawless.
Completeness of delivery: Amp, power cord, extensive owner's manual.
Human interactions: Always entertaining and enlightening. Few designers in the field are as generous with their knowledge or as tolerant of ignorant questions.
Pricing: A good value and most likely half of what a competing valve amp would cost.
Final comments & suggestions: Like all amps in this range, the M2 runs hot. If your room is small and in Florida, you'll want good ventilation or air conditioning.
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