Recommended Components

Editor's Note: Because of the disparity between typical tube and solid-state "sounds," we have split Class A for separate power amplifiers into two subclasses. Nevertheless, even within each subclass, Class A amplifiers differ sufficiently in character that each will shine in an appropriate system. Careful auditioning with your own loudspeakers is therefore essential. Except where stated, output powers are not the specified powers but rather those we measured into an 8 ohm resistive load. All amplifiers are stereo models, except where designated.

A (Solid-State)

Audionet Max monoblock: $30,500/pair
Designed and built in Germany, the solid-state, class-AB Max is based on what manufacturer Audionet refers to as Ultra-Linear-Amplifier (ULA) topology, developed for medical applications. Its MOSFET output stage draws from separate power supplies for its positive and negative swings—there are also separate mains-transformer windings for positive and negative voltages—and outputs 400W into 8 ohms, 700W into 4 ohms, or 1100W into 2 ohms. Both single-ended and balanced inputs are provided on the rear panel, while the front panel is home to a digital display that reports temperature, DC offset voltage, and various fault conditions. (If the Max goes completely tits up, it automatically disconnects itself from the mains supply.) Listening through Wilson Alexia speakers, JVS found the Maxes to sound darker and less colorful than his reference Pass Labs XA200.8 monoblocks, but noted the tightness with which they depicted bass-drum thwacks—and concluded that, "if your preferences lean toward hard rock, driving jazz, or propulsive/explosive music of any genre, [the Maxes] are must-hears." JA observed that, apart from a very low input impedance on its balanced input, "Audionet's Max performed very well on the test bench." (Vol.40 No.7 WWW)

AVM Ovation MA8.2 monoblock: $31,990/pair
The solid-state, class-AB Ovation MA8.2 is a fully balanced, push-pull design whose output section uses 24 N-channel and 24 P-channel MOSFETs, each bank drawing power from its own 1000VA toroidal mains transformer. The amp, which uses a full 30dB of negative feedback, is specified as offering 600W of output power into 8 ohms; according to AVM's Udo Besser, depending on loudspeaker impedance, up to 40W of that output will be in pure class-A. JVS described the sounds of CDs, SACDs, and digital files played through the MA8.2s as "clean, clear, and neutral," though he felt the sound was, at times, "a bit more spare than I'm used to." That said, the amp's speed earned from JVS special praise. Writing from his test bench, JA concluded that the Ovation MA8.2's impressive specifications are, if anything, conservative, and declared his pleasure at measuring such a "well-engineered" amplifier. (Vol.40 No.4 WWW)

Ayre Acoustics MX-R Twenty monoblock: $33,500/pair ★
In the time since the introduction of their original MX-R monoblock, for which Wes Phillips had high praise in Stereophile's April 2007 issue, Ayre Acoustics discovered the suitability to high-performance domestic audio of the half-century-old "diamond" circuit configuration of bipolar transistors. An advanced version of that innovation, the Double Diamond output stage, along with Ayre's recently developed AyreLock voltage-regulator refinement, and a few other new refinements, have now been applied to Ayre's top-of-the-line power amplifier. The result is the MX-R Twenty, a 300Wpc flagship named in honor of Ayre's 20th anniversary. Describing the MX-R Twentys' performance in his system, JA opined that, no matter what he played, the MX-R Twentys "simply stepped out of the way of the music, letting it flow unimpeded." And bassist JA couldn't help being impressed by how well the MX-R Twentys reproduced a sound he knows so well: "[Jaco Pastorius's] detuned low F and C were so well formed and so palpable that they rolled from the speakers and across the floor to me!" Reporting from his test bench, JA described the Twenty as "significantly quieter than its predecessor," and declared the Ayre "a true high-resolution amplifier." (Vol.38 No.8 WWW)

Bel Canto Design e.One REF600M monoblock $4990/pair $$$
Bel Canto's REF600 monoblock is similar in size to their REF1000M, which KR reviewed in the March 2009 issue, but the newer amp trades the older's ICEpower-based class-D output section for the newly ubiquitous Hypex nCore class-D module, which develops 300W into 8 ohms or 600W into 4 ohms—specs identical to those of the MPS1 power-amp section of the Bel Canto Black (described elsewhere in this edition of "Recommended Components"). Writing of his experience with the REF600Ms driving his Bowers & Wilkins 802 D3 loudspeakers, KR observed that they "never ran out of steam with any sort of music at any listening level, and remained absolutely consistent in tonal balance and clarity." Kal also described the amp's price as "eye-openingly" low. JA's measurements uncovered a remarkably high (109.9dB) unweighted signal/noise ratio, power outputs higher than the manufacturer's specs, and "impressively low levels of distortion and noise." In a Follow-Up in September 2017, HR reported on his experience with the Bel Canto monoblocks, ostensibly borrowed to drive his current-hungry Magnepan .7s (which Herb had never heard sound "more precise, powerful, or three-dimensional" than with the post-break-in REF600Ms). After trying them with five different loudspeaker models, he wrote, "the Bel Cantos offered up weight, body, punch, and crystalline detail better than some of the most expensive amplifiers I know." (Vol.39 No.10, Vol.40 No.9 WWW)

Benchmark AHB2: $2995 $$$ ★
Named for the late Allen H. Burdick, the engineer whose work formed the basis for its design, Benchmark's AHB2 is a compact (11" wide by 3.8" high by 9.3" deep), lightweight (12.5 lb), solid-state amplifier. The AHB2 makes use of THX Corporation's Achromatic Audio Amplifier (AAA) technology, in which a low-power feed-forward amplifier drives a low-bias class-AB output section, the latter energized by a system of class-H power-supply rails that deliver power in response to demand—thus creating an extremely efficient amplifier capable of robust power output (100Wpc into 8 ohms, 190Wpc into 4 ohms, or a strapping 380W when strapped for mono, per Benchmark's specifications). In his listening tests, KR discovered "much more apparent low-level detail in already-familiar recordings"—a characteristic he credited to the Benchmark's evident noiselessness—and a tonal balance that "sounded more 'right' than any" of the other amps he had on hand. In measuring the AHB2 and attempting to confirm its very high specified signal/noise ratio, JA observed nonlinearities in his testing equipment that "haven't affected the measured performance of other amplifiers I've tested, but they were detectable with the AHB2's very low intrinsic distortion and noise." His conclusion: "an extraordinary amplifier." KR's last word: "bargain." In a late-2018 Follow-Up, JCA reported on using this amp with Benchmark's DAC3 HGC D/A processor (see elsewhere in "Recommended Components"); in this context, and in comparison to his reference amp, he wrote, "Although I preferred the AHB2's denser presentation of certain details, it's not clear to me which is truer to the source." (Vol.38 No.11, Vol.41 Nos.10 & 11 WW)

Boulder Amplifiers 2150 monoblock: $110,000/pair
The new 2150 monoblock—it replaces Boulder's 2050 monoblock, introduced in 2000—is among those relatively recent designs that operate in class-A for their full rated power, yet do so by means of microprocessor-based circuits that continually adjust bias in response to current draw, load, and other parameters. Consequently, the 2150 runs relatively cool. Each 2150 uses global feedback (proudly: its manufacturer suggests that manufacturers who don't use feedback simply don't know how to do so properly), boasts an output section containing 80 bipolar devices, requires an AC cord compatible with its 32A IEC socket, has heatsinks machined into the sides of its chassis, and weighs an extraordinary 220 lb (319 lb in its shipping crate). For all that bigness, the Boulder amp kept out of MF's way, sonically: "It was about as tonally neutral a piece of electronics as I've heard here, neither warm nor cool." MF also praised the Boulder's "unrestricted microdynamic excursions" and its "well-controlled, ultra-solid, well-defined low-frequency response." Writing from his test bench, JA noted that the difficult-to-move Boulder "measured so well that it taxed the capabilities of my Audio Precision SYS2722." (Vol.40 No.2 WWW)

Bricasti Design M15: $18,000
With its CNC-machined aluminum case and its purposeful-looking feet—the latter presumed by JVS to discourage the use of ancillary isolation products—the 90-lb Bricasti M15 stereo amplifier evinces the DNA of its similarly chunky stablemate, the M28 monoblock. Like that older amp, the solid-state M15 is fully balanced—both single-ended (RCA) and balanced (XLR) inputs are provided—and operates in class-AB. It's fully analog, using traditional power-supply technology, and each channel's output section employs 24 bipolar output devices, for a specified power output of 125Wpc into 8 ohms. JVS praised the Bricasti's "colorful if toned-down" sound and, with some music, heard "excellent three-dimensionality," but felt that on some recordings strings "lacked shine," while others "felt diminished by the muzzled bite of percussion." Still, he noted in his conclusion that the M15 "could be just what the doctor ordered" for a system or room that tends toward the harsh or the bright. JA's measurements suggested that the Bricasti "comfortably exceeded" its specified power output, but was surprised when the amp shut down while being tested into a low-impedance load: "It is clearly less happy driving 2 ohms than it is higher impedances." See also JA's Constellation review in this issue. (Vol.41 Nos.7 & 10 WWW)

Bricasti Design M28 monoblock: $30,000/pair ★
Made entirely in Bricasti's factory in Massachusetts, the solid-state M28 is a fully balanced, dual-differential design from its XLR input to its speaker outputs. Its specified 200Wpc into 8 ohms comes courtesy of two banks of 12 bipolar transistors each, and its CNC-machined case ensures that, in Bricasti's words, "each element mates to a purpose-built location within the chassis." MF described the M28 as "a powerful, beautifully built amplifier" and praised it as "faultlessly smooth and rich in the critical midrange," adding that "[e]ven a diehard tube lover would be impressed." But he found the M28 to be "more smooth and sweet than fast and tight," and suggested that its top end was "insufficiently fast and precise for my taste." Reporting from the lab, JA said: "Its measured performance reveals Bricasti's M28 to be a worthy stablemate to the company's superb-measuring and equally superb-sounding M1 D/A processor." For his Follow-Up, JA auditioned the Bricasti amps in his own system, and described the sonic balance achieved by the combination of Bricasti amps and KEF Blade Two loudspeakers as being "a little on the polite side." But he also found that the GamuT RS7 speakers "proved a synergistic match," the M28s producing "an enormous sweep of sound at the climaxes." His conclusion: "used in an empathetic system . . . [the M28s] will produce true reference-quality sound." (Vol.38 No.5 & 8 WWW)

Constellation Centaur II 500 Stereo: $55,000
For five times the price of their Inspiration Stereo 1.0 amplifier (see elsewhere in "Recommended Components"), Constellation Audio offers the Performance Centaur II 500 stereo amp, with 2.5 times the power (500Wpc into 8 ohms) and nearly three times the weight (150 lb). As with a relative few of audio history's most distinguished solid-state amps, the Centaur uses all N-channel transistors; here, they're built into single-ended modules, themselves combined into what Constellation calls their Balanced Bridged topology. Both balanced (XLR) and unbalanced (RCA) inputs are provided, as is a switch allowing the user to bypass the amp's input-voltage gain stage, for systems in which the Performance Centaur II 500 is preceded by a Constellation preamp. Used to play a recording of Rachmaninoff's Symphony 1 through a review pair of Wilson Alexia Series 2 speakers, the Constellation provided JA with "a transparent window on the acoustic of the Amsterdam hall where the recording was made," with stable and appropriately scaled imaging of solo instruments. And with Tidal's mighty Akira speakers, the Constellation offered "superb" dynamics: "The timpani blows . . . rocked me back in my chair." Reporting from his test bench, JA noted the amp's significantly-lower-than-specified input impedance but also its slightly-higher-than-specified (550Wpc) power into 8 ohms. Quoth the editor: "a powerhouse of an amplifier." (Vol.41 No.10 WWW)

Constellation Inspiration Stereo 1.0: $11,000
Although casework machined from aluminum is far from rare among high-end power amplifiers, the enclosure of Constellation's Inspiration Stereo 1.0 is a slight but welcome departure from the norm, aesthetically and functionally: on each side of the amp are nearly 200 cooling holes machined into inner and outer side panels, the two panels offset so that the inner and outer holes don't quite line up with each other—a source of visual interest that apparently doesn't compromise the effectiveness of the internal heatsinks. Of far greater interest is this 200Wpc class-AB amp's sound quality, which LG described as "more vivid, refined, and transparent than my reference amps." LG also noted being impressed with the "speed, pitch definition, and harmonic balance" of instrumental sounds in the lowest octaves, though he reported that the lowest tones "didn't thunder or shake the room" as he might have wished. In measuring the Inspiration Stereo 1.0, JA observed higher-than-specified power output—340Wpc instead of the claimed 200Wpc—and praised the Constellation's "quite respectable measured performance." (Vol.41 No.4 WWW)

Dan D'Agostino Master Audio Systems Progression Monoblock: $38,000/pair
The star of Dan D'Agostino's Progression line of amplifiers—products only slightly less exalted than those in the company's Momentum line—the Progression Monoblock is actually D'Agostino's most powerful amplifier, delivering 500W into 8 ohms, 1000W into 4 ohms, or 2000W into 2 ohms. This class-AB amp retains D'Agostino's signature venturi heatsinks, in which a series of vertical holes is drilled straight through solid billets of aluminum. Each Progression Monoblock weighs a considerable 150 lb and, as JVS discovered, tends not to run more than warm to the touch: "On colder days, I still needed to heat the room." JVS also discovered the Progression Monoblocks' fine performance: "Not only did [they] nail the savagery of the high-driving piccolos and brutal bass without breaking a sweat, they also kept every instrument in control and balance while conveying a realistic, airy soundstage." In measuring the Progression Monoblock, JA confirmed the amp's very high power output but discovered distortion products suggestive of low bias current in its output stage: curious for an amp from a designer famous for favoring class-A (ie, high-bias) architectures. (Vol.40 No.10 WWW)

Lamm Industries M1.2 Reference monoblock: $32,490/pair ★
The 110W M1.2, with tubed front end, MOSFET output stage, comprehensive short-circuit protection, and high/low impedance settings, offered "unflinching honesty in conveying the true nature of the music that passed through it," said PB. "Utterly continuous and coherent from top to bottom," the M1.2 combined resolution and transparency with harmonic completeness, timbral richness, and glow. JA concurs. Compared to the Classé CT-M600 and MBL 9007, the Lamm had a more robust, less delicate sound, but nevertheless impressed JA with its three-dimensional soundstaging and midrange richness. "This is a great amplifier," he decided. "Highly recommended." In his Follow-Up, JCA observed that "the Lamms didn't bring out every tiny nuance . . . but, boy, are they musical! In that way, they deviate from the stereotype of a big-boy amp." (Vol.28 No.2, Vol.35 No.4, Vol.37 No.6, Vol.39 No.12, Vol.41 No.10 WWW)

Mark Levinson No.534: $20,000
The 350Wpc, class-AB No.534 stereo amp can be seen as a less expensive alternative to Mark Levinson's 400W No.536 monoblock (see below). In fact, apart from the balanced bridge topology of the No.536, which enables its separate banks of bipolar power transistors to function together in driving a single loudspeaker, and the very different parts counts and circuit layouts in their input and driver boards, a No.534 and a single No.536 are very much alike. LG spent time with both, and declared the No.534 "a compelling stereo amplifier" with a "tirelessly wide" dynamic range and the ability to "easily [reproduce] the deepest bass chords of pipe-organ recordings." His verdict: "the No.534 delivers the No.536's sound and build quality at two-thirds the price." (Vol.41 No.5 WWW)

Mark Levinson No.536 monoblock: $30,000/pair
The class-AB, 400W (into 8 ohms) No.536 is distinguished by many of the same technical characteristics seen in other recent Mark Levinson products, including a direct-coupled, fully discrete signal path and the fact that, while the No.536 uses some negative feedback, its overall gain structure is designed for linear open-loop performance before that feedback is applied. Each monoblock contains two fully differential output stages in a bridged configuration claimed to bolster current output and maintain stability into very low-impedance loads, and the No.536 is specified as operating in pure class-A up to 3W. LG praised the No.536 for exerting "a superior grip" on the woofers of his Revel Ultima Salon2 speakers, and for reproducing deep organ-pedal notes "with unflinching control and solidity." Larry also noted that "the midrange blossomed when my speakers were driven by the No.536es," and pointed to their ability to throw "a wide, deep soundstage." LG's conclusion: The No.536 is "equaled by only one or two other amplifiers in memory." JA's summary: "Overall, the Mark Levinson No.536 measured very well, its performance uncompromised by its ability to deliver very high powers into low-impedance loads." (Vol.40 No.7 WWW)

MBL Corona C15: $20,000/pair ★
Although it employs a custom-specified version of the popular Hypex class-D amplifier module, the Corona C15 monoblock, which offers 280W into 8 ohms, 520W into 4 ohms, is, according to its designer, a "Linear Analog Switching Amplifier," the overall design concept of which entails the use of a linear rather than switch-mode power supply—itself built around a toroidal transformer with generous mu-metal shielding. (The latter is presumed to contribute to the amp's 48.5-lb weight.) Indeed, in his measurements, JA confirmed that "the C15's transfer function appears to remain relatively consistent with both frequency and output current"; also unusual for a class-D amp was the C15's admirable output-impedance behavior: JA discovered that its low-pass function into loads of 4 and 2 ohms was "very similar to its 8 ohm behavior." In his listening tests, JA found that the C15's "intrinsic character was all about control, especially the tight control of low frequencies"—a quality that served well the amp's pairing with the somewhat rich-sounding woofer alignment of the Vivid Giya G3 speaker, but less so the Joseph Audio Perspective, with which the MBL sounded clean but lean. Especially when combined with the classic Rogers LS3/5a, JA observed "superb imaging definition and stability" with the good-looking MBL amps. (Vol.37 No.6 WWW)

Moon by Simaudio 860A: $15,000 ★
The solid-state, balanced Moon 860A, capable of delivering 200Wpc into 8 ohms, is biased to operate in pure class-A up to 5W, beyond which the dual-mono 860A slides into class-AB. Its DC-coupled output circuits employ 12 bipolar transistors per channel. Among the 860A's many performance characteristics that caught FK's attention, "most noticeable was the bass: subterranean, articulate, complex—and musical, not just a mush of bass notes." But FK hastened to add that neither that nor other individual sonic traits were the deal-sealers: "What most impressed me . . . was my sheer pleasure of listening to music." In his measurements, JA discovered that the 860A's power output was closer to 180Wpc than the specified 200Wpc; otherwise, he wrote, the amp "lives up to Simaudio's reputation for excellent audio engineering." (Vol.38 No.8 WWW)

Moon by Simaudio 888 monoblock: $118,888/pair
For our reviewers, 2018 was The Year of Lifting Dangerously: first the 255-lb EgglestonWorks Viginti loudspeaker, then the 300-lb ASR Emitter II Exclusive integrated amp, and now the 250-lb Moon by Simaudio 888 monoblock—500 lb per stereo pair! In the Moon's case (haw), much of that weight comes from the massive, elaborate, one-piece aluminum castings that comprise the amp's side panels, each one enfolding, damping, and heatsinking the 32 output transistors per channel. Each Moon 888 also includes two massive, potted power transformers, 12 big, custom-made power-supply storage capacitors, and a cast-aluminum chassis capable of supporting all of the above. With the 888W (into 8 ohms), class-AB Moons in his system, MF delighted in "the highest level of sound quality in my system, passing along warmth or chills, transparency or sludge, grain or greatness, as dictated not by the electronics but by the recording itself." We can't tell you how the 888 fared on JA's test bench because it was too heavy to make the trip—John packed up his battery of test gear and measured the amp chez Mikey, where he confirmed its low impedance, high output power (he measured not 888W but 990W), "excellent" in-audioband signal/noise ratio, and overall "good measured performance." (Vol.41 No.8 WWW)

Pass Laboratories XA25: $4900
The lowest-priced amplifier in Pass Laboratories' XA series, the XA25 strips away all inessentials—front-panel meters, balanced inputs, excessive output power—and provides the listener with a three-stage amplifier in which output power is generated by a single pair of transistors per channel, operating in push-pull class-A. Output is specified as 25Wpc into 8 ohms or 50Wpc into 4 ohms. HR tried the Pass with a great variety of speakers, starting with a rebuilt pair of original Quad ESL electrostatics: "Through the Quads, the XA25 radically improved the tactile presence of music and musicians, their voices and instruments," he wrote. "It made the Quad's legendary midrange more solid, dynamic, and well defined than I'd ever thought possible." Another, very different panel speaker came next: "Driven by the Pass Labs XA25, the Magnepan .7s did real-life natural with ease and élan." Additionally, HR found, "bass weight and organ power were well reproduced by a speaker not famous for these traits." But even that pales in comparison to Herb's adventures using the Pass amp to drive his DeVore Orangutan O/93s: "I began to realize that the XA25 is the most transparent amplifier I've ever heard." His verdict: "reasonably priced, strong beyond its power rating, and positively revelatory." Writing from his laboratory, JA noted that the Pass far exceeded its rated power output, concluding that "the XA25 performed well on the test bench, as I've come to expect from Pass Labs." (Vol.41 No.2 WWW)

Pass Laboratories XA60.8 monoblock: $13,500/pair
In reviewing Pass Laboratories' solid-state XA60.8 monoblock amplifier, JCA echoed JA's earlier published response to the amp's predecessor, the XA.60.5: "the best amplifier I've heard." Each 88-lb XA60.8 operates in pure class-A, made possible in part by the amp's massive aluminum heatsinks and no-less-massive steel mains transformers. (Indeed, the most obvious change from XA60.5 to XA60.8 is a weight increase of 22 lb—per channel.) Output is specified as 60W into 8 ohms, doubling to 120W into 4 ohms. JCA thought the Passes sounded "sweeter, warmer, more delicate" than the more expensive monoblocks that preceded them in his system, but reserved his greatest praise for their spatial prowess: "I'd never heard an amplifier make such an obvious difference. . . . To walk into this room while a good recording was playing was to enter an immersive aural apace." JA, now acting as measurer rather than reviewer, wrote from his test bench that the "well-engineered" XA60.8 "considerably exceeded" its rated output power, delivering 150W into 8 ohms at 1% THD, and he praised its A-weighted signal/noise ratio of 93.6dB: "This is a quiet amplifier." JCA's conclusion: "I am smitten." (Vol.40 No.12 WWW)

Pass Labs XA200.8 monoblock: $42,000/pair
That a single Pass XA200.8 monoblock should weigh almost precisely the same as the average European adult (it won't catch up with the average North American adult for another 20 lb) should come as no surprise: This is a 200W (into 8 ohms) amplifier that operates in class-A, and does so without any sort of microprocessor-controlled sliding-bias scheme. That takes big heatsinks and a good deal of design cunning, and the XA200.8 shows evidence of both, the latter in its DC-coupled front end and distinctive application of input-stage feedback, the former in its . . . well, in its big heatsinks. JVS, who found himself wearing a back brace by the time he'd packed up the review samples and sent them away, agreed with those visitors who described the sound of the XA200.8 as "organic." He wrote that the Passes seemed to "illuminate music from within" in a way that complemented the artistic brilliance of the musicians. He noted the XA200.8's lack of sweetening or softening, and concluded by praising it as "a masterpiece of amplifier design." After measuring the XA200.8, JA expressed his belief that it lacked sufficient bias current to operate in class-A for its full output power, yet nonetheless praised it for its "well-balanced audio engineering." (Vol.41 No.8 WWW)

PS Audio BHK Signature 300 monoblock: $14,998/pair ★
Named for its designer, Bascom H. King, the BHK Signature 300 is a hybrid monoblock with a tubed input stage and a MOSFET power stage, and is specified to deliver 300W into 8 ohms. In a departure from the usual practice, all of those output transistors are n-channel devices, implementation of which requires cunning—and, according to King, a bit of global feedback. Both single-ended (RCA) and balanced (XLR) inputs are provided on the rear panel, where one also finds a removable panel for access to the amp's two dual-triode tubes. In MF's words, "the BHK Signature 300 is a powerful, nimble-sounding amplifier that immediately drew me in with fast, punchy, tight bass; rich, liquid mids; and delicate, transparent highs—all effectively well integrated into a coherent whole." On JA's test bench, one of the BHK Signature 300 monoblocks became unstable, and while it didn't fall and break its hip, it stopped working. JA carried on with the other monoblock, describing it as "a well-designed, well-engineered powerhouse of an amplifier, though it will perform at its best with speakers having an impedance of 4 ohms and above." In 2017, JCA embarked on an interesting pairing—PS Audio's "big brute" amps with the sensitive, easy-to-drive DeVore Fidelity Orangutan O/96 speakers—and discovered that while the DeVores didn't necessarily benefit from such a surplus of power, he "slightly preferred the BHKs' weight" to the "airier, spacier" sound heard with his Leben tubed integrated amp. (Vol.39 No.2, Vol.40 No.8 WWW)

PS Audio Stellar M700 monoblock: $2998/pair $$$
The Stellar M700 incorporates the latest version of PS Audio's Gain Cell—described as "a proprietary, fully differential, zero feedback, discrete, class-A MOSFET circuit"—using it to drive a class-D power module from the Danish ICEpower company. The Stellar M700 has both balanced (XLR) and single-ended (RCA) inputs, weighs a mere 13 lb, and is specified as providing 350W into 8 ohms. RD used a pair of M700s to drive his Monitor Audio Platinum PL300 II loudspeakers, and described the amp as having "very little sound of its own, and nothing that I could describe as a distinct class-D character." The PS amps impressed RD with their distinctly smooth sound—"tubelike wouldn't be entirely inappropriate as a description"—and noted that they were able to drive the Monitor speakers "to volume levels as high as I could tolerate without amplifier or speakers ever sounding distressed." In measuring the Stellar M700, JA noted that it slightly exceeded its rated power output, and concluded that it "offers respectable measured performance." (Vol.41 No.2 WWW)

SAE 2HP-D: $19,995
Named for its output as expressed in horsepower—seriously!—the 2HP-D is the first new amplifier in over a decade to wear the once-ubiquitous SAE badge. This solid-state amp operates in class-AB—one assumes the D in the model name refers to its fully dual-mono architecture—and is supplied in a sturdy enclosure that studiously avoids the excess mass and filigree of other high-end powerhouses. That said, the 2HP-D's one concession to style is the large fluorescent screen on its front panel, which simulates the power meters that distinguished SAE's Old Testament amps. MF found the 2HP-D to be "spectacularly fast, taut, and lean in a very pleasing way . . . with a sound that was, overall, tonally neutral." Writing from his test bench, JA confirmed the SAE's staggeringly high power—"with just the left channel driven into 2 ohms, the amplifier clipped at 2kW (27.0dBW). Wow!"—as well as its "extremely low levels of both harmonic and intermodulation distortion." (Vol.39 No.10 WWW)

Theta Digital Prometheus monoblock: $12,990/pair ★
Joining the growing ranks of class-D amplifiers with perfectionist aspirations, the Theta Prometheus combines the Hypex Ncore NC1200 module developed by noted class-D specialist Bruno Putzeys with a linear power supply designed by Theta's own David Reich. (The latter—or at least its enormous toroidal transformer—accounts for the amps' 54.5-lb weight.) Specs include 500Wpc, less than 0.001% THD+N at 1W, and an output impedance said to be extremely low. According to LG, "The most striking things about the Prometheus's sound were its huge dynamic range and bass impact." He also noted that "The Prometheuses projected a broad, detailed, involving, three-dimensional soundstage," and "the midrange response blossomed when my [Quad ESL-989] speakers were driven by the Thetas." LG's conclusion: "My last impression of the Prometheus was the same as my first: It's one of the best-sounding amplifiers I've heard in my listening room." RD concurs. Following the Theta's time on his test bench, JA wrote, "The measured performance of Theta Digital's Prometheus is superb, even for an amplifier with a class-D output stage." (Vol.38 No.3 WWW)

Vandersteen M7-HPA monoblock: $57,200/pair
The M7-HPA is designed for use with Vandersteen's Model Seven loudspeaker: this High-Pass Amplifier rolls off its output below 100Hz, in deference to the onboard 400W amp that powers the Seven speaker's 12" dual-cone subwoofer. Other distinctions abound, including the M7-HPA's single-billet aluminum chassis with elastomer-suspended internals, twin-tube balanced input stage, single-ended solid-state output stage, 10-stage power supply with extensive protection circuitry, and (silent) pump-actuated liquid-cooling system in place of heatsinks. JA's very positive comments on the sound of the M7-HPA are, of course, inseparable from his comments on the Model Seven itself (see elsewhere in this issue's "Recommended Components"), but he did try using the Vandersteen amps with a pair of KEF LS50s, whose limited bass extension rendered the pairing technically—if not economically—reasonable. JA noted in particular that the amp "sounded sweet without any attenuation of the high frequencies," and concluded by wondering what a full-range M7-HPA might sound like. Note that, during the review, both M7-HPA samples shut down owing to physically broken plate-load resistors—failures linked to rough handling of JA's much-traveled demonstration pair. Writing from his test bench, JA noted that the Vandersteen monoblock not only measures well for an amp with no feedback, "it measures well, period." (Vol.39 No.5 WWW)

Ypsilon Hyperion monoblock: $93,000/pair
The 209-lb Hyperion monoblock is a two-stage hybrid design with a low-impedance dual-triode tube for its input stage—a 6H30 or a 5687 will do—and MOSFETs for its output section. The two stages are joined by a custom-wound transformer that also serves as the phase splitter for the push-pull output architecture, and the amp is fitted with a 6CA4/EZ81 rectifier tube—an oddity in an otherwise predominantly solid-state product. Output power is specified as 370W into 8 ohms, with the first 100W assured as being in class-A, and 650W into 4 ohms. As for the influence of that input tube, MF found that "the Hyperion's additive quality wasn't . . . identifiable as what's usually thought of as tube sound, but rather as a subtle harmonic and textural generosity that I think most listeners would find very pleasing." Mikey added that "the Hyperion strikes the ideal balance between tube-amp richness and flow and solid-state quiet, authority, and dynamic swagger," and proclaimed it "among . . . the best-sounding power amplifiers I've heard, and it's the most musically enjoyable of the lot." JA, writing from his test bench, painted a somewhat different view, having found an ultrasonic response peak, a linear increase in distortion with increasing power output, and a failure to meet the power-output specs with less than 2% THD. (Vol.41 No.4 WWW)

A (Tube)

Air Tight ATM-300R: $16,995
AD, who has "never heard a single-ended 300B amp that I disliked," was especially smitten by this compact yet remarkably heavy (54 lb) stereo amplifier: a three-stage design in which 12AU7 and 12BH7 dual-triode tubes—one each per channel—are used as voltage-gain stages and drivers, respectively. The output tubes are a single 300B per channel, specified to output 9W each. AD's review sample was equipped with all Electro-Harmonix tubes, but the ATM 300R is available with Takatsuki 300Bs for an additional $3000. Distinctively, the ATM 300R uses global feedback, but tapped from the primary rather than secondary side of each channel's Tamura output transformer. AD heard from the Air Tight "startling . . . details of musical nuance," "lusciously textured and colorful" strings and woodwinds, "realistic force" in its reproduction of timpani, and the ability to sound surprisingly big. He also made special mention of the ATM 300R's way with orchestral double basses, noting their "depth and power, and far greater clarity of pitch than I've heard through any other 300B amplifier, bar none." Writing from his test bench, JA described the Air Tight as "well engineered," praising in particular its apparently "excellent output-transformer design." (Vol.42 No.2 WWW)

Audio Research Reference 160M monoblock: $30,000/pair
Introduced in 2018, the Reference 160M is a mono amplifier that wrings 140W from a push-pull quartet of KT150 pentode tubes operated in class-AB, with phase inversion and voltage gain provided by a pair of 6H30 dual-triode tubes. Yet for all that tubiness, the REF 160M offers switch-selectable balanced and single-ended inputs—during testing, JA was surprised to see that the latter mode offered considerably more gain—and employs a modern auto-bias system that both compensates for tube wear and makes possible easy swapping of tubes among various pentode types. In another modern touch, the REF 160M lets the user choose between Ultralinear and Triode modes at the flip of a switch (in Triode mode, the amp outputs 75W). With a pair of REF 160Ms driving his Wilson Alexia 2 floorstanders, JVS reported sound that was impressively "natural, full-range, and beautiful," including "fully fleshed-out" colors and an ability to convey "hard-to-distinguish details and sonorities . . . with clarity." Writing from his test bench, JA reported evidence of only "a modest amount" of global feedback, and commendably higher-than-specified output power with the REF 160M in Ultralinear mode and speaking to the world through its 8 ohm outputs (there are also taps for 4 and 16 ohms). (Vol.41 No.10 WWW)

BorderPatrol P21 EXD: $14,450 with 300B tubes
The BorderPatrol P21 EXD is a 300B amp with a twist: although its present-day reputation stems from its appearance in single-ended amplifiers, here the hallowed 300B output tube is used in pairs, for push-pull operation, producing what designer Gary Dews claims to be an output of 20Wpc. The EXD model reviewed by KM is an upgraded version of the company's basic P21, with better parts and cryogenic treatment of its copper chassis and interstage and output transformers. It is a fixed- rather than auto-bias design, does not employ global feedback, and has an outboard power supply that itself weighs a chunky 35 lb—virtually as much as the amp itself. Used with DeVore Fidelity Orangutan O/93 and Klipsch Heresy III loudspeakers, the BorderPatrol amp provided KM with deep bass that was "tighter and tauter" than he hears from his Shindo Laboratory Haut-Brion (also push-pull, also 20Wpc), plus "super-'black' backgrounds, solid dynamics, excellent imaging, and a special way with texture and touch." Through the DeVores in particular, the P21 EXD provided more "speed, clarity, and slam," although Ken noted that he missed "the warmth and tone of [the Shindo's] lower-end fundamentals." His conclusion: "transparent, dynamic, and a serious truth-teller." Measurer-in-chief JA noted output impedance high enough to have audible consequences (of the nongood variety), plus higher-than-expected harmonic and intermodulation distortion: "I was disappointed by the BorderPatrol P21 EXD's measured performance." (Vol.41 No.7 WWW)

Doshi Monoblock V3.0: $35,995/pair
The solidly built Doshi Monoblock V3.0 uses four KT150 pentode tubes in class-AB to output 160W into 4 ohms, and is adaptable for KT88 or 6550 tubes. (Designer Nick Doshi says the amp remains in class-A up to 120W.) The Monoblock V3.0's toroidal mains transformer is made in the US, and its frame-style output transformer is made in the UK, by Sowter. The latter has only a single secondary—according to Doshi, "the problem with multiple [loudspeaker] taps in amplifiers is, if your amplifier has feedback around the outputs, then what tap do you take feedback from?" The small-signal tubes, for voltage gain and phase inversion, are a single 12SJ7 and a pair of 12SN7s, while all rectification is solid-state. The amp has both unbalanced (RCA) and balanced (XLR) inputs, the latter buffered by op-amps, and the output section is fixed-bias. JVS used a pair of V3.0s to drive his Wilson Alexia 2 speakers—he reported preferring the sound with the tubes biased at 190mV—and enjoyed their signature "warm, glowing, nonfatiguing, well-controlled, finely detailed sound," though he felt that the sound was "a mite bass-shy." JA reported from his test bench that the Doshi's measured performance "indicates conservative audio engineering." (Vol.41 No.11 WWW)

Jadis JA200 Mk.II monoblock: $35,900/pair
The JA200 Mk.II is an updated version of Jadis's original JA 200 (the space in the latter name is not a typo), a two-chassis, 160W monophonic tube amp that itself is no stranger to longtime readers of Stereophile. The new amp, based on the same circuit as its predecessor, is specified to deliver 160W into 8 ohms and is also divided between power-supply and signal boxes—that's four enclosures per stereo pair—but adds an important twist: In the JA200 Mk.II, the 10 output tubes are operated in auto-bias rather than fixed-bias mode. (The latter represents a confusing bit of nomenclature: a fixed-bias amp is one whose bias voltage requires periodic adjustment.) JVS noted that the pair of Jadis amps—20 output tubes in all!—put out less heat than his class-A Pass Laboratories XA200.8 monoblocks, and praised them for their total lack of noise. More to the point, Jason heard from the Jadis amps a sound that was "smooth, supremely well balanced, and captivatingly beautiful," though he noted that frequency extremes were not as well served as with his Passes, resulting in a lack of sparkle with some recordings, less impactful bass with others. With the JA200 Mk.II on his test bench, JA discovered very good squarewave reproduction and an absence of ringing, indicating "an excellent output transformer" (Jadis designs and winds their own); he also commented on the "high standard of construction." But he noted that the amp approached its rated output power only from its 2-ohm taps, and regretted its "poor linearity at high frequencies." (Vol.41 No.2 WWW)

Luxman MQ-88uSE: $5995 $$$
Luxman's limited-edition MQ-88uSE—only 100 have been made—is that rarity in contemporary high-end audio: a modestly powered tube amplifier unburdened by microprocessor-fueled gimmicks or a ghastly, massive, machined-aluminum case. The fixed-bias, push-pull, class-AB Luxman uses two KT88 pentode tubes per channel, operated as triodes, producing 25Wpc; ECC802 and ECC803 dual-triode tubes supply input gain and phase inversion. Power-supply rectification is solid-state, and output transformers, mains transformer, and power-supply choke are all designed and made by Luxman. The chassis is old-school minimalist, and a tube cage is supplied. The amp is mostly hardwired, and the build quality is first-rate. With the MQ-88uSE driving his DeVore Orangutan O/93 speakers, AD reported superb musicality—excellent musical momentum, flow, and pacing—and a sense of touch by which "every [harp] arpeggio was a tactile delight." AD's conclusion: "brilliantly musical and unmistakably high value . . . grab one while you can." In his Follow-Up, JA described the full set of measurements he made of the Luxman; his observations included higher-than-specified power (30Wpc into 8 ohms) and excellent squarewave reproduction ("a tribute to the high quality of its output transformers"). His conclusion: "an excellent example of a classic push-pull tubed design." (Vol.41 Nos. 9 & 11 WWW)

McIntosh MC275: $6000 $$$ ★
The revived 75Wpc MC275 preserves the look of the original while adding modern innovations. Chimneys are used to cool the tubes by convection, and three circuit boards have been replaced by a single board on which are mounted all components, tube sockets, and power-supply parts. ST: "I heard all the dynamic quality, all that aliveness of the original, plus a level of transparency that brings the MC275 definitely into the 21st century." Sam bought the review sample. The fifth incarnation of the 75Wpc MC275, originally introduced in 1961, retains the first incarnation's classic appearance and its use of four KT88 power-output tubes, three 12AX7 input tubes, and four 12AT7 driver tubes. New are a stainless-steel chassis, balanced inputs, and gold-plated, five-way binding posts. While the MC275's two hefty transformers utilize the same "unity-coupled circuit" invented by McIntosh in 1947, the copper wiring is now insulated in a more durable synthetic material. Though dynamic expression was restricted and bass notes were "a bit muddy" in dense musical passages, the MC275 produced "stunning" soundstage depth and "spooky" intertransient silences, said FK. Meanwhile, the MC275's superb signal/noise ratios led JA to conclude that "Good audio engineering is timeless." Compared to its predecessor, the Limited Edition version of McIntosh's MC275 ($6500) has beefier binding posts, a more conveniently positioned power switch, and a gold-plated rather than stainless-steel chassis. In addition, a new output-transformer winding process has resulted in wider bandwidth, increased damping factor, and improved linearity. RD noted an ideal top-to-bottom tonal balance and an impressive sense of rhythmic drive, concluding, "The MC275LE is simply a wonderful-sounding amplifier, able to bring out the best from a wide range of loudspeakers." He bought the review sample. (Vol.27 No.7, Vol.33 No.10, Vol.35 No.10 WWW)

Music Reference RM-200T Mk.II: $7900 ★
Made in the US, the 100Wpc RM-200 Mk.II has the same basic physical and electrical architecture as the original, but uses better output transformers, adds a capacitor-forming function to extend tube life, and has a revised power supply. The fully balanced design features a high-power, bipolar, solid-state input stage and tubed driver and output stages. It uses two matched pairs of KT88 (standard) or 6550 (optional) output tubes, and a pair of 6BQ7 drivers. Though it lacked the slam and bass authority of more powerful solid-state amps, the RM-200 Mk.II produced airy highs, well-defined bass, and a lush midrange. "When the RM-200 Mk.II was in my system, I wanted for nothing," said MF. JA noted "superb measured performance for a tubed design." Now comes with hand wound output transformers; tube bias balance control, add $800. (Vol.34 No.12 WWW)

Rogue Audio Stereo 100: $3495
If you like tube amps and you also harbor a fondness for audio-frequency transformers, you may well love the Rogue Stereo 100, which greets the input signal with a Jensen line-level transformer—this as much to buffer unbalanced signals as to couple balanced signals that enter via the Rogue's XLR jacks—then hands off to dual-triode–based input and driver stages with a pair of KT120 output tubes per channel, operating in class-AB push-pull, with the user's choice of switch-selectable Ultralinear and Triode modes. The fixed-bias, solid-state–rectified Stereo 100 outputs 100Wpc in Ultralinear, 60Wpc in Triode; in either mode, the Rogue kept HR's Harbeth M30.2 speakers "obviously well-tempered and well-nourished, power-wise. Both modes delivered detailed sound that was neither warm nor cool, just well balanced. The bass was tight and clean, maybe slightly lean, with no overhang or fuzzy harmonic additives." In measuring the Stereo 100, JA noted "superbly square" squarewaves in both modes of operation, and quite respectable signal/noise ratios, though power output was a bit lower than specified in both modes. Herb's conclusion: "the Stereo 100 is everything . . . that we could hope for in a 21st-century tube amp." (Vol.41 No.11 WWW)

Shindo Haut-Brion: $10,995 ★
Like earlier models, the latest Haut-Brion uses two matched pairs of the rare 6L6GAY pentode tube to deliver 20Wpc. The output section is a fixed-bias design, with a regulated bias supply and individual adjustment pots for each of the four output tubes, while the output transformer is a C-core Lundahl model made exclusively for Shindo. Unlike earlier models, the new Haut-Brion has three 6AW8A triode/pentode tubes per channel, uses a pair of Alps 250k ohm potentiometers, and forgoes global feedback. Though it lacked some low-frequency tightness, the new Haut-Brion created an enormous soundstage and showed impressive force. "The amp was the pizzicato king," said AD. (Vol.35 No.2 WWW)

B

Akitika GT-102: $488 (assembled); $314 (kit) $$$
As HR mused in his review of the solid-state AkitikA GT-102, "Jeez, Louise—should I even bother to drive a $7700 pair of [Joseph Audio Pulsar] speakers with a $488 amp?" That question is made more poignant by the fact that the GT-102 is available in kit form for only $314—far less than many of you would dream of spending on an interconnect or even a cable riser. To ensure that ours was an exemplary sample, for the sake of measurements if nothing else, HR tried a factory-built sample and found the 60Wpc GT-102, which is based on the same power op-amp as the well-regarded 47 Laboratory Gaincard integrated amp, had a "warmer, darker" sound than that of his Pass Laboratories XA25. The AkitikA didn't make much headway with the difficult-to-drive Magnepan .7 planar-magnetic speakers, but it partnered well with Falcon Acoustics' LS3/5a minimonitors, Herb being especially impressed by "how attractively it rendered the complex harmonic structures of . . . saxophone, harmonica, violin, and marimba." His conclusion: "Super-highly recommended." Writing from his lab, JA noted that the AkitikA "is happier driving higher impedances, though this drawback must be considered in the context of its very affordable price." (Vol.41 No.7 WWW)

Audio Alchemy DPA-1M monoblock: $3990/pair
The resuscitation of Audio Alchemy brings with it a new line of relatively affordable electronics—including the Peter Madnick–designed DPA-1M monoblock. A class-D amp based on Hypex's UcD modules, this compact amplifier (10.4" wide by 3" high by 11.5" deep) is specced at 325W into 8 ohms and 500W into 4 ohms. When used to drive his Wilson Sabrina loudspeakers, the DPA-1Ms impressed RD with "their clear, open sound, resolution of fine detail, dynamics, and freedom from coloration," though he felt the (more expensive) Theta Digital Prometheus monoblocks reproduced music "with more body." Writing from his test bench, JA praised the Audio Alchemy for its "astonishingly high output power for such a small, lightweight amplifier," but cautioned that speakers whose impedances drop below 4 ohms "are best avoided." (Vol.39 No.9 WWW)

Cary Audio CAD-805RS monoblock: $15,995/pair
On the cover of the January 1994 Stereophile, the original Cary CAD-805 monoblock—a single-ended triode amp that offered 27Wpc—posed next to a 300Wpc Krell amplifier; above them, a headline fairly taunted: "If either of these amplifiers is RIGHT . . . the other must be WRONG." A quarter-century later, that Cary monoblock endures as the CAD-805RS. Now as then, the user can select between a single 845 or 211 output tube: both are supplied, and both deliver 27W in class-A, which is how this amp operates under most conditions. But when more power is required the Cary slips into class-A2, and there the output tubes differ, the 845 delivering up to 50W for short durations, the 211 up to 70W. The CAD-805 offers adjustable global feedback (0–10dB), apparently high voltage gain, and separate output taps for 4, 8, and 16 ohm loads. In his listening observations, AD expressed a preference for the 845 tubes, but noted from both tube types a bass range that was slightly too generous but nonetheless sublime, giving impact and taut tone to timpani and rewarding him with electric-bass lines that were always engaging and never sluggish. Consistent qualities were believably rich timbral colors and textures and spatial performance in which "centrally positioned voices and solo instruments popped forward from the mix . . . [with] pure, not-unexpected single-ended magic." Reporting from his test bench, JA noted output impedance that was "relatively low for a single-ended triode amplifier," yet also confirmed that load-impedance–related modulation of the amp's frequency response resulted in a low-bass boost. (Vol.42 No.1 WWW)

EleKit Tu-8600RS: $1785 as reviewed
Basic price is $1185. See HR's review in this issue.

Emotiva XPA-2 Gen3: $999 (two-channel version) $$$
The XPA Gen3 is a modular class-H amplifier—its input stage tells its power-supply stage when and when not to stoke the fires of its output stage—but it isn't just a stereo amp: it can be ordered with anywhere from two to seven channels. (The two-channel version costs $999; each additional channel adds $200 to the price. Channels can be added only by Emotiva or an authorized Emotiva dealer.) HR tried a two-channel XPA Gen3—300Wpc into 8 ohms, 550Wpc into 4 ohms—with a variety of different speakers. The pairing with Zu Soul Supremes was less than dreamy, but listening to Miles Davis through the hard-to-drive Magnepan .7s, Herb wrote that "the bell of his trumpet was right there in front of me," and observed that the Emotiva "held and guided the Magnepan .7s with almost a lover's touch: not too tight, not too loose." Then again, through the Technics SB-C700S speakers, the sound of the Emotiva had HR reaching for words like "hard" and "transistor sound." In measuring the XPA Gen3, JA discovered that the percentage of THD+noise climbed "precipitously" above 15kHz, though he acknowledged that it offered very high power "at a very affordable price." (Vol.40 No.8 WWW)

First Watt J2: $4000
Described by founder and designer Nelson Pass as a "kitchen-table" enterprise, First Watt is dedicated to making low-power, class-A amplifiers in which JFETs are the output devices of choice. Their latest amp, the 25Wpc J2, impressed HR with its "grainless images and head-shaking rhythms. Every aspect of the music—pace, presence, performance—was fully exposed." The J2 didn't have quite the vividness of Herb's tubed Line Magnetic LM-518IA, but made up for it with its dependable musicality: "Every day, [the First Watt J2] made me and my favorite loudspeakers extremely happy. Highly recommended." In measuring the J2, JA discovered a relative lack of linearity into lower impedances, but declared that, "with higher-impedance speakers, the J2 will sing!" (Vol.39 No.10 WWW)

First Watt SIT-3: $4000
First Watt's most recent amplifier takes its name from the Static Induction Transistor, a type of output device claimed to share certain characteristics with vacuum tubes. The amp's designer, Nelson Pass, commissioned a now-defunct maker of semiconductors to produce for him an exclusive run of these transistors, and the SIT-3, produced in a limited edition of 250 units, may prove to be the last amplifier designed to use them. The SIT-3 is a single-ended amplifier in which the input signal has its voltage gain boosted by only an onboard step-up transformer; the amp's single active stage provides only current gain. The product of all this simplicity is a decent enough 18Wpc, but with less gain than most other amplifiers (11.5dB vs most other amps' 25+ dB). HR tried the First Watt with a variety of loudspeakers—the DeVore O/93, the Harbeth P3ESR, and, perhaps surprisingly, the Magnepan .7 proved the most compatible—and declared the SIT-3 "a modestly priced masterpiece" and "one of the two or three finest-sounding amplifiers I've heard anywhere, at any price." JA's measurements did nothing to dispel that notion: "Although [its] relatively high level of second-harmonic distortion, which is a deliberate design choice, is controversial, the First Watt SIT-3 is a well-engineered amplifier." (Vol.42 No.2 WWW)

Margules Audio U280-SC Black: $6000 (stereo), $10,800/pair (monoblock)
Designed and built by a family-owned company in Mexico City, the Margules U280-SC Black is a tubed amplifier that uses a total of four KT88-family power pentodes. The amp is switchable between stereo and monoblock modes, and between Ultralinear and triode operation, with these power outputs: Ultralinear stereo, 60Wpc; triode stereo, 30Wpc; Ultralinear mono, 120W; triode mono, 60W. In all settings, power-tube performance is monitored and maintained by an Active Servo Control bias technology, hence the SC in the model name. JVS used a pair of Margules amps in monoblock mode to drive his Wilson Alexia speakers and a pair of Harbeth M40.1s, and found the latter to be the far better mates. At their best, with the Harbeths, the Margules amps rewarded Jason with "colors [that] were naturally and ideally saturated," and "believable" timpani attacks. His conclusion: "a wonderful class-A tube amp whose modest power output belies its ability to deliver one grand, splendid performance after another." But when the Margules amp took its place on the test bench, JA's eyebrows rose. He "was not impressed by the . . . U280-SC's measured performance when it was used as a conventional stereo amplifier." That said, JA's measurements uncovered lower distortion and higher power when the amp was operated as a monoblock. (Vol.40 No.11 WWW)

Mytek Brooklyn Amp: $2495
Mytek's first commercial amplifier for the domestic market, the Brooklyn Amp is a compact (8.5" wide) class-D design specified as delivering 250Wpc into 8 ohms or 600W into 8 ohms when bridged for use as a monoblock. (Bridging can be done by the user, via internal DIP switches.) The Brooklyn's output section, based on an amplifier module from the Danish firm Pascal A/S, is described by Mytek as heavily modified; it draws DC from a switching power supply, with nary an old-fashioned mains transformer in sight. Both single-ended (RCA) and balanced (XLR) inputs are provided. Used in stereo mode, the Brooklyn Amp provided KM with "sharply resolute" sound and "a persistent sense of truthfulness, striking resolution that was never analytical, spacious soundstages, superb dynamics, and some of the 'blackest' backgrounds I've ever heard." Switching from a single, stereo Mytek to a pair in bridged mode, Ken noted that "images were larger, and the weight behind those images had increased at least fivefold" and "greatly increased the flesh and blood of the sound of a single Amp, with no loss of dynamics." In measuring the Brooklyn, JA confirmed KM's observation that the amp runs warm for a class-D product; that said, he wrapped up by describing the Mytek as "a tiny powerhouse offering respectable measured performance." (Vol.41 No.9 WWW)

NAD Masters Series M22: $2999 ★
With an output stage based on the Hypex NC400 NCore module, the class-D M22 is rated at 300Wpc into 8 ohms, and is described by its manufacturer as being DC-coupled from end to end, without so much as an output-blocking relay. Our review sample's packaging was of notably high quality, and the compact M22 impressed KR with its solid feel—but, as with NAD's similar Masters Series M27 amplifier, he was frustrated by the lack of tactile feedback in the soft-touch standby switch recessed into the M22's top front edge. That hurdle cleared, KR set about listening, and he praised the M22 for conjuring "a big, wide soundstage populated with colorful instruments and voices." In KR's words, the M22 "is more than fair value in view of its compact size, excellent build quality, the wallop necessary for staggering volume levels, a tolerance for driving difficult loads, and, most of all, its transparent sound." From his test bench, JA reported that the M22 delivered more power than specified, and was so low in noise that he had to alter his test regimen to ensure that he was measuring actual distortion—which was itself very low. His conclusion: "It is the very model of a modern class-D amplifier!" A "slight loss of delicacy" keeps the M22 from Class A, feels KR. (Vol.39 No.3 WWW)

PrimaLuna Prologue Premium: $2199
PrimaLuna's lowest-priced stereo power amplifier, the ProLogue Premium, uses solid-state rectification, two 12AU7/ECC82 input tubes per channel, and two EL34 pentodes per channel operating in class-AB, to produce a grand total of 35Wpc into 8 ohms. (KT88, KT120, and KT150 pentodes can also be used, to respectively provide 40Wpc, 44Wpc, and 48Wpc). As in its other amps, PrimaLuna's Adaptive Auto-Bias circuitry is used to monitor power-tube performance and eliminate the need to buy tubes in matched pairs. Using the ProLogue Premium with his Falcon Acoustics LS3/5a minimonitors, HR found that it "made a tight, glorious, extremely communicative partnership—exactly as the LS3/5a forum folk said it would." He also discovered that the PrimaLuna supplied "the most impactful bass I have yet heard from the KEF [LS50]" speakers, and that the ProLogue Premium driving the Zu Soul Supreme speakers created "a system that sounds as if it should cost at least $21,000, not less than $7000." Writing from his test bench, JA concluded that the PrimaLuna "measures about as well as can be expected from a classic tube design that uses a push-pull pair of [EL34s]," but cautioned against using it with lower-impedance speakers. (Vol.39 No.11 WWW)

Editor's Note: There are no amplifiers listed in Class C and D.

K

McIntosh MC462, Verity Montsalvat Amp-60.

Deletions
darTZeel NHB-458 monoblock & Schiit Ragnarok replaced by new versions not yet auditioned; Classé Sigma Mono monoblock Parasound Halo JC 1 monoblock discontinued; Channel Islands Audio E?200S, Spec RPA-W7EX Real Sound, not auditioned in too long a time.