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.
Arcam FMJ P49: $5750
Now that British company Arcam has been acquired by a Canadian pro-audio firm, they've begun manufacturing their highest-end products in the US, and the FMJ P49 is among the first of these. Specified to deliver 200Wpc into 8 ohms, the FMJ P49 operates in class-G: Depending on the demand placed on it by the input signal, the power supply switches between low- and high-voltage rails for the output transistors, thus vastly improving efficiency and reducing heat. (The transition occurs at approximately 50Wpcabout the limit one might expect from a pure class-A amp with a similar output section.) JA commended the Arcam for "meet[ing] a high standard" of clarity and transparency. The FMJ P49 exhibited excellent control of the very lowest frequencies, and retrieved subtle details sufficiently well that, with one 50-year-old track, "even some print-through" from the original tape was audible. JA declared the P49 "a well-engineered amplifier," with no problems arising from its distinctive power-supply arrangement. (Vol.38 No.11 WWW)
ATC P1: $3499 ★
No strangers to the making of solid-state amplifiersATC has, for decades, specialized in building self-powered speakers for the pro and domestic marketsthe British firm now sells a 150Wpc standalone power amplifier. The P1 is said to offer wide bandwidth (2Hz400kHz) and a high damping factor (400), plus true balanced inputs alongside its unbalanced RCA jacks. (ATC recommends the former for best performance.) After spending a number of weeks with the P1, JM concluded that "the P1 did not editorialize on the music; it just delivered it." Indeed, his time with the ATC P1 led JM to wonder if perhaps another recommendable amp was, by comparison, adding a bit of "zip" to the sound, "and that perhaps the ATC P1 was telling the story straight." JM concluded: "I am unaware of any other amplifier built from discrete components with this level of build quality, and from a firm with a record of accomplishment similar to ATC's, that offers so much excellent wattage at such a comparatively low price." (Vol.37 No.10 WWW)
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 swingsthere are also separate mains-transformer windings for positive and negative voltagesand 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 thwacksand 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: $29,990/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: $29,500/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 ohmsspecs 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" W by 3.8" H by 9.3" D) and lightweight (12.5 lbs) 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 demandresulting in an extremely efficient amplifier capable of robust power output (100Wpc into 8 ohms, 190Wpc into 4 ohms, and a strapping 380W when strapped for mono, per Benchmark). 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 noiselessnessand a tonal balance that "sounded more 'right' than any of [the other amps on hand]." In measuring the AHB2 and attempting to confirm its specified very high 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." (Vol.38 No.11 WWW)
Boulder Amplifiers 2150 monoblock: $99,000/pair
The new 2150 monoblockit replaces Boulder's 2050 monoblock, introduced in 2000is 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 lbs (319 lbs 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 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)
Classé Sigma Mono monoblock: $8000/pair
The rare class-D amplifier designed entirely in-houseas opposed to being based on an OEM output module such as those from Hypex and Bang & Olufsenthe Classé Sigma Mono delivers a whopping 350W into 8 ohms and is equipped with a rear-panel line-level output jack for driving a subwoofer (or whatever else). Auditioned by KR with his Bowers & Wilkins 802 D3 Diamond loudspeakersa model that, per Classé, was developed using their ampsthe Sigma Monos produced no detectable character: "Across the audioband, there was no part that stood out from the rest. . . . my reference recordings all sounded disarmingly natural and devoid of distractions." As KR said, "As Goldilocks said, this one sounds ju-u-u-st right." (Vol.39 No.10 WWW)
Dan D'Agostino Master Audio Systems Progression Mono: $38,000/pair
See JVS's review in this issue.
darTZeel NHB-458 Monoblock: 170,000 CHF/pair ★
Made in Geneva, Switzerland, the 450W (530W at actual clipping) NHB-458 measures 18" H by 11" W by 20" D, weighs 154 lbs, and has a deep-gold, brushed front panel and retro-industrial, red-anodized case. Tinted glass side panels allow magnetic fields to escape the chassis and reveal the amp's enormous cylindrical transformer. The large rear-panel heatsink has a cutout for connections that include RCA and XLR inputs, as well as a Zeel BNC 50 ohm input, for use with darTZeel's NHB-18NS preamplifier and Playback Designs' SACD player and DAC. It combined outstanding speed, precise high-frequency transients, and unlimited dynamic range with powerful bass and unsurpassed transparency, said MF. "The darTZeel NHB-458 is easily the finest power amplifier I have ever heard in my listening room," he concluded. At a price! (Vol.35 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 musicpace, presence, performancewas 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)
Krell Solo 575 monoblock: $22,500/pair
With their top-of-the-line Solo 575 and its similarly new stablemates, Krell has made a conspicuous return to class-A amplification, but with a twist: Krell's iBias signal-tracking technology is claimed to make possible an amp that runs in class-A yet takes up less space, runs cooler, and delivers significantly more power than class-A amps of yore. In MF's system, the 575W Solo 575which offers balanced (XLR) and unbalanced (RCA) inputs as well as Krell's proprietary CAST input socketproduced not the richness, suppleness, and maxed-out sustain he associates with traditional class-A amplification, but rather a speedier attack and a "taut bottom end [that] produced always-entertaining rhythmic drive and pleasing musical flow." As Mikey says, especially if you listen mostly to rock, "you need to hear the Solo 575s." JA noted that the Krell inverted the polarity of signals appearing on both its XLR and RCA inputsand observed power output that comfortably exceeded the manufacturer's specifications. His conclusion: "Amplifiers don't get much better." (Vol.38 No.10 WWW)
Lamm Industries M1.2 Reference monoblock: $27,590/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 WWW)
Luxman M-700u: $8995
Unlike the discontinued Luxman M-600A, a 30Wpc class-A amplifier whose sound JM described as "sweet," Luxman's recent M-700u is a 120Wpc class-AB design. The M-700u employs Luxman's Only Distortion Negative Feedback (ODNF) circuit, whereby negative feedback is applied only in the presence of, you guessed it, distortion. For JM, the M-700u proved to be "an amp for all seasons," and while it wasn't, in his words, "out-and-out euphonic," it did allow pianos to sound "more rounded and continuous" than the Channel Islands Audio E200S, itself another JM favorite. (Vol.38 No.6 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 supplyitself 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, complexand 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)
Parasound Halo JC 1 monoblock: $8990/pair $$$ ★
MF heard exactly what this high-power400Wpc specified, 586W at clipping!John Curl-designed amp's specs showed: "ultra-wide bandwidth, high-current capability, low, low noise, a high S/N ratio, and a fast slew rate, among many other indicators of outstanding amplifier performance . . . .There was an honesty to the overall tonal and harmonic presentation that transcended technological stereotypes." MF found the overall sound to be powerful, refined, smooth, organized, dynamic, transparent, and rhythmically supple, if a little on the subtly warm and rich side of the sonic spectrum, but decided that this was not at the expense of transient speed and resolution of detail. "Perhaps some listeners will find the JC 1 too refined and perhaps a tad polite, but I didn't." "Rocks for sure," says ST, adding that with the amp driving the Triangle Magellans, he found the "bass firmed up, the sound wasn't strained in any way, and there was an overall sense of ease. Dynamic ease. Listening ease. Just ease. Compared to the Halcros, the Parasound JC 1s brought the soundstage forward. Tonally, the Parasounds were magnificent . . . with no trace of solid-state hardness. And the amps weren't even broken in." "The Parasound JC 1 is one of the finest high-powered solid-state amps I've heard," said ST. "Think of it as a 25W class-A amp that does 400W class-AB when pushed." Matched with the JC 2 preamp, the JC 1s presented even greater holographic detail and transparency. The Parasound Halo JC 1 traded the Moscode 402Au's snappy, vivid tonality and larger soundstage for "quiet precision," clarity, and focus, said WP. The Halo JC 1 traded the Aesthetix Atlas's creamy midrange for greater bottom-end heft and top-end extension, said WP. Compared to the Bryston 7B SST2, the Parasound had deeper bass, tighter images, faster transients, and greater low-level resolution and microdynamic delicacy, said MF. A favorite of JA's, who was equally impressed by how the JC 1 performed on the test bench: "This is excellent measured performance. The Halo JC 1 is not only the best amplifier to come from Parasound, it ranks up there with the best high-end heavyweights," though WP felt that while the Halo JC 1 exhibited grace and delicacy compared with the much more expensive Luxman B-1000f, it lacked some impact, drive, resolution, and detail. Stereophile'sand Sam Tellig's"Joint Amplification Component" for 2003. (Vol.26 Nos.2, 6, & 12, Vol.30 No.12, Vol.31 No.3, Vol.32 No.9, Vol.33 No.1, Vol.34 No.2 WWW; see also JA's review of the YG Carmel 2 in Vol.38 No.12 WWW)
Pass Labs XA60.5 monoblock: $11,000/pair ★
Designed by Nelson Pass, the XA60.5 is rated to deliver 60W into 8 ohms (130W into 8 ohms at clipping) and uses Pass Labs' balanced, single-ended, class-A Supersymmetry circuit topology. Housed in the same case as the XA30.5 stereo amplifier, it has a gray-anodized aluminum front panel with a large, blue-illuminated meter that indicates the output stage's current draw. The interior is dominated by a large Plitron toroidal power transformer flanked by two circuit boards, each carrying 10 pairs of complementary power MOSFETs. Though it lacked some bottom-end authority, the XA60.5 produced a natural, transparent overall sound, with especially beautiful mids and highs, said JA. "It is the best-sounding amplifier I have ever used," he concluded. (Vol.37 No.1 WWW)
PS Audio BHK Signature 300: $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 cunningand, 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 highsall 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 pairingPS Audio's "big brute" amps with the sensitive, easy-to-drive DeVore Fidelity Orangutan O/96 speakersand 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)
SAE 2HP-D: $19,995
Named for its output as expressed in horsepowerseriously!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-ABone assumes the D in the model name refers to its fully dual-mono architectureand 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)
Siltech SAGA: 3 Box System ($37,500 each) Total Package: $112,500 ★
With their unabashedly expensive Structural Amplifier Gain Architecture (SAGA), cable specialists Siltech took a new approach to the two-box power amp: This is neither a pair of monoblocks nor a split between an amplifier and its power supply, but rather a split between a voltage amplifier and a current amplifier. The former, called V1, is a battery-powered tube amp that's switchable between triode and pentode modes, while the latter, P1, is a mains-powered solid-state amp. More unusual is the P1's Apollo Light Drive technology, in which bias current for the output transistors is produced by training high-intensity LEDs on a solar panel: Take that, orthodoxy! According to MF, in comparison with other top amps of his recent experience, "the SAGA sounded as if it had tightened the turnbuckles of tonality, space, and, especially, rhythm 'n' pacethe sound was positively exhilarating." But tightened turnbuckles have their price: "Soundstage depth was somewhat foreshortened, with images between the speakers that normally appear well behind the speaker plane presenting themselves closer to a line drawn between them." Still and all, MF concluded, "As soon as I pulled [the SAGA] from my system, I began to miss it." JA's measurements, spread over two sample sets, revealed "idiosyncratic" performance, prompting the recommendation that "this expensive amplifier must be auditioned with the prospective owner's own speakers. Stated price is for the V1 (voltage amp) plus P1 (current amp). C1 (preamp) costs $37,500. Entire system costs $112,500. (Vol.37 No.10 WWW)
Theta Digital Prometheus monoblock: $12,000/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 latteror at least its enormous toroidal transformeraccounts 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 technicallyif not economicallyreasonable. 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 resistorsfailures 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)
Air Tight ATM-1S: $10,450
Among the audio world's artisanal power amplifiers, only a few have more impressed AD than the Air Tight ATM-1S, the earliest version of which was made by Japan's A&M Limited way back in 1987. Today's ATM-1S uses two EL34 pentode tubes (configured as triodes) per side, for 36Wpc into 8 ohms. It also features solid-state rectification, dual-channel input-level controls, Hashimoto output transformers, a distinctly easy-to-use tube-biasing system, and a high level of build quality (including point-to-point wiring). AD found the ATM-1S to sound enjoyably warm and lush in his low-powered-ampfriendly system, yet with "an appropriate sense of [musical] momentum and drive." He praised the Air Tight for sounding "forceful and present when listened to at lower volumes," while noting that it remained "poised and free from gross colorations" when pushed. In his measurements, JA noted excellent squarewave response with short risetimesbut, on the downside, "drastically higher levels of second and higher harmonics in the left channel," a problem that may have been caused by a bad tube or damage in shipping. AD summed up: "The Air Tight ATM-1S is among the few power amps I'd care to live with." (Vol.37 No.11 WWW)
McIntosh MC275: $5500 $$$ ★
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)
Miyajima Labs Model 2010 OTL: $10,000
Built into "some of the loveliest steel casework" AD has ever seen, the Model 2010 from Miyajima Laboratory is a stereo OTL amplifier that uses a total of eight 6080WC dual-triode tubes to produce 7Wpc. The fixed-bias 2010 also uses two 12AX7 dual-triodes, two 12AU7 dual-triodes, solid-state rectifiers, and a host of vintage and vintage-style parts, including 18 NOS Black Cat capacitors. Used as a stereo amp, the 2010 impressed AD by sounding "distinctly open and transparent" while lacking "nothing in the way of color or texture." Driving AD's DeVore Orangutan O/96 speakers, the 2010 surpassed the similarly priced and powered Shindo Cortese in top-end sparkle while giving up nothing in terms of "superb touch and force." Adding a second 2010 and strapping the amps for monoresulting in 16Wpcdelivered what AD described as some of the best sound he has had in his home: "The monoblock experience seems to be the way the Miyajima 2010 was meant to be heardand I loved it." Full Class A status refers to the use of two 2010s. AD suggests that, even at nearly $20,000 for a stereo pair, the Miyajima represents very good value. (Vol.37 No.7 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)
PrimaLuna DiaLogue Premium: $3199 ★
Rated to deliver 25Wpc in Triode mode or 42Wpc in Ultralinear, the extremely versatile DiaLogue Premium is a push-pull design that uses six 12AU7 and four EL34, KT88, or KT120 power tubes. Like other PrimaLuna models, it offers: an LED-based Bad Tube Indicator system for the output tubes; a Power Transformer Protection circuit; an Output Transformer Protection circuit; an AC Offset Killer circuit intended to eliminate hum; and an Adaptive Autobias circuit. With every recording AD played, the DiaLogue Premium exhibited an open, clear, and dramatic overall sound, with good tonal balance, realistically weighty bass, and remarkable spatial depth. "A very strongly recommended amplifier and a hell of a good value," AD concluded. Due to the amp's high output impedance and its rising distortion when the load impedance drops below the nominal value of the output-transformer tap, careful matching with the user's loudspeakers is mandatory, cautioned JA. (Vol.37 No.2 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)
Thöress 300B monoblock: $12,995/pair
The single-ended, feedback-free, hand-wired Thöress 300B mono power amplifier, made in Aachen, Germany, delivers 8W into 16 ohms. Its unique input section is designed around a pair of new-old stock Telefunken EL803S pentode tubes used as triodes, and its fixed-bias output stage is set at the factory: Apart from its power switch, the 300B's only user control is an anti-hum pot for its DC-heated output tube. The Thöress 300B, whose build quality impressed AD as "masterful," delivered "an exceptionally good sense of touch" in his SET-friendly system. It also "distinguished itself as a 300B-tubed amplifier with a little more treble extension than usual," and earned praise as "a lively, colorful, tactile, well-textured, unambiguously musical amp." JA declared that, "Overall, the Thöress 300B monoblock measures well for a SET amp." Mr. D concluded: "Highly recommended for anyone who has both the means and a sympathetic playback system." (Vol.38 No.4 WWW)
VTL Siegfried Series II Reference monoblock: $65,000/pair ★
VTL's Siegfried Series II Reference weighs an abdominal-wall-challenging 200 lbs and uses a dozen 6550 beam tetrode tubes to produce 330W in triode mode or 650W in tetrode mode. High-tech touches abound: A microprocessor delays and ramps up the output section's B+ rail on power-up and enables (reasonably) fast switching between triode and tetrode modes. And rather than use an output transformer with multiple secondariesan approach that designer Luke Manley considers a sound-sapping compromisethe Siegfried Series II Reference has a trannie optimized for a 5-ohm load, plus a four-setting feedback control to adjust the damping factor as needed. In MF's system, where tetrode was the preferred mode (with damping factor set to Medium, and with occasional forays into triode), the VTL sounded three-dimensional, with "slam not in spades, but in dump trucks full!" MF reveled in the VTLs' midrange creaminess, while acknowledging the slightly greater transparency of the darTZeel NHB-458 monoblocksand noted that the Siegfried II Reference was "as neutral and uncolored as any tube amp I've listened to." (Vol.37 No.5 WWW)
VTL MB-450 Series III Signature monoblock: $22,500/pair ★
Rated to deliver 425W (tetrode) or 225W (triode) into a 5 ohm load, the MB-450 III uses eight 6550 output tubes, a 12AT7 input tube, and a 12BH7 driver. Revisions to the Series II include a redesigned, fully balanced differential input stage, a lower-impedance output stage, premium Mundorf capacitors, and a shorter, faster, fully balanced negative-feedback loop. While the VTL sounded soft and "tubey" in triode mode, its tetrode performance was marked by an expansive top end, unusually fast attacks, clean decays, and well-controlled bass. "A significant evolutionary advance" over its predecessor, said MF. Because it provides lower distortion into higher impedances, the MB-450 III will sound best with higher-impedance speakers, JA advised. (Vol.34 No.4 WWW)
Audio Alchemy DPA-1M monoblock: $3990/pair
The resuscitation of Audio Alchemy brings with it a new line of relatively affordable electronicsincluding the Peter Madnickdesigned 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)
Channel Islands Audio E200S: $2500 $$$
California-based Channel Islands Audio, aka CIAudio, manufactures a variety of solid-state electronics, the E200S being at the top of their line of stereo power amplifiers. This class-D amp offers single-ended (RCA) and balanced (XLR) inputs, its differential input stage driving a set of Bruno Putzeys's well-regarded Hypex modules that produce 200Wpc into 8 ohms. JM described the sound of the E200S as "coherent from bottom to top," adding that the CIAudio amp had "high resolution, authoritative bass control, and, yes, tonal neutrality in spades. It also projected a very large soundstage with a very quiet background." JM suggests that the E200S deserves our $$$ rating for very high value. (Vol.38 No.6 WWW)
Emotiva XPA Gen3: $999 (two-channel version)
The XPA Gen3 is a modular class-H amplifierits input stage tells its power-supply stage when and when not to stoke the fires of its output stagebut 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 Gen3300Wpc into 8 ohms, 550Wpc into 4 ohmswith 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)
Linear Tube Audio ZOTL40 Mk.II: $5800
See HR's review in this issue.
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 feelbut, 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 distortionwhich 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 partnershipexactly 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)
Raven Audio Spirit Mk.2 monoblock: $26,995/pair
Designed in Korea by S.E. Han and built in Texas by tube enthusiast Dave Thomson, the Spirit Mk.2 is a hand-wired mono amplifier that uses a push-pull pair of 300B direct-heated triode tubes to produce 26Wpc. The Spirit Mk.2 is tube-rectified, thanks to a pair of 5AR4 diode tubes; they and all six of the amp's small-signal tubes are new-old stock (NOS) items from Thomson's collection. In his own flea-watt-friendly system, AD found the Ravens comfortable with high playback levels, at which they "maintained their poise and clarity," and praised the amps' "flow and general clarity of rhythmic purpose." AD's verdict: "I believe that this Raven deserves more than a second look and listen." (Vol.38 No.5 WWW)
Spec RPA-W7EX Real Sound: $5995
Created by a pair of Japanese audio designers with experience in both small-scale DIY tube-amp projects and large-scale consumer-audio manufacturing, the RPA-W7EX Real Sound is a class-D amplifier that swims against the class-D tide. Rather than relying on IcePower or NCore modules, the Spec uses a hybrid approach, integrating the driver and DirectFET MOSFET of International Rectifier's AUDAMP4 with a PowIRaudio module. From there, attention is focused on tuning the amp with select paper-in-oil capacitors and hardwood slabs strategically placed on its steel case. (The wooden parts also form the Spec's three support feet.) In KM's system, the RPA-W7EX provided so much gain that he couldn't turn the volume knob of his Shindo preamp past 7 o'clock; the remedythe need for which was evidently foreseen by Specwas the use of the company's H-VC1 hardwired external volume control ($400), a special jack for which is found on the amp's rear panel. Once the RPA-W7EX was up and running, KM reported that it "had qualities I usually associate with tube amplification: sweet 'n' saturated tonal colors and palpable instrumental textures, coupled to startling microdynamics that left me slack-jawed in wonder." The RPA-W7EX did not, however, impress on the test bench: JA reported that "the Spec RPA-W7EX emitted more RF interference than I have encountered with other class-D amplifiers," and concluded his remarks by describing the Spec as "not an amplifier that can be universally recommended, I feel." In a Follow-Up, AD praised the Spec for challenging his tubed Shindo Laboratory Haut-Brion in the areas of color, texture, and scale, while noting the RPA-W7EX's rather different tonal balance of "less bottom-end weight [and] a little more midrange sun." (Vol.39 Nos. 3 & 4 WWW)
Editor's Note: There are no amplifiers listed in Class C and D.
Jadis JA200 Mk.II, Pass Labs XA60.8, PS Audio M700.
Fi 421A no longer available; Shindo Cortese revised; Constellation Centaur monoblock, Dan D'Agostino Momentum monoblock, Lamm ML2.2 monoblock, Lamm ML-3 monoblock, Moon by Simaudio 880M monoblock, Shindo D'Yquem monoblock, all not auditioned in a long time.
A linear amplifier is an electronic circuit whose output is proportional to its input, but capable of delivering more power into a load. The term usually refers to a type of radio-frequency (RF) power amplifier, some of which have output power measured in kilowatts, and are used in amateur radio. Other types of linear amplifier are used in audio and laboratory equipment.
Linearity refers to the ability of the amplifier to produce signals that are accurate copies of the input, generally at increased power levels. Load impedance, supply voltage, input base current, and power output capabilities can affect the efficiency of the amplifier.
Class-A amplifiers can be designed to have good linearity in both single ended and push-pull topologies. Amplifiers of classes AB1, AB2 and B can be linear only when a tuned tank circuit is employed, or in the push-pull topology, in which two active elements (tubes, transistors) are used to amplify positive and negative parts of the RF cycle respectively. Class-C amplifiers are not linear in any topology.
There are a number of amplifier classes providing various trade-offs between implementation cost, efficiency, and signal accuracy. Their use in RF applications are listed briefly below:
- Class-A amplifiers are very inefficient, they can never have an efficiency better than 50%. The semiconductor or vacuum tube conducts throughout the entire RF cycle. The mean anode current for a vacuum tube should be set to the middle of the linear section of the curve of the anode current vs grid bias potential.
- Class B can be 60–65% efficient. The semiconductor or vacuum tube conducts through half the cycle but requires large drive power.
- Class AB1 is where the grid is more negatively biased than it is in class A.
- Class AB2 is where the grid is often more negatively biased than in AB1, also the size of the input signal is often larger. When the drive is able to make the grid become positive the grid current will increase.
- Class-C amplifiers are still more efficient. They can be about 75% efficient with a conduction range of about 120°, but they are very nonlinear. They can only be used for non-AM modes, such as FM, CW, or RTTY. The semiconductor or vacuum tube conducts through less than half the RF cycle. The increase in efficiency can allow a given vacuum tube to deliver more RF power than it could in class A or AB. For instance two 4CX250B tetrodes operating at 144 MHz can deliver 400 watts in class A, but when biased into class C they can deliver 1000 watts without fear of overheating. Even more grid current will be needed.
Although class-A power amplifiers (PA) are best in terms of linearity, their efficiency is rather poor as compared with other amplification classes such as “AB”, “C” and Doherty amplifiers. However, higher efficiency leads to higher nonlinearity and PA output will be distorted, often to extent that fails the system performance requirements. Therefore, class-AB power amplifiers or other variations are used with some suitable form of linearization schemes such as feedback, feedforward or analog or digital predistortion (DPD). In DPD power amplifier systems, the transfer characteristics of the amplifier are modeled by sampling the output of the PA and the inverse characteristics are calculated in a DSP processor. The digital baseband signal is multiplied by the inverse of PA nonlinear transfer characteristics, up-converted to RF frequencies and is applied to the PA input. With careful design of PA response, the DPD engines can correct the PA output distortion and achieve higher efficiencies.
With advances in digital signal processing techniques, digital predistortion (DPD) is now widely used for RF power amplifier subsystems. In order for a DPD to function properly the power amplifier characteristics need to be optimal and circuit techniques are available to optimize the PA performance.
Most commercially manufactured one to two kilowatt linear amplifiers used in amateur radio still use vacuum tubes (valves) and can provide 10 to 20 times RF power amplification (10 to 13 dB). For example, a transmitter driving the input with 100 watts will be amplified to 2000 watts (2 kW) output to the antenna. Solid state linear amplifiers are more commonly in the 500 watt range and can be driven by as little as 25 watts.
The maximum Amateur Radio Output is dependent on the licensed location, usually 1,500 to 2,250W. This is achieved, usually, with a linear amplifier. Large vacuum-tube linear amplifiers are based on old radio broadcast techniques and generally rely on one or more vacuum tubes supplied by a very high voltage power supply to convert large amounts of electrical energy into radio frequency energy. Linear amplifiers need to operate with class-A or class-AB biasing, which makes them relatively inefficient. While class C has far higher efficiency, a class-C amplifier is not linear, and is only suitable for the amplification of constant envelope signals. Such signals include FM, FSK, MFSK, and CW (Morse code).
Broadcast radio stations
The output stages of professional AM radio broadcast transmitters of up to 50 kW need to be linear and are now usually constructed using solid state technologies. Large vacuum tubes are still used for international long, medium, and shortwave broadcast transmitters from 500 kW up to 2 MW.