When I began writing for Stereophile, I dreaded doing comparisons. They were stressful and tedious—and what if I got them wrong? But I quickly learned: Not only do readers enjoy comparisons, they need them. How else might they imagine the relative merits of the component under consideration? Once I realized this, I began acquiring a range of reference amplifiers, such as the low-power Pass Laboratories XA25 and PrimaLuna ProLogue Premium, the high-power Bel Canto Design e.One REF600M monoblocks, and the single-ended Line Magnetic LM-518 IA. I also collected a range of affordable, popular loudspeakers: DeVore Fidelity's Orangutan O/93, Falcon Acoustics' LS3/5a, KEF's LS50, Magnepan's .7, Zu Audio's Soul Supreme—as well as the most uncolored and coherent two-way dynamic loudspeaker I've heard, Harbeth's M30.2 ($6495/pair).
But conspicuously missing from my audio menagerie has been a fast, neutral, 100Wpc tube amp to put more pop, fire, and maybe a little glow, into the Harbeth M30.2s.
After surveying what's available at working-class prices, I decided to start my auditions with Rogue Audio's all-tube, 100Wpc Stereo 100 amplifier ($3495).
As I was told by Rogue's founder, president, and chief engineer, Mark O'Brien,
"The topology of the Stereo 100 begins with the transformer-coupled input stage. We use a very fancy Jensen transformer to get the balanced signal in. Many companies use a FET or other solid-state device on the input stage, but I prefer the sound (or lack thereof) of a high-quality transformer. This is an expensive way to couple the balanced signal, but it sounds really good, and has the added benefit that we can connect the RCA inputs through it to achieve much of the same noise-reduction benefits as the balanced inputs. The next stage is a 12AX7 voltage amplifier followed by a 12AU7 driver tube. We use the JJ Long plate Euro-spec versions (ECC802S and ECC803S) for all of these small-signal tubes.
"The output stage consists of a push-pull pair of Tungsol KT120 output tubes that can be operated in either Ultralinear or Triode mode via a switch on the rear panel. We use custom power and output transformers designed around the Stereo 100 circuit. Output power is 100Wpc in Ultralinear mode and about 60Wpc in Triode. The power supply is one of my favorite design areas, and is truly critical to the audio performance. Your readers might know that when you are listening to a power amp, you are effectively listening to the sound of the power supply being modulated by the audio signal.
"There are five separate power supplies in the Stereo 100—all of which utilize pi filtering to reduce ripple, thus providing a very low noise floor. Both of the Stereo 100's high-voltage supplies use large electrolytic capacitors for storage, all of which are bypassed by high-quality film capacitors. We also use very–high-quality parts throughout the Stereo 100, including aluminum/oil Mundorf coupling caps, lots of Vishay and PRP resistors, as well as Cardas hardware. The overarching design goal was to create an excellent-sounding, well-built power amp with a useful amount of power. To that end, I try to design a very accurate, low-distortion circuit and then just let the tubes add the magic."
Besides all that, the Stereo 100 is big and heavy: 18" wide by 7" high by 19" deep and 65 lb. It's remarkably old-school in incorporating no newfangled automatic tube-biasing circuits; instead, it requires the user to manually adjust the standing current (bias) for each of its four KT120 tubes. According to the manual, this is to be done during initial installation, following 15 minutes of warmup to stabilize the tubes, and then every four months thereafter.
It was easy to remove the vented, black-crinkle-painted top plate and spot the clamp-mounted bias tool just south of the four toggle switches (one per tube) and their associated bias pots, which can be adjusted by turning a brass screw. Mounted at the center of the steel subchassis is the conspicuously cool, vintage-looking milliamp meter, which, along with the tubes, can be easily seen and admired while looking in from above. Setting the bias is accomplished by flipping a tube's toggle switch from Run to Set, slowly turning that tube's bias screw until the meter reads close to 40mA, then flipping the switch back to Run before going on to the next tube. The entire idiot-proof process was fun and took me less than 10 minutes.
On the Stereo 100's rear panel are separate pairs of five-way binding posts, for loudspeakers with nominal impedances of 4 and 8 ohms; single-ended (RCA) and balanced (XLR) inputs; an IEC power inlet; and, to left of center, a toggle switch that lets you choose between triode and Ultralinear connection of the KT120 output tubes. The Stereo 100 is warranted for three years, its tubes for six months.
Sorry folks, but I must interrupt myself to tell a little side story. Today's ubiquitous short warranties for tubes remind me of when, in the 1990s, I bought some McIntosh tubed gear from a woman who was selling her late husband's stereo. She'd sent me a photo of the preamp and radio as they sat on a shelf in her living-room closet. When I asked about the amplifier(s), she told me that the original receipt listed them as "two McIntosh MC30s," but that she "didn't know what amplifiers looked like."
When I visited her house, on Long Island, the system, playing FM radio, sounded big and effortless. A pair of Altec A7 Voice of the Theatre horn speakers were hidden behind 6'-high gold-threaded grilles, mounted flush on opposing sides of an enormous flagstone fireplace.
When I extracted the industrial-grade Altecs from their tombs, I discovered, sitting on the cement floor behind each, a monoblock Mac. Like the horn speakers, they were thick with dust, rust, and mouse turds—and still working. Through the blanket of dust came the faint orange glow of their original General Electric 6L6 tubes. These once-shiny tube classics had been powered on continuously for over 30 years.
A six-month tube warranty?
I plugged the Stereo 100 directly into a wall socket using Rogue's stock generic cord. After experimenting with wiring, I finally settled on Triode Wire Labs' Spirit interconnects and Auditorium 23's speaker cables, a combination that seemed to deliver the richest, most naturally lively, most three-dimensional presentation.
The Stereo 100's rear-panel toggle lets you choose between connecting its KT120 tubes' screen grids to an Ultralinear (UL) tap on the output transformer's primary or to the tubes' anodes, creating faux triodes. Per the owner's manual, the Stereo 100 is specified to output 100Wpc in UL, but no power rating is given for Triode mode. I assumed it would be about 30Wpc, but it appears I was wrong.
"When triode-connected, the Stereo 100 makes about 60W," Rogue's Mark O'Brien told me. That put a smile on my face: I have a historical tendency to favor triode operation. Fueled with anticipation, I began my serious listening by comparing the Stereo 100's Triode and UL modes with the Harbeth M30.2s connected to the Stereo 100's 4 ohm taps.
The first recording that fully captured my attention was a spectacular recommendation by Stereophile's Jason Victor Serinus: Chimère, a recital of French, German, and American art songs sung by soprano Sandrine Piau (24/96 FLAC, Alpha Productions 397/Qobuz). In these beautiful performances, the shimmering tones of Piau's voice set against the vibrating weightiness of Susan Manoff's concert grand let me compare Triode and UL operation.
In either mode, the Harbeth M30.2s were 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.
Ultralinear mode was more sharply focused than Triode. In UL, music's empty spaces seemed too empty, in a slightly unnatural way. I felt I was receiving less information, that those empty spaces had once been occupied by texture, color, light, and shade. UL seemed higher in contrast. In Triode, voices and instruments felt denser, more tactile, more a part of their surroundings. UL was more forceful and immediate, but occasionally peaky or hard. UL maximized bass punch and string plucks, emphasized keystrokes and Manoff's left hand; Triode favored the upper octaves, emphasizing decays, sustains, and harmonics. Manoff's pedaling was more obvious in Triode.
UL drew my attention to the tempo; Triode showed me more mélodie Française.
I preferred Triode mode because the most obvious difference between it and UL was an important one: Triode delivered bigger, deeper, more CinemaScope soundstages populated with denser, more vivid three-dimensional aural images.
The differences just described were actually subtle, and both modes of tube operation were enjoyable, but for different reasons. More obviously, in both modes, the Stereo 100's sound was unsubtly not tube-like. In a blind test, I doubt I could identify it as a tubed amplifier.
Comparison: Pass Laboratories XA25
The sound of the Rogue Stereo 100 was not as strikingly transparent as that of Pass Laboratories' XA25 25Wpc amplifier ($4900), nor did it feel more powerful. Nor was it as alive, open, or dynamic. That doesn't mean I found it inferior. Watt for watt, the XA25 stands tall and truthful next to any amplifier putting out less than 100W. So will the Stereo 100. Both played diverse music with the authority of excellent design.
To my taste, driving my speakers in my room, the triode-connected Stereo 100 seemed nigh on perfect. It had a way of disappearing, leaving vital, living music hanging in the air between the Harbeth M30.2s. The XA25 may be more transparent in the sense of the air between performers being freshly oxygenated and free of haze, but I can always tell when the Pass is driving my speakers—I can feel its hyperdynamic presence. The Rogue was more unobtrusive, readily disappearing from my awareness. Driving the Harbeth M30.2s, both were true reference-quality amps.
Comparison: PrimaLuna ProLogue Premium
The Rogue Stereo 100 also proved more "invisible" than my long-beloved PrimaLuna ProLogue Premium 35Wpc amplifier ($2195). The Rogue added less of the friendly magic of the PrimaLuna's kinder, gentler, more romantic EL34 tube sound—which I love. To me, the ProLogue Premium, which operates only in UL, is exceedingly tactile in its reproduction of music: It enhances soprano voices in a way that makes singer and song more beautiful and physically tangible. It can also add an occasional hard bite to a high note, or a moment of blur to a bass line.
Meanwhile, the Rogue Stereo 100 sailed smoothly and elegantly through even the most highly modulated bass and treble passages. Bass was consistently tight and detailed, and treble always relaxed, big-sky open, and finely rendered. (Forget bandwidth specs—solid-state amps are never as elegant or as transparent in the highs as a properly designed tube amp.)
Driving the Harbeths in Triode mode, the Stereo 100 delivered a greater sense of easy-flowing, unrestricted power than the UL-connected PrimaLuna. This easy-flowing power reproduced sopranos' high notes with zero compression or strain. Triode mode delivered pianos that were more full-size and solid, more anchored in the recording venue than through the PrimaLuna. Overall, the Rogue Stereo 100 plus Harbeth M30.2s reproduced sopranos, pianos, and big orchestras with relaxed precision, realistic timbres, and understated beauty. It put reference quantities of bang, fire, and a subtle little glow into the Harbeth monitors. Rogue Audio Stereo 100 + Harbeth M30.2 = my new reference amp-speaker pairing.
With: KEF LS50 speakers
The first time I heard KEF's LS50 minimonitors ($1499/pair), I was with friends. When they asked me what I thought, I laughed: "I'm glad I'm not in the business of making $5000/pair speakers!" The LS50's agile bass, focused imaging, and conspicuous eight-octave coherence seemed as tight and right as anything I'd ever heard from a pair of speakers with 5" woofers.
Now, as I listened via the LS50s to Morphine's Cure for Pain (24/96, Ryko/Qobuz), Mark Sandman's saxophone was displaying such a beguiling low-frequency textural growl that I had zero urge to replace the 35Wpc PrimaLuna ProLogue Premium with the 100Wpc Rogue Stereo 100. But I forced myself, and discovered a new level of clarity that let Sandman's voice step forward and reach out of the fog. The Stereo 100 reduced the saxophone's growl, but instead showed me the air coming out of its bell. The articulation of Sandman's voice improved. Microphone positions were easier to pinpoint.
Lately, I've been deeply appreciating Jimmy Smith—I can't get enough of his fast-moving grooves or the rich harmonics of his Hammond organ. I'm especially entranced by his 1972 album Bluesmith (16/44.1 FLAC, Verve/Tidal), which showcases Smith at his purest and freewheeling best. Eric Miller's production perfectly frames Smith's dreamy keyboard blues with Ray Crawford's guitar and Leroy Vinnegar's double bass. "Absolutely Funky"—a demo-quality recording—opens slowly and quietly, Vinnegar methodically walking the strings of his double bass for a long 40 seconds; then, out of this vibrating bass-filled darkness, Smith's electric organ hits like lightning. Sparks fly! The Stereo 100 gave Smith's keyboard sparks every bit of electric fire they needed—and none extra—and made the LS50s sound newly vivid and impressively solid.
The Rogue-KEF combo did an outstanding job of recovering the basic pleasures of every musical genre. LS50 + Stereo 100 = a powerful good marriage.
With: Magnepan .7 speakers
Still streaming Bluesmith, I connected Magnepan's .7 quasi-ribbon panel speakers ($1395/pair) to the Rogue Stereo 100. This time with "Absolutely Funky," I felt Vinnegar's double bass physically against my chest and face. Smith's Hammond was huge in the space in front of me. When I tell you that plucked double-bass strings sounded visceral through one of my minimonitors, I mean lower-case visceral. Now, suddenly, through the 5'-tall Magnepans, bass was VISCERAL. Smith's band was big and tall. The Stereo 100 (in Triode!) was driving my beloved Maggies, making all of Bluesmith feel like an extended dream of head-nodding beat, high tactility, and scrumptious instrumental tones.
In both Triode and UL, and at the modest SPLs of 75–85dB/2m I listen at, the Rogue easily adapted to the Maggies' current-hungry load, with no hint of clipping or compression. In UL, there was no odd-order harmonic bite. The Rogue-Magnepan sound was hypertextural, superspacious, luxurious. I'd forgotten how much I like speakers that move lots of air, that deliver instruments and voices in something close to actual size. Except for some fluffiness in the bass and some roundness on the edges of transients, the Rogue-Maggie mashup made recordings sound surprisingly real and completely engaging.
Remember those McIntosh MC30 tube amps that were left turned on for 30 years and still sounded wonderful? Well, I doubt the new KT120 tubes in Rogue Audio's new power amplifier will last over 262,980 hours, as the G.E. GT6L6GCs in those ancient beauties did, but I'm certain that the Stereo 100's tank-like build quality, vividly neutral dynamic sound, and timeless styling make it a worthy successor to McIntosh's most adored classic. In my view, the Stereo 100 is everything—sonically, mechanically, aesthetically—that we could hope for in a 21st-century tube amp. Everything about it screams "End-game tube amp, long-term keeper!"
Sidebar 1: Specifications
Stereo, push-pull, class-AB1, tubed power amplifier with transformer-coupled inputs.
Tube complement: two 12AX7 (voltage amplifier), two 12AU7 (driver), four KT120 (output). Inputs: 1 pair single-ended (RCA), 1 pair balanced (XLR).
Outputs: 2 pairs (4 and 8 ohms) five-way speaker binding posts per channel.
Power output: 100Wpc into 8 ohms (Ultralinear mode, 20dBW), 60Wpc into 8 ohms (Triode mode, 17.8dBW).
Frequency response: 5Hz–50kHz, ±1dB.
Input impedance: 75k ohms. Sensitivity: 1V RMS.
Dimensions: 18" (455mm) W by 7" (180mm) H by 17" (400mm) D. Weight: 60 lb (27.2kg).