pf logo POSITIVE FEEDBACK ONLINE - ISSUE 36
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rogue audio

M-150 amplifiers

as reviewed by Chip Stern

 

 

 

 

CHIP STERN'S SYSTEM

LOUDSPEAKERS
Dynaudio Confidence C1. Acoustic Zen Adagio, Joseph Audio RM25si Signature Mk2 & RM7si Signature Mk2, Meadowlark Swallows, Epos ELS-3, Alon/Nola Li'l Rascals

ELECTRONICS
VTL 5.5 vacuum tube preamp, Rogue Audio Stealth phono preamp, Manley Massive Passive Vacuum Tube Parametric Equalizer, Rogue Audio Magnum 99 vacuum tube preamp, Mesa Tigris, Rogue Audio M150 Vacuum Tube Monoblocks. and Bel Canto Ref1000.

SOURCES
Upscale Audio Ah! Njoe Tjoeb 4000 vacuum tube CD player (in 24/192 Super Tjoeb configuration), California Audio Labs CL-20 DVD-CD Player, McCormack UDP1 Deluxe universal disc player, Rega Planar 25 turntable with Rega RB600 tone-arm and Grado Statement Master cartridge, and Musical Hall Trio.

CABLES
Acoustic Zen Silver Reference II interconnects, Audioquest Panther interconnects and CV-6 speaker cables, Monster Cable Sigma Retro Gold interconnects and speaker cables, JPS Labs Superconductor 2 interconnects and speaker cable, JPS Labs Superconductor 3 interconnects and speaker cables, JPS Labs Aluminata, Kaptovator, Digital and Analog AC, Acoustic Zen Gargantua II and Absolute AC.

ACCESSORIES
Two PolyCrystal equipment racks, a PolyCrystal amp stand and PolyCrystal speaker stands. Power line conditioning: Equi=Tech Q650 and 2Q Balanced Power Isolation Transformers, Monster Cable AVS 2000 Automatic Voltage Stabilizer. JPS Labs Kaptovator outlet centers, Mondial Magic-Splitter, NEC CT-2070S monitor, Ringmat 330 and Signal Guard II isolation stand (turntable), Shakti Stones (electromagnetic stabilizers), PolyCrystal cones, Argent Room Lens, Echo Busters Bass Busters and absorptive and diffusive panels, Grado RS-1 Stereo Headphones

 

There is something very Noel Coward, darling, something fundamentally snooty about certain members of our high end audio congregation that has always seemed sadly counter-productive to a wider acceptance of this spiritual calling of ours—this haughty attitude which holds that gear below certain price points is simply unworthy of them and those who suggest otherwise invite derision and contempt. Of course, exclusivity and limited access go a long way towards cementing the reputation of certain products, we understand that. However, as a result, on those infrequent occasions when the uninitiated find themselves down wind from some sauntering high end nimrods doing the George Sanders-two-step, the aroma of pretension and self-entitlement is so strong, they figure why even bother to get involved since we couldn't possibly drop, fifteen, twenty, thirty grand on a pair of monoblocks.

Or as my worthy constituent John Potis recently wrote in an e-mail communiqué: "I refuse to buy into the whole mystique designed to suck in the suckers who need to feel that they're more sophisticated than the average audiophile."

Now I'm not going to front: it's just as snooty in a willfully obverse, Cory Greenberg-styled, faux-populist manner to summarily dismiss speaker cables, interconnects and power cords as undifferentiated hype, and likewise to discriminate against pricey gear—jumping to the equally bogus conclusion that one could not indeed find absolute performance nirvana in the higher priced spread. Still, I wish I had a fiver for every time a manufacturer confided to me that his dealers were balking: "You should raise the prices on your gear. It's too inexpensive…no one will take it seriously." It's worth noting at this point that my esteemed colleague John Potis recently had the opportunity to review the Bryston 28B-SST Monoblocks for 6moons—1000 watts of muscle amp glory (into 8 ohms) for $15,000 a pair—and his enthusiasm for their aural attributes knew no bounds. Believe me he hated to pack those babies up: "They out-tube a lot of tube amps in their performance," he enthused to me just the other night.

Well, imagine my enthusiasm when I got a chance to spend an extended conjugal visit this past summer and fall with the Rogue M-150 Monoblocks, which out solid-state a lot of the best SS amps in my estimation, such is their taut, steely bass control, smooth, linear, extended bandwidth, clear, airy, transparent high frequency detail and utter lack of untoward colorations. And believe me, I hated to pack them up just as much as John did when saying goodbye to the Brystons ...however, I haven't given up hope quite yet—and am presently making inquiries into the body organ market in search of ready cash. Anyone out there need a kidney?

Still, was I ever surprised to discover that my enthusiasm for the Rogue's value and absolute musicality sullies my reputation among some august members of the audiophile college of cardinals. A good friend of mine recently purchased some no-compromise full range speakers based in some part upon my experience and recommendations. He subsequently related to me how an "upscale" audiophile dealer of our acquaintance—currently stalking unsuspecting consumers just due south of Hudson's Bay—recoiled in horror at his decision...mostly, one may reasonably conclude, because my friend hadn't purchased the speakers from him. Well, when my friend parried this thrust with a pointed rejoinder about how "Chip Stern spoke very highly of them," Cardinal Richelieu came right back at him with this disdainful dismissal: "Chip Stern," he snickered, "he has no credibility—he uses $4000 monoblocks."

Well, fancy that. Now over the years, this carbon-based life-form has heard all manner of whispers concerning his unseemly corporeal presence upon this tiny green orb: oh, that Chip Stern sure is a character; wow, Chip can be pretty temperamental; Chip sure is high maintenance—but never has anyone ever suggested that I am naught but a philistine, subject to eternal damnation because for all of my high end audio posturing I am but a low class shitheel, with a predilection for budget gear retailing for only $4000 a component. Only $4000—let no man write my epitaph.

And while there is nothing to be gained by calling out this popinjay in a more pointed, public way, the notion he personifies—that musicality is synonymous with uber-expensive gear—is all-too pervasive an idea in high end circles, and needs to be periodically challenged. Again, I don't have anything against hot-damn, cutting-edge technology, cost-is-no-object gear ...bring it on, but can we get real for a moment? I mean, system synergy anyone; room coupling—making your compromises work for you? The joys of high value, high performance gear?

Our focus shouldn't be about creating even more silly hype around expensive components, but about how gear makes music, whether it retails for $400, $4000 or $40,000. Those harboring similar delusions can stop reading right now.

Jaded Is So Overrated, Dare To Be Happy

For the rest of you, might I suggest the possibility that this jaded approach to high end audio is terribly overrated and that one should dare to be happy? Which begs the question: "Is it possible to find happiness and musical satisfaction in tube monoblocks selling for only $4000 a pair?" Only $4000 a pair? Did I just say that? Hell, it's possible to put together a drop dead cool system for $300-400 a component if you have a sense of how to make your compromises (and your room) work for you, but that's another story (to wit, the remarkable $999 Music Hall Trio CD Receiver, coming soon to a Positive-Feedback Online review near you).

Now Mark O'Brien (shown above in Chip's listening room), chief honcho of the well-regarded Rogue line of amplification, has always had the courage of his convictions, and the savvy to design high end vacuum tube circuitry with an ear towards delivering elevated levels of aural quality at something approaching real world prices. What has made the Rogue product line so successful over the past decade, even as one company after another has fallen by the wayside, is the realization that it's not enough to come up with great prototypes, you have to build with an ear towards consistency and reliability and repeatability (tube rollers of the world, please take note), and that if you spec out a sensible long-term parts/performance strategy, you can make gear more affordable, without neglecting your most lofty musical aspirations. And if you pay close attention to the pointed feedback of end-users in the marketplace, you can further refine and evolve your product line over time.

My first encounter with the Rogue line occurred some years back in the pages of Stereophile, where I was especially taken by the bang for buck value and musicality of their $1399 Rogue Sixty-Six preamp, and what a nice, no-compromise match it made with my reference power amp of that time, the Mesa Baron. It had a friendly, forgiving triode quality which tamed some of the Baron's biker bar aggression, retaining the visceral low end punch, midrange layering and rhythmic drive, while affording an element of control and refinement to the top end—all the while significantly extending the amp's dynamic head room. Yet for all the sweetness the Sixty-Six afforded, this wasn't some sort of sloppy seconds, butterscotch sundae kind of my-fi triode line stage straight off of the boat from the Wuhan province, wearing its tube colorations garishly on its sleeve, but a good partner in the signal chain, with exemplary levels of control and resolution at an affordable price.

Moving on to review the $2399 Magnum Ninety-Nine preamp in Stereophile, I was captivated by the vastly enhanced levels of resolution, detail, clarity and control …it proved significantly more revealing and less forgiving than the Sixty-Six line stage, and thus not quite so happy a match for the Baron as the sweeter, more subjectively "tubey" Sixty-Six, though a far more engaging partner for the more refined, sophisticated succession of amps which would temporarily touch down in my reference system. The Rogue Magnum 99 compared favorably with my long-standing reference preamp, the VTL 5.5, yet much as I adored how well the VTL worked with the now departed McCormack DNA-500, I found that I lacked for nothing when teaming the Rogue Magnum 99 with my now-departed successor to the Mesa Baron, the Musical Fidelity Nu-Vista 300.

However, out of all the Rogue gear I evaluated for Stereophile, I was most taken by the $3495 Rogue Magnum M-120 monoblocks, which back in the day, before the roll-out of their more formidable Zeus design, represented Rogue's top-of-the-line amplification offering, and a dandy amp it was, evincing what I've since come to hear as Rogue's performance signature: a nice balance between refined tube tonality and solid state clarity and control.  That is to say, while the Magnum M-120s' midrange had the kind of butter-cream layering, tactile sweetness and benign overload characteristics one covets in a good tube amp, the firm low end focus, weighty bass impact, snappy rhythm and pacing, and smooth high frequency response were not simply emblematic of tube performance but of musical performance.  And as very conservatively rated at 60 watts triode and 120 watts Ultralinear, the Rogue M-120 amplifiers offered substantial current drive for a wide variety of loudspeakers, and were an exceptionally musical partner to the original Joseph Audio RM33si.  

Still, I had a few reservations: while the top end was not overly muted in a tube sense, the smooth sweet quality of the M-120 could get a touch grainy when pushed too damn hard, as is my wont. Also, as the amp employed cathode biasing, it tended to run fairly hot, and while I never really wanted for current, clearly the M-120 was dissipating a fair amount of power in the form of heat, which could of course take its toll on the life of output tubes (in the case of the M-120, 6550s). And finally, while I loved the straightforward form and functionality of the M-120's, in terms of user-access, I used to chide Mark O'Brien about his penchant for hiding certain devices and functions inside his chassis like a church mouse sequestering kernels of corn in some obscure corner of a barn—such as the M-120's internal triode/Ultralinear switches which one could access only by removing one of front-mounted power tubes and reaching inside to access a mini-toggle switch.

Despite this detail being part and parcel of my text, Stereophile editor John Atkinson proceeded to undertake a series of technical measurements based upon the M-120 running in triode. An honest faux pas, but as a consequence of this oversight, when the Recommended Components issue came out, the blurb suggested that Rogue had overstated the output power of the amp at 120 watts, when in fact it measured more like 100 watts per side. However, that figure of 100 watts per side referenced the amp's triode output as John Atkinson had measured it, an output which Rogue had conservatively rated at 60 watts per side; the quoted figure of 120 watts per side was for its Ultralinear mode of operation, which John never listened to nor referenced for his measurements (and which one might thenceforth conclude to be more powerful than Rogue's conservative claim of 120 watts).

All of these issues were subsequently addressed in the development of the Rogue Zeus, and while the M-120's successor the M-150 shares the basic look and configuration of its forbearer—such as the steel chassis and the thick 3/8" aluminum faceplate with its heavy, centrally located, aluminum eye-in-the-sky button functioning as a slow-start/turn-on mechanism—the M-150's basic topology and circuitry differs substantially from the M-120.

"The output section is entirely different," O'Brien states emphatically. "When you have cathode biasing it's somewhat simpler, at least from a circuit standpoint, but there are quite a few more components in the M-150. Also, we have that built-in meter in the back of the amp which is a nice feature, where any 10 year old could learn to set the bias correctly. Among the other significant things that we did differently from our competitors is that each of the output tubes is individually balanced. I always wonder how you can get away with biasing one pair or one phase of tubes, because you're winding up with an average current being divided by the tube. And so, it really doesn't mean a lot if one tube is current-hogging, for instance. So with the M-150 amplifiers as you bias each tube separately, tube-matching is a whole lot less critical. If one tube fails you don't need to change them as a set, and you know that each tube is running spot on.

"In addition, we still employ mu-follower technology in most of our amplifiers for inter-stage coupling: to get a really nice signal transfer from the input section to the output section with a good amount of gain and low distortion; it's a very nice intermediate stage—a driver stage—in an amplifier. We also went to a balanced input section, so that the amplifier could be run in balanced mode. And we use a real fancy little transformer, to transformer-couple the input, which does a couple of interesting things: it's a nice way to have a balanced input where you can actually run the single-ended/RCA mode across the same transformer, and it really lowers the opportunity for any sort of a ground-loop, because your ground is now isolated."

Another benefit of this circuitry is that it also allows users with pronounced tube-rolling proclivities to indulge their Jones to the max, once you loosen the four screws which affix a protective faceplate to the set of individual mini-toggle switches just below the bias meter, which disengage each individual tube from the circuit, allowing you to employ a mini-screw driver to adjust the tube bias for each individual potentiometer to 40mA, and then switch them back into operation. Audibly, to this listener, the switch from 6550s in the M-120 to KT88s in the M-150 seemed to make for a sweeter, more linear sound and a more stable, efficient operation. O'Brien cautioned me not to over-generalize.

"Yeah, but you can run 6550s, or KT88s or KT90s, or EL34s in the M-150 and it's not as huge a difference as you would think, although the EL34s do make for quite a different sound. EL34s sound great in there, but if we designed the transformer exclusively around the EL34 tube it would have slightly different parametric values; but they're pretty close, so that the EL34s work quite nicely. As for the move from the 6550 to the KT88? I view them as essentially identical in terms of many characteristics, though I guess it comes down to a sonic flavor more than anything else. The KT88 to my ears sounds a hair more lush than the 6550s but maybe less so than some other people's 6550s. So to me it comes down to reliability and sonics, which is why we really like the Electro-Harmonix KT88s. Now there are KT88s out there that are a couple of hundred dollars a pair, and obviously we can't put those in a $4000 amplifier, and I've heard some, and frankly I'm not sure we would even want to if they were the same price. Likewise, there are several good 6550s out there that will work splendidly with the M-150 and several other brands of KT88s as well. Still, we really like the KT88, so that's what it ships with, but I don't begrudge anyone using 6550s."

And so when I first began using the new Rogue M-150 monoblocks, the most telling of my first impressions was how much cooler these new amps ran than the M-120s, and as a result, how well they held their bias and how stable and rock-solid the long-term operation of the amps were—and how the M-150's performance parameters had much more in common with Rogue's flagship, no-compromise Zeus power amp, than the M-120s. Ironically enough, given my earlier sense of their solid-state attributes, the M-120s performance seemed more palpably tube-like by comparison with the more powerful, more linear new generation of M-150 amplifiers.

"That's an interesting observation, Chip, and I wouldn't disagree with you," Mark O'Brien reflected during our phone interview, "and I don't know why that is, frankly. The distortion figures for both amps measure quite well and are well below what we understand to be audibility. So why one sounds a little bit more lush and tube-like, I don't know. What I do know is that the M-150 is definitely a more transparent, dynamic, powerful amp. 

"Don't get me wrong, there are an awful lot of happy M-120 owners out there," O'Brien points out. "And if I bought last year's Corvette, I would be surprised if it weren't in some ways better than the one I owned. So it's an evolutionary process, and frankly you come out with quite different designs every once in a while, and that's what happened here.

"One of the few things the M-150 amplifiers have in common with the old M-120 monoblocks is the form factor. It's a nice convenient size, shape and weight to fit into many people's systems, yet it's large enough to accommodate a really beefy power supply and a bunch of tubes and all the stuff you need for a big amplifier. And that's really all they have in common, even though they look really similar. So we came from that direction, adapted the Zeus circuitry—cut it in half, because of course the Zeus is a dual-mono stereo chassis—and scaled it back slightly less in terms of complexity than in terms of power so it could fit on the M-150 chassis. One of the things we found with the M-120s is that when it comes to power supplies—with the caveat that this is all within reason and scaled accordingly—bigger is generally better. So we basically took the M-120 power supplies, and doubled them.

"The M-120 amplifiers were a really nice sounding, nice looking and generally nice performing amplifier to own with a couple of exceptions ...we became less enamored with the cathode biasing system over the years for a number of reasons: you can't get the same kind of power output for a given amplifier with cathode bias, and while variable bias takes a little bit more tweaking, if you design the amp properly, as I think you found, you don't need to bias the amp all that often. You wind up with an amp that runs at a little more efficiently so it puts less of a strain on the output tubes, and with a lower noise floor, which was one of the things we wanted to do when we set out to design our new amplifiers—make them quieter, more powerful and more reliable. So we went away from the cathode biasing with the Zeus, and never looked back. And that Zeus design, in fact, trickled all the way down from the M-150 to our smaller Atlas amplifiers."

Surely for those with space constraints and those lacking a readily available forklift, the form and function of the M-150 is a welcome trickle down from the massive Zeus, weighing in at roughly 60 pounds per monoblock, and so scaled as to easily come to rest astride your average amp stand, whereas the Zeus tips the scales at 200 pounds, and while that degree of authority is plainly apparent in any side-by-side bake-off, in the end it's a question of how big is your room, how sensitive are your loudspeakers, and how much amp do you really need. And the answer is that for most standard-issue humanoids, the Rogue M-150 monoblocks will more than rock your world.

"It's scaled back slightly, sure, so that one person could pick it up and move it...60 pounds per side is still formidable, but it isn't 200 pounds. The Zeus is a beautiful piece of gear, but it isn't for everybody. The M-150 amplifiers were intended to be a set of amplifiers that people can integrate into a very reasonable system both in terms of price and the physical extent of the system. Twenty years ago people didn't think about an audio system of such scale that it essentially took over the room. And people still would like a great sounding system that isn't comprised of two hundred wires with these immense amps in the middle of the floor. The M-150 amplifiers are a nice reasonably sized set of amps that can either be put side-by-side on a shelf or tucked in between the loudspeakers without being too obtrusive.

"I mean, there is a reason we make a more expensive amp," O'Brien explains in drawing comparisons between the relative power output of the M-150 amplifiers and the mighty Zeus. "They are both quite similar sonically, but you get a weightier more room filling bottom end with the Zeus—still you'd need some much better speakers to hear that difference. Through a set of bookshelves they should sound pretty much identical. The power supplies in the M-150 amplifiers are scaled back considerably from what's on the Zeus, so while they have lots of headroom, they aren't capable of the same kinds of current swings and 1200 watt transients the Zeus can deliver. There's plenty of headroom in the M-150 amplifiers, but still, you can hear the difference in the Zeus when you really, really crank it up, because some of those low frequency transients can suck up tremendous amounts of power.

"Still, I haven't heard anyone complaining that the M-150 amplifiers are lacking for dynamic reserves of power. If you add the two monoblocks together I don't think you'll find too many 120 pound tube amplifiers for $4000 that have the kind of ability the M-150 amplifiers have. As for whether or not the M-150 amplifiers are amplifier enough for someone's system, I think the efficiency of the speakers has more to do with that than anything else. I mean if you are running out of power with 100 watts triode or 150 watts Ultralinear, you are either in an awfully big room or you have some particularly low response loudspeakers. I mean being in your room, Chip, we've played some incredibly loud music without experiencing any audible distortions."

Listening Notes

So what else do we need delineate technically about the Rogue M-150 amplifiers before explicating some of our listening experiences?

The Rogues employ four KT88 output tubes per side, with a single 12AX7 functioning as the input tube and a pair of 12AU7 employed as driver tubes. This is where the use of mu-followers comes into play, or as O'Brien explained to me some time back: "It's a way of taking two tubes—or two halves of a dual-triode, in our case—and using one half as a constant current source. Because the current can't change, the voltage is amplified. We employ two 12AU7s as driver tubes, whereas most driver circuits employ only one. This allows the tube to function in a very linear portion of its operating range, which means that in bumping up a fairly small signal to 10V, 20V, or more, you're driving the output stage without adding any significant distortions. The other benefit is that mu-followers really excel at rejecting any power-supply noises."

Elsewhere on the back panel are two pairs of beautifully machined, gold-plated 4 ohm/8 ohm speaker taps, a choice of RCA or XLR inputs, a switch which allows users to toggle between triode and Ultralinear modes, a ground lift switch, a 15-amp power inlet and the main fuse.

Straight away, having spent the better part of my audio life in the single-ended mode, I was happily transported to a new level of resolution by being able to employ the VTL 5.5 pre-amp's fully balanced output stage for the first time, with the M-150's balanced input section. And while audiophiles possessing more extensive experience with balanced gear will not be surprised by this observation, when I hooked up a long run of JPS Labs Superconductor 3 balanced interconnects, what was already excellent synergy between the M-150 and the 5.5 preamp just bowled me over. The entire presentation was fuller and more expansive, with more clean gain, greater detail and an even bigger soundstage. O'Brien demurred when I made note of this palpable enhancement to him.

"I don't doubt that at all, but again, the input for the RCA's being essentially the same as the XLR inputs, I think that is more likely to be a product of a difference at the preamp end than at the amplifier. It's just that our RCA stuff goes through the exact same circuit as the balanced, which is probably not the case for the preamp—where you're probably picking up another +3 dB of gain with even lower distortion and a better signal to noise figures."

Of course, without the option of balanced XLR inputs, the option of a fully balanced output stage on the VTL 5.5 would be irrelevant, and I'd never have discovered how an excellent sounding preamp could sound even better.

"Well, that is absolutely true and a great point in general. In configuring the M-150, we didn't want to leave out people who wanted to run balanced, and as many pre-amps do operate better in balanced, we wanted to be able to accommodate them."

Furthermore, the ability to switch on the go (without turning off the amp) between triode and Ultralinear operation, is another appealing aspect of the M-150, allowing end-users the sonic flexibility to configure the output of these amps to best suit your speaker and room requirements, or simply to partake of a more appropriate sonic match for the musical characteristics of analog and digital sources. O'Brien sums up the nature of triode and Ultralinear operation, and his reasoning for employing them thusly:

"Ultralinear represents a different way the output transformer is connected to the output section of the tube circuit. There are basically three ways to hook up the output section: one is pure tetrode or pentode mode, which gives you the most power but also the highest level of distortion; then there is triode mode, which gives you the lowest distortion but affords you only about half the power of pentode or tetrode. Ultralinear is a way of hooking up the output transformer so there are some taps on the primary side that get hooked into the tube circuitry that allows you to get pretty close to the tetrode power output, but with almost the same low distortion levels as triode. To me, when I hear amps that are designed as either tetrode or triode and you can switch in between, while the triode may sound very good, the tetrode always howls a little at me. So for us at Rogue, we don't favor tetrode: we like to go with Ultralinear output sections that are switchable with pure triode. To me the differences between those two modes are fairly subtle and different kinds of music or different kinds of speaker systems may sound better in one mode than another, which is why we allow users to switch between them."

When I first began playing with the Rogue M-150 amplifiers, I spent a good deal of time eye-balling the relative attributes and trade-offs between these different output sections, both of which are exceptionally dynamic and musical, and conservatively rated at 100 watts in triode and 150 watts in Ultralinear.

There is something about the trio recordings, particularly when employing guitar, bass and drums, that I love to immerse myself in, especially given both the level of musical interplay and the amount of ambient space and instrumental detail such a minimalist format affords listeners. And so, employing my favorite reference CD recording of the moment, the eponymous collaboration between Bill Frisell, Ron Carter, Paul Motian on Nonesuch, I went back and forth between triode and Ultralinear countless times to zero in on their relative merits.

First off, I was pretty nonplussed by how much power and drive the pure triode output section had, and I seldom found myself wanting for power in the sense of volume or sheer drive—the differences were just as often marked by a different sense of presence and perspective in triode as compared to the enhanced wham-bam-thank you slam of Ultralinear.

The triode mode seemed to reveal more harmonic information about Ron Carter's bass; the overtone series and the nuances of his touch seemed a little more fleshed out, a tad richer, while in Ultralinear, the leading edge of his transients, the snap, crackle and pop of his attack was more palpable and a touch more forward in the mix, as if we had moved from our table in the back of the room, and snuck up to cop a seat right astride the bandstand.

Listening to a voluptuously spectral performance of the old mountain song "Pretty Polly," I suppose the triode mode was more textured, more airy and transparent, sweetly extended on top with a nice sparkle, smoothly detailed but not at all bright—in a word musical. In triode mode the M-150 throws a very deep, expansive soundstage, with pinpoint image specificity and illumination, all neatly centered and focused inside the loudspeakers. Switching over to Ultralinear, there seemed to be a little less flow to the performance, but significantly more palpable dynamics and a greater sense of presence (that is to say, immediacy); and while not quite as textured, transients seemed to have bit more drive and pep, and rhythmic elements in the music were more pronounced. I also took note of how the soundstage seemed to extend beyond the borders of the loudspeakers, a larger, more airy presentation, if not quite as transparent.

Referencing another famous trio recording, Jimi Hendrix's A Band Of Gypsies (Capitol), the audience perspective is more like first balcony, and you have not only a literal sense of the acoustic venue but of the immense amplified sound coming up off the stage let alone through the Fillmore East PA system. And while triode proved very satisfying, the switch to Ultralinear delivered a sense of physical impact and aliveness that had me shaking my head in disbelief, as the Dynaudio Confidence C1 loudspeakers seemed to disappear and I was immersed in a you-are-there presentation of the event. Not only did the lateral dimensions of Hendrix's sound seem to extend in an all-encompassing manner throughout the lateral domain, but the late Buddy Miles' kick and snare had a most believable thump and crack while there the fleshy, chakra-massaging projection and immediacy of Billy Cox's enormous Fender Jazz Bass was rendered with a most convincing sense of scale. The triode mode, comparatively speaking, was more laid back, but Cox's bass still projected like a velvet fist. In either mode the M-150 amplifiers conferred a low–end authority to the diminutive Confidence C1 that precluded any fantasies I might have harbored about investing in sub-woofers and crossovers. The way everything coupled to the room it was like, "Hey, who needs it—me happy." It's alive, alive I tell you.

So, which mode did I prefer? Well, I liked them both, though I must confess that for most of my listening, triode had more than enough drive, with hypnotic layers of midrange texture, harmonic detail and dimensionality that communicated a more realistic, nuanced, intimate presentation of the music. However, there were plenty of times, dear listener, when I went through a long stretch of music, thinking, "Well, let me switch up from triode to Ultralinear," only to discover that I was already in Ultralinear, which speaks volumes about what O'Brien characterized as the subtle differences in the two output modes. Again, I would sum up the differences in presentation as matters of scale, thus, if I was listening to acoustic jazz combos, string quartets, solo instrumentalists or more intimate, sparely orchestrated vocal recordings, triode filled the bill. If I was listening to heavy rock or funk, a more processed, kick-ass style of music, or larger ensembles, such as a Buddy Rich big band or a Mahler Symphony—where the music would benefit from wider dynamic swings and more rhythmic authority—well then, more Ultralinear please.

However, in either mode, there was terrific bass control, exceptionally accurate imaging, a naturally engaging tonal balance and a top-to-bottom linearity throughout the frequency ranges that proved richly detailed without being analytical—to a thrilling degree on solo piano recordings, such as the Zenph Re-Performance, Gould 1955 Goldberg Variations (Sony Classical). Re-performance? Que es esta, kimosabe? Well, audio engineers took pianist Glenn Gould's historic studio performance of Bach from 1955, which was rendered in glorious Mono, and converted it into MIDI-files, thus encoding each note, phrase, dynamic nuance and manipulation of the pedals, then re-recorded it live in Gould's own recording studio on a specially modified piano designed to translate all of this software voodoo into a convincing live performance in a super acoustic space, recorded direct to DSD as both a Stereo Surround version and a Binaural Stereo version—mastered in a Hybrid SACD format.

Well, the Zenph Re-Performance of Gould is a remarkable post-modern technical achievement that functions as an engrossing artistic statement. And the manner in which the Rogue M-150 (running JPS Labs' uncompromising Aluminata AC cords) completed the signal chain of a McCormack UDP-1 Deluxe (outfitted with an Acoustic Zen Absolute AC cord and a one meter run of JPS Labs Aluminata interconnects), the VTL 5.5 preamp (also outfitted with the Acoustic Zen Absolute AC cord, and a seven meter run of JPS Labs Superconductor 3 balanced interconnects) and the Dynaudio Confidence C1 Mini-Monitors (hooked up to M-150 amplifiers with JPS Labs Superconductor 3 Speaker Cable) was just magical.

Without discounting individual contributions from each aspect of the signal chain, the Rogue M-150 distinguished itself by rendering a realistic portrait of an acoustic piano with unerring clarity, dynamic veracity and image specificity, beautifully balanced and even from top to bottom, with no untoward frequency dips or peaks. Nor did one ever have a sense of the tubes fluffing up or sweetening any aspect of the presentation; instead—as I have often noticed with the M-150—there was an entrancing richness and luminosity to the midrange that revealed layers of overtones and inner details, without short-changing the immediacy of the initial attack, or losing the forest for the trees. That is to say, all at once this listener felt (here's that word again) immersed in all manner of small acoustic nuances and the very interplay of instrument and room, while preserving the greater organic architecture of the performance as a whole. Shorter and more simply, it looked like a piano, it sounded like a piano, it felt like a piano, and the end results were music. The synergy between the M-150 monoblocks and the Confidence C1 loudspeakers was breathtaking, and I never felt wanting for dynamic reserves of power: in listening to the Gould recording I never felt as though I was listening to a sound system, but a musical performance. Praise the Lord.

And not surprisingly, any amp that can render the tonal complexity of an acoustic piano convincingly without colorations or loss of detail, is going to do justice to vocals, and I can truly say that on recording after recording I never felt as though the amp was getting in the way or stamping any vocal performances with any particular sonic signature. As such, the Rogue M-150's authentic foundation in bass (not as boom or as woof or as bloat), laid the groundwork for this listener to truly experience a sense of the varied component parts in a vocal performance (diaphragm, chest, throat, larynx, nose, teeth, tongue and mouth) without necessarily causing you to fixate on any one aspect—even though one could readily zero in on which technical element was more prominent in the singer's delivery.

The great pianist Herbie Hancock's Grammy Award Winning River: The Joni Letters (Verve) is a case in point. A far hotter, higher gain, close-miked acoustic presentation than the aforementioned Zenph Re-Performance, the piano is recorded very full and up inside the strings, the better to focus the listener on Herbie's gloriously nuanced touch, the rich play of overtones and inner voicings coming up off of the soundboard. The Rogue's ability to project a vocal while fleshing out all the related instrumental details without everything kind of bleeding together into tube-mush is among its most notable attributes. I love how on "Court and Spark" Nora Jones' eggshell floats so free and unencumbered, projected out front and center all intact and textured like a good amp should; then there is the joy of contemplating the contrasting qualities of Wayne Shorter's chesty soprano saxophone with the throaty, slightly nasal projection of Nora's vocal in the same relative instrumental range—both rendered clearly and distinctly. Likewise, on "Tea Leaf Prophecy" Joni Mitchell herself makes an appearance, and onc can discern how much more the diaphragm and mouth are in play to produce her smoky, mezzo-soprano delivery, while on "Amelia" the more fully-formed, full-bodied components of Luciana Souza's technique are there for all to hear, just as plain as day. And how about Leonard Cohen's subterranean intonations on "The Jungle Line," emanating from somewhere down below the chest cavity, and deep within the Earth's molten iron core, as Herbie's ritualistic bass patterns invoke something ancient and ineffable and timeless—and talk about bass...turn me on dead men. Again, the M-150 imposes nothing and reveals all, getting out of the way and allowing the performance to shine through—with a distinctive musical character, that is utterly unobtrusive in a manner which both bespeaks yet belies its vacuum tube pedigree.

Conclusions

Okay, so I'm an unabashed enthusiast, I admit it—I think these are perfectly splendid amps...an exceptional value at any price, let alone one so modest by audiophile community standards. Over time, I've found little to quibble about with the Rogue M-150 Monoblocks' sound, build quality or stability, and believe you me it was not for lack of trying.

After their initial set-up and bias, just to be on the finicky side, I biased them at two week intervals. Initially I found that of the four tubes in the right channel one had drifted down about 10mA, while the other had fallen around 5mA; in the left channel a few of the tubes has drifted down less dramatically. Two weeks later I found that the same two tubes in the right channel had fallen off a touch, while there was negligible drift in the left channel.

From that point on, I pushed continuous musical signals for weeks at a time, and then let them lay dormant for long stretches. I generally pushed them pretty hard, though seldom in my auditions did I ever feel as though I made them break a sweat, let alone exceed Vlad The Impaler limits. I can recall maybe one time, when I was operating under the influence of a fine red wine from Tuscany, that I tossed on Italian percussionist Jonathan Faralli's DVD-V audio disc of Percussion XX (Arts Music) to reference one of my demolition derby specialties, the "Canto" from Elliot Carter's "Eight Pieces for Tympani." This 24/96 recording, done live to optical disc, has an enormous dynamic range, and the leading edge of the opening transient is enough to cave in your chest. This one time I think I had the preamp floored at around midnight, and I thought I discerned just the slightest touch of break-up, like stepping on an egg-shell, but I was being especially brutal...I got up, backed down on the volume to around 11:30, and the Rogue tracked the transient perfectly, with a most convincing attack and decay...nor did it short-change the more subtle ambient elements for handling that big current swing.

Anyway, after several months of such treatment, at the conclusion of one particularly extended weekend of continuous music, the unit having been on light playback for a week, it occurred to me that it'd been a long time since I biased the tubes. Well, imagine my surprise when I discovered that all the tubes had held their bias, save for one in the left channel, which had drifted down just a hair.

I was pretty shocked. Nor did I have cause to replace any of the output tubes over the course of several months. I came to the conclusion that the Rogue M-150 Monoblocks when configured with the stock KT88s were damn near bullet-proof. The M150 never seemed to get too damn hot, didn't eat tubes, ran stable and glitch-free, and reproduced every variety of music with a sense of ease, power and elegance—highly resolved, with every level of detail, coherent and fleshed out.  

Are they the best tube amps I've ever heard? Well ...I'm a rabid enthusiast but that would be a bit much. To my ears, they are superbly musical and as the heart of my reference system, left me wanting for almost nothing. Generally running in triode, and paired with my Dynaudio Confidence C1 mini-monitors (employing a McCormack UDP-1 Deluxe as my primary source), I seldom found myself thinking longingly about sub-woofers or more power, and the C1 are loudspeakers that like some current behind them: the totality of the experience, the realism of images, the sense of dynamic immediacy and of a believable acoustic space, the swelter of accurate harmonic detail and the manner in which the entire system seemed to melt away was so convincing that there were times I found myself shouting out loud. I mean, hot damn, this may not have been absolute perfection, but it was close enough for jazz—and all I needed, thank you very much.

Okay, let's return to Planet Earth for a second so that I might clarify things: while the build quality and performance of the 60 pound per side Rogue M-150 Monoblocks are in no way, shape or form compromised, they were not conceived as purely no-compromise instruments, but as scaled down versions of no-compromise amps. The 200 pound Rogue Zeus, with its immense, vibration-free chassis, tank like construction, massive power supplies and output transformers clearly has more robust authority and is capable of more dramatic current swings. I've heard the Rogue Zeus a number of times paired with full range, floor-standing loudspeakers such as the Meadowlark Blue Heron 2 and the Vandersteen 5A, and the top-to-bottom bandwidth, the immensity of the soundstage, the sheer speed and power of that bottom octave brought a tear to my eye. Double your pleasure, double your fun right...well, reality check: the Rogue Zeus sells for double what the M-150 does, much as the big Vandersteen 5A sell for more than twice what the diminutive Confidence C1's do—hey, you get what you pay for (well, this being high end audio, make that sometimes you get what you pay for).

Just to let you know that I am drawing upon a good frame of reference in adjudging the Rogue M150 monoblocks, there are a number of world-class, high-power (and relatively high-powered) tube amps out there by the likes of Balanced Audio Technology, VTL, Manley, Audio Research, Cary, Conrad Johnson, McIntosh, Nagra, Lamm, Luxman and VAC that have made my skin tingle when I got to audition them at hi-fi shows or in friends' homes, not to mention any number of balls-to-the-walls solid state amps that also rock my world. In terms of cutting edge technology, well...sometime before I die I hope to enjoy a conjugal visit with the Balanced Audio Technology VK-150SE Monoblocks (150 watts per side, single-ended triode amps on steroids) and hearing the 800 watt flagship VTL Siegfried Reference Series Monoblocks at a couple of high end shows and dealers, I just about lost my mind (hearing their literal antecedents, the 1250 watt VTL Wotans at my good friend Jonathan Scull's crib I did in fact lose my mind ...damn you, Scull). Nor are we immune to the charms of single-ended triode amps, such as the amazing Art Audio Carissa or a rousing 300B push-pull ball-wiper such as the Canary CA 160 Monoblocks, both of which I got to hear at John Potis' crib.

But for this audio writer, given my taste for more traditional dynamic loudspeakers, and my experiences as a drummer, I tend to gravitate towards a more visceral power source to communicate the immediacy of music and the power of real bass transients as I experience them in the live domain. And yet I also crave the kind of textured midrange sweetness and high frequency detail that good tube circuitry can deliver with such sublime intimacy.

These twin qualities of power and intimacy have always been manifest in Rogue amplifiers, pre-amps and integrated amps, and for me, dollar-for-dollar, pound-for-pound and watt-for-watt, the Rogue M150s offer end-users as profound a combination of tube refinement and real-world power as I've ever heard at their price point, with performance and reliability parameters that can stand favorably with the best tube amps in the world.

What final caveats might I offer? Hey, if you can afford more amp, if you have the room for more amp, if you have the kind of speakers which require more amp, then son...you need more amp. And there are some hellacious models out there for you to choose from, including the Rogue Zeus. However, for most carbon-based life forms, with modestly configured rooms such as mine (14' x 20" x 10'), the Rogue M-150 Monoblocks will more than satisfy your nutritional requirements for volume and resolution, and keep you grinning from ear to ear for years to come.

But then there is the issue of, for want of better word, I shall fabricate one ...tubeiness. Not tubby, but tubey. Some folks I know favor a much more pronounced tube tonality, a lusher, richer midrange with a more romantic perspective; hell, there are many folks who as a matter of religious dogma, flat-out reject the notion that high powered tube amps can ever hope to sound as good as low-powered tube amps. In some cases they favor exceptionally sensitive loudspeakers, such as horns. I have certainly enjoyed my encounters with such gear, but my tastes run in contrary directions. Now according to Mark O'Brien, the Rogue M-150 amplifiers will happily accommodate EL34 output tubes, so they might very well deliver a more luscious presentation, but you'll have to consult another end-user, because I was so happy with the manner in which the stock KT-88 output tubes sounded and performed, that I took a very contented cow attitude: if it ain't broke, don't fix it.

I would not characterize the sound of the Rogue M-150 outfitted with stock KT88s as exceptionally tubey, but rather as exceptionally musical. Their nuanced depth of detail and resolution is uncommonly smooth, natural, frequency extended, balanced and linear. I have spoken of the M-150 amplifiers' top end as silky, but I have also spoken of its sparkle—and yet it is in no way bright. And of course, the Rogue's muscular low frequency extension belies the old wives tales about how tube amps don't deliver a believable low end—the M-150 amplifiers not only convey the weight and dynamic pop, but the speed and focus of real bass—what's more, their low end performance is the foundation for an immense sound stage. In a sense, these qualities bespeak a degree of control that we more readily associate with the best in solid-state.

So would a tube fanatic not named Chip Stern really gravitate to the Rogue M-150 amplifiers? Well, in the absence of the Rogues, I got to listen at some length to all of my favorite reference recordings with the exact same system, only with other amps in their stead, such as my venerable Mesa Tigris, a low-powered pure Class A tube integrated amp (28 watts in 2/3 pentode [2 x 6V6]-1/3 triode [4 x EL84] mode with like 8dBs of negative feedback). And in preparation for another Positive Feedback review, I've been doing a lot of listening of late to the Bel Canto Design e.One REF1000 Digital Amplifiers. Now we don't want to get into a Pillsbury bake-off here, let's just say that the REF1000 amplifiers are in class of their own, much as the Rogue M-150's are, but that they couldn't be less alike (save that these monoblocks both retail for $4000 a pair ...oh the shame of it, wallowing in such "modestly priced" equipment). We are still just getting to know the REF1000 amplifiers, so lets just say leave it by saying they do some things differently and some things better and some things not quite as well (by my lights) than the Rogues, and which you prefer is more a matter of personal taste than overall quality ...both rock.

However, one thing about the REF1000 that John Potis has hipped me to, and which I am still getting used to, is the quality of the midrange, which is exceptionally warm and detailed. Tube-like? Oh, gee, let me work on that for a while, if you will, but let's just say that this ain't your standard issue solid-state amp, but something quite remarkable and unique ...and addictive.

Why then do I reference the Bel Cantos in explicating my final, definitive feelings about the Rogue M-150? Because in journeying through my core collection of reference audition discs, I began to notice things about what I was hearing in the midrange which brought my experiences of the Rogue M-150 rushing back in vivid detail. As is often the case in high end audio, we tend to hear things more plainly in their absence than in their presence.

So what I am trying to convey, and I am speaking directly to those of you who might believe the M-150 amplifiers are not tubey enough for your tastes, is that having lived with the Rogues for so long, and having just spent a short period of time with an amp that is its literal opposite, I recognize anew those qualities in the Rogue's depiction of the midrange—such as the layering, texture, depth of field, pinpoint resolution, image specificity, and incredible detail—that are unmistakably, irrefutably tube-like.

Well, duh, it is a tube amp.

Yeah, yeah, yeah, I guess it's just that I've heard so many people characterize the KT88s as not sounding particularly tubey ...and boy is that a load of crap. No the Rogue M-150 ain't a triode tube amp where everything runs warmly together like caramelized onions, but rather is like a freshly harvested Vidalia sweet onion, all of the separate elements crisply, sharply, sweetly, piquantly defined and distinct...yet with an organic balance and coherence that never calls undue attention to itself. The Rogue M-150's midrange is accurate and revealing, with a luminosity and a luster, and an endlessly expansive soundstage that I've found myself missing every day since they've been gone.

Oh, well, we soldier on. In conclusion, while I can readily summon up vivid images of several fantastic amps with distinct performance parameters and sound signatures that I find most fulfilling—maybe even more fulfilling—I cannot for the life of me imagine, nor have I ever heard, any tube amps at the Rogue's price point with more drive and nuance, which are easier to listen to, or possess a more pleasing, musical tonality and coherent presentation than the M-150 amplifiers—they are simply glorious. And even after a modest price increase for 2008, they still retail for "only" $4495. Not expensive enough for some august members of our little congregation, to be sure, but a little slice of heaven for the rest of us.  Amen. Chip Stern

M-150
Retail: $4495

Rogue Audio
web address: www.rogueaudio.com

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