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Reviewer: Srajan Ebaen
Source: Zanden Audio Model 2000P/5000S; Opera Audio Reference 2.2 Linear
Preamp/Integrated: ModWright SWL 9.0SE; Music First Audio Passive Magnetic; Bel Canto Design PRe3; Wyetech Labs Jade

EQ: Rane PEQ55 active merely below 40Hz
Amp: 2 x Audiosector Patek SE; Yamamoto A-08S; Canary Audio CA-308s; FirstWatt F3 & F1; Bel Canto e.One S300; Eastern Electric M-520
Headphones: AKG K-1000 w. hard-wired Stefan AudioArt harness; audio-technica W-1000
Speakers: Zu Cable Definition Pro; Anthony Gallo Acoustics Ref 3.1

Cables: Zanden Audio proprietary I²S cable, Zu Cable Varial, Gede, Libtech and Ibis; Stealth Audio Cable Indra, MetaCarbon & NanoFiber [on loan]; SilverFi interconnects; Crystal Cable Reference power cords; double cryo'd Acrolink with Furutech UK plug between wall and transformer
Stands: 2 x Grand Prix Audio Monaco Modular four-tier
Powerline conditioning: 2 x Walker Audio Velocitor S fed from custom AudioSector 1.5KV Plitron step-down transformer with balanced power output option
Sundry accessories: GPA Formula Carbon/Kevlar shelf for transport; GPA Apex footers underneath stand, DAC and amp; Walker Audio Extreme SST on all connections; Walker Audio Vivid CD cleaner; Furutech RD-2 CD demagnetizer
Room size: 16' w x 21' d x 9' h in short-wall setup, with openly adjoining 15' x 35' living room

Review Component Retail: $18,995/pr

The ultimate amp?
B.K. Butler's Monad is a nomad. It goes solitary places and ascends to where none have dared climb before: a nearly anything-goes power plateau of 100/200 stable 8/4-ohm watz. Single-ended zero NFB Class A watts to be precise. From a lone 300B. I know, I know. That spec is way off the plausible charts. It instantly puts tube hounds on alert. Or should. Code nuclear orange. Had B.K. one wee dram -- or five perhaps -- of potent bootstrap brew too many? Or is he a bloody genius? What with a self-professed 18 hours daily in the lab building these (and other Butler amps) not as quickly as he sells 'em, inebriation at the work bench is highly unlikely. Which leaves the genius part.

Monads have been in secret demand. This has terminally clogged the mile-deep review request pipeline. That's a very dandy problem for any boutique amp maker to have. Sell more than you can make, no real advertising required except for -- apparently stellar -- word of mouth. Hence for well more than a year, mum's been the word on the Monad. Just like the famously mysterious smile of the Mona Lisa. With whom the Monad shares the first four letters. And perhaps the genius level of its maker. With the plugged-up review pipe line finally past constipation, B.K.'s IQ can now be cross-examined in the audiophile court. Does his amplifier deliver what it claims?

For my earlier review of B.K.'s TDB 2250 amplifier, I had managed to wrest a kind of white-paperish statement from our reclusive lab rat. To wit, "using a very low
distortion, discrete Class A front end and voltage amplifier of my own design, the single-ended output is DC-coupled into a pair of triodes (both enclosed in the 6SL7 bulb). These triodes are set up for maximum thermionic action which in tandem produces both the necessary drive and output biasing for a pair of complementary push-pull output current multipliers. Their output signals are then passively summed for the speaker output. The tubes are continuously operated at or near electron-emissive saturation which produces the pleasing, slightly non-linear tube overload characteristics ... required for the tube sound. The other condition ... is that the tubes must be interfaced directly with the speakers. Hence, the tubes receive a reflected and proportional DC load as they play into the output current multipliers. [The omitted parts refer to a famous research paper Mr. Butler references. It explores the reasons why tubes sound different from transistors and posits two specific requirements under which tubes come out ahead - Ed.]

"For those who are wondering, the answer is yes. Parts of this description apply to my rare 300B-based Monad monoblock amplifiers. But not all. The A100 is not simply a single-ended version of the 2250's output circuit with a 300B inserted. I prefer to keep a few things under wraps." B.K. then summarized the advantages of his novel circuit as follows:
  • No output transformer
  • Power amplifiers of any practical wattage level can be readily designed with this technology.
  • No excessively high voltages. High quality lower-voltage-rated supporting components may generally be used (resistors, capacitors, diodes etc.)
  • Drastic weight reduction. (Allows for 5 channels x 150w or more in a single chassis as in our 48 lb. TDB-5150 model)
  • No maintenance. No periodic bias adjustments or re-tubing required. The amp may be left on continuously or switched via internal relay as desired by the customer.
  • Low heat production. The thermionic tube auto biasing reduces operating and standby idling temperatures to about the same or lower than solid state amps of similar power ratings.
  • All Tube sound: This proven new vacuum tube technology delivers as promised!"

Too good to be true? For the bridgeable-to-800 TDB 2250, most certainly not. As my review stated, this amplifier does indeed sound very much like a powerful push-pull valve amp. Except that the only valves used are low-level small triodes. That constellation of opposites induces quite the brain freeze in properly jaded 'philes. In short, Butler's tubes get to act like high-maintenance babes. Without the maintenance. Too bad that concept doesn't translate. In the real world. In the audio world, it's now been translated to operate in single-ended, zero negative feedback fashion. With what's arguably the most famous direct-heated triode of them all. While ordinary Bs will work just fine, for best results, it is recommended that one use the 300Bs supplied with the amps. [Below with PipeDreams at CES 2006.]

For parts, there's "Sprague polypropylene capacitors, Vishay-Dale mil-spec 1% resistors; internal circuitry interconnections of the positive military-type screw connector style utilizing #10 hardware. A 700-watt toroidal power transformer. Exceptionally large heat exchange area on the internal heatsink as coupled into the large bulk and mass of the CNC custom chassis for remarkably cool operation." There's a blue-lit dimmer-controlled laser-cut logo as power indicator. There are silver circuit boards and Rhodium-plated silver contacts. Protection goes to the ends of the Yangtze river. It includes input fuse protection; thermistor protection against AC input line transient voltage spikes and surges; soft-start vacuum tube 5VDC warm-up for the 300B tube heater filament; 5 to 8 sec. turn-on and warm-up transient delay circuit that keeps speaker relay open until outputs have stabilized; DC offset monitoring circuit on the speaker outputs; short circuit protection; and an over-temp circuit breaker.

Back in spec land, there's a lowish input sensitivity of 1.4V for full 100-watt/8-ohm output. 47K input impedance. Dimensions of 17" W x 12.5" D x 10.5" H. 46 lbs weight. Worthy then of its very own short sentence so as to punctuate its distinctiveness is the power supply. It rates an absolutely gargantuan 2 Farad (2,000,000 or two million uF) to act as the immovable rock of Gibraltar. Battery-like, it is claimed to be essentially immune to powerline tweaks such as conditioners and fancy power cords. And then there's yet another giveaway to the Monad's unusual stature in the triode jungle. A claimed S/N ratio, A-weighted, of 105dB. Jimminy Cricket, that's CD or preamp territory and not even remotely close to -- never mind within -- the usual 'hood of no-feedback single-ended tube amps.

The ultimate amp? Does that header begin to seem sensibly relevant? On paper, the Monad appears to offer remarkable load invariance (the American Cabasse importer confided that the Monad is the only tube amp he's ever tried that could properly drive and control the big Cabasse models). It promises low-distortion SET sonics yet transistor muscle. It's got enough power to drive all but the most inefficient beastly speakers. Though offering in excess of 10 times its usual power, B.K.'s 300B will never need replacing. Unless you drop something on its purty head and break it. In fact, this 300B isn't "fully turned on" as we usually understand it. There's no B+ voltage applied but instead, some kind of reverse bias. Though how this hybrid/parallel implementation of valve/transistor devices in the output stage really works remains a mystery. Usual hybrids employ tubes in the front, transistors in the back. The Monad puts both tube and transistors in the back. Where the speaker sees and loads them. Without the usual output transformer.

That's another very distinctive trait of this single-ended tube amp. In fact, Butler calls such signal-path iron "hundreds of feet of phase-shifting, musicality-robbing wire and many pounds of induction-generating core iron to overcome". It's of course widely known that the quality of the output transformer is perhaps the greatest variable in valve amps. Nobody disputes that. Yet many would argue that it's precisely the presence of a -- superior -- output transformer that bestows on such tube amps their particular sonics. Certainly McIntosh and Audiopax would concur. After all, both firms use output transformers even for their transistor amps.

Whether demon or deva in the end, the Monad doesn't use an output transformer. That other toroid on deck isn't a transformer at all. It's an aesthetic balancing act hiding the massive capacitor bank. That doesn't rate the Monad a conventional OTL either. Unless a headphone amp, OTLs run endlessly paralleled tube banks to substitute for the speaker impedance matching usually handled by an output transformer. As a direct function thereof, nearly all OTLs run hot as sin. Think tube forest on fire. Not so the Monad. It's a high-power single-ended tube amp with neither transformer coupling nor typical OTL architecture. Instead, it uses B.K.'s "newly patented current-multiplying DC coupling circuitry that allows the 300B output driver tube to directly operate into a fixed ratio-reduced speaker output load. This novel design results in the production of authentic triode tube characteristics many times greater in power than the normal capability of 300B tubes yet retains all their superior musical qualities."

That's of course a deceptively simplistic explanation. It fails rather miserably at explaining why nobody chanced upon this solution a lot earlier. If it were really that simple. Butler's current patent rather suggests the opposite. Remember, most patents for tube audio applications predate the birth years of plenty of our readers. How many undiscovered ways to insert a tube in an audio circuit are there left? To see a 21st century patent awarded in this field is rare then. With neat categorizations eluding us like mad -- what to call the Monad, exactly -- it is some form of unconventional hybrid circuit. In one fell swoop, our nomad seems to eliminate what to many is plain wrong with conventional tube amps: excessive heat; excessive weight; power issues; load issues; distortion issues that arise from load issues; bass issues that arise from impedance and power issues; and pricing that's utterly inconsistent with measured performance and real power.

That latter point is of course fiercely debatable. Approaching twenty-thousand bills, the Monads are blatantly priced as über amps. Part of that cost surely ties to their near fanatical enclosures. Those are milled from 6" thick solid aluminum billet to eliminate any and all visible screw heads, provide mechanical integrity and "immunity from stray RF" and act as unified heat sinks. They purportedly require "hundreds of machine shop hours" to produce. If the Monads not only spec but perform as tube-based über amps, the pricing is in line with current dealer network convention. Though no less prohibitive. Prohibitive in fact is often the operative term when it comes to acquiring anything that's truly exclusive, hand-crafted and of superior quality. As such, each Monad is indeed hand-built by its inventor. Accordingly, it's in short supply. Best think of the Monads as custom creations. They just happen to be in production. They're not complete one-ups as the hot-rodded Cadillac for Billy Gibbons with Tube Driver Blue mobile amps above is.

Does this tie-in with car audio strike a shocked note of discord with your personal HighEnd-worthy überamp-bitions? It shouldn't. There perhaps is no better testing ground for a tube amp designer's real-world mettle than the bump & grind whereby imperfect road conditions and spirited driving rattle the dental fillings in tube amps. Save perhaps for the abuses musicians suffer upon their amplifiers on stage and on the road. Which B.K. has built for some twenty-odd years for quite the list of celebrity guitar wranglers as well. In my book, Butler's credentials then are exactly what you'd hope to see if no-nonsense engineering and reliability were to be in your thermionic future.

To this day, I get the occasional Mesa Baron owner e-mail. Invariably, their amps continue to go strong, a testament to their coliseum genetics. Getting high-end 2-channel love as a guitar or car amp maker is of course another matter altogether. That hitch -- plus protracted hand-holding of indecisive audiophiles and pathetic sales regardless when compared to his professional division -- prompted Mesa's Randall Smith to eventually pull the plug on the legendary Baron and Tigris. B.K. Butler positions his Monads quite differently. First off, they're unapologetically expensive. This might get them the necessary respect. They forgo feedback, switchable modes and use the most audiophile-approved triode rather than lowly pentodes. They're also single-ended, not push/pull. All ultra kosher.

Should one further assume that the Monads will measure more respectably on the bench than the wildly adjustable Baron did in his day? This will be for future reviewers to determine, those who enjoy the attachment of graphs to their subjective findings. Lastly, the Monads are accompanied in the lineup by multi-
channel amps of far lesser cost. Think TDB home series. This opens doors with dealers. It gets the Butler brand into audiophiles' eyes. Finally, if the basic circuit is robust enough to find favor with multi-channel custom installers where reliability, high SPLs and monster bass are hard-core prerequisites, the Monads might also have the perception barrier thing licked. The 100-watt power rating that cleanly doubles into 4 ohms then is mere icing on that cake. (Jerry Kindela's TDB 5150 5-channel Butler amp review for Home Theater Magazine certainly suggests that tubes and monster sound tracks have finally met. The magazine's measurements show "that the TDB 5150's left amplifier channel, with two channels driving 8-ohm loads, reaches 0.1 percent distortion at 219.8 watts and 1 percent distortion at 250.5 watts. Into 4 ohms, the amplifier reaches 0.1 percent distortion at 347.0 watts and 1 percent distortion at 393.7 watts... the signal-to-noise ratio with 2.83 volts driving an 8-ohm load from 10Hz to 24kHz with A weighting was -119.91.") That's spectacular benchmanship for a $3,295 5-channel tube amplifier. It underlines that for once, a valve amp designer's claims correlate closely with independent tests.

If we extrapolate similar accuracy for the Monad specs, we arrive at 1/10th the usual THD figures, suggesting not only a far more powerful than usual 300B amp but also one that's significantly more linear. Which begs a question. If the sonics of traditional zero feedback SET amps are lawfully wedded to a particular harmonic distortion behavior -- that's the perhaps most widely shared opinion about why they sound as they do -- would suppressing this behavior by 90% kill off that peculiar recognizable SET sound? Can you have one without the other?

100 degrees of separation?
While on peculiar, there's another deeply held SET belief. Namely that the pursuit of power invariably misses the real magic of low-power triodes (Wavac perhaps being the exception here). Taken to the extreme, this belief posits that a 100-watt SET is a SET no longer. This is what's most peculiar about the Monads. They don't rely on a rare, ultra high-voltage triode like an 833 to obtain power. They don't parallel ten or more 300Bs in single-ended mode to get there. They do it the time-honored way. By using one 300B. That's traditionally good for about 8 watts of premium go juice. Yet the Monads make a cool 100 watts from the same device. That's 10 times the power from the unbreakable rule. With 1/10th its distortion. Sans distortion-minimizing feedback.

Were I mathematically inclined, I'd feel compelled to say something extravagant: that it's exactly 100 degrees which separate the Monads from conventional 300B amplifiers. To non-engineer mortals, it's all a bit mindboggling and fantastically confounding. Yet anyone who'll check show listings can readily confirm that the speakers mated to Monads at trade events are not the high-efficiency sort SETs usually thrive on. Rather, they're always perfectly normal denizens you'd expect to see strapped to a Jeff Rowland amp. No doubt about it, the Monad sisters rewrite a handful of rules and expectations to enter utter dreamland. Just consider this case evidence even prior to any listening. The least you'd have to give B.K. Butler is that he's done away with convention and created something inescapably, inarguably unique. This has never been done before. To call it monad then isn't perhaps as far-fetched.

About the actual 300Bs supplied, I learned the following: "My 300Bs are made by ShuGuang in China. I request special hi-gain, low microphonic hand-selected units. The WEs are very good too but I found that the SGs have a bit more body overall and are not quite as microphonic. Microphonics are not a major problem with the Monads, however as the tubes run at speaker level, I prefer to optimize that aspect of their performance." Since I had a pair of current production WEs, I could make that exact comparison myself.

Added B.K. in a later correspondance: "I have a rather unique set of performance requirements in the Monad design. When I dial in everything just right, (it's all so esoteric but really an amazing and exciting process), I conjure up a predominant 2nd-order harmonic that is almost sine-wave perfect throughout the entire frequency response range and power bandwidth with virtually zero phase shift. It rises up like a phoenix from the flat-line distortion read-out as I adjust about 5 different parameters at once. I know of no other amp in the world that produces this kind of purity in a desirable musical distortion wave - and only tubes produce this. I can tell when a tube is good by how 'pure' and symmetrical this generated harmonic is. When the 2nd order is right (between .5 and 1% predominant in the THD read-out), the sound becomes heavenly. It took literally hundreds of hours to discover the formula to extract this ideal condition from the 300Bs. They do it better than any other tube I've tried. Hence the decision to make the A100 Monad with a 300B. There's little chance that anyone else will ever discover my little 'trade secret' of 'conjuring up' the 2nd harmonic."

My non-technical mind translated this statement as follows: the 300B has been inserted into the circuit such that one harvests from it purely the desired THD behavior. This leaves drive and power requirements to transistor output devices. The "nearly plate-saturated" tube merely sees the load as a fixed (mirrored) constant to produce the sought-after harmonic distortion response. Otherwise it remains divorced from the flux and strain of the usual drive requirements. Mr. Butler has thus distilled the aroma of the tube. He maintains it in a protected condition. It is unafflicted by the usual nonlinearities enforced by loudspeaker interaction. Plainly then, Butler's 300B operates in an idealized state. This is similar to how actual distillation captures and concentrates the spirit of a substance while it burns off impurities in the process.

Mr. Butler found this statement to closely approximate the truth but added the following to make it more specific: "As I've mentioned before, Mr. Russel O. Hamm gave me some great insight into the philosophy of what makes tube sound in addition to my practical work with pro musicians on stage and in the studio over many years. The advantage that my new patented designs have in general (and the Monad in particular) is that I can use highly regulated power supplies to nail the perfect thermionic sweet spot of the tube and then use the current multiplying potential of modern, ultra linear output devices to do the heavy lifting. This elusive non-linear sweet spot comes and goes in conventional tube amplifier designs depending on many factors including B+ voltage, bias current, loading, power output level and output transformer problems of inductance and phase shift etc. The Monad shines supreme in her ability to create -- in my opinion -- a continuous state of ideal conditions for the sweet spot operation of the 300B independent of the load, frequency or power level." (These parameters are hand-tuned by the designer for each Monad and allow for tube rolling without readjustments.)