Boulder Amplifiers 2150 monoblock power amplifier

"I'm a recording engineer, so I value accuracy," said a panelist in a discussion—titled "How to Read Between the Lines of Audio Advertising"—at last October's Rocky Mountain Audio Fest. I, too, was on the panel, which was moderated by Brent Butterworth, a writer for the SoundStage! Network of online audio magazines.

"Accuracy is overrated," I interjected from the other end of the dais. "Accurate to what? To your sonic tastes? To what you hear on your preferred loudspeakers? Other than one's personal preferences, I'm not sure the term accuracy has much meaning."

I then qualified those statements somewhat, saying that gross tonal colorations—like the ones that afflicted many of the demo systems at RMAF—were unacceptable.

In audio, this debate has always and will always amount to a sonic free-for-all. That's part of both the fun and the frustration of the hobby. I've heard absolutely thrilling sound from a tiny vintage tube amplifier putting out just a few watts to drive EJ Jordan Designs "full-range" speaker arrays, supplemented with carefully dialed-in subwoofers. The action, so to speak, appeared between the speakers, a few feet in front of the single seat, on a miniature soundstage on which appeared the most living, breathing Ella Fitzgerald I'd ever heard. She was, within the constraints of the system, brought to life.

Yes, it was a tiny visitation. Yes, dynamics and visceral bass were MIA. But what was there—able to be enjoyed by only the person seated in that sweetest of sweet spots and by no one else in the room—was, again, thrilling. It became even more so when the system's owner swapped out his "everyday" tubes for a pair of ultra-rare vintage tubes saved for special occasions.

Did that system produce "accuracy"? Had you heard what I heard, you'd have to say "yes"—but accurate to a specific aspect of sound reproduction, to the exclusion of other such factors. I have a few friends who prefer that kind of sound, and who look at me with poorly disguised derision, or the kind of concern usually reserved for someone with a life-threatening illness, when I remind them what my version of accuracy requires: big speakers capable of producing realistic SPLs, wide dynamic swings, big soundstages, and gut-socking bass—all to the exclusion, to one degree or another, of the aspects of sonic realism that they prefer.


Some audiophiles prefer looking into the sound; others want it projected outward toward them. There's room for all at the audio table, but I don't expect to convert a Quad ESL 57 enthusiast into a Wilson or Magico or YG Acoustics fanatic. Nor should the EJ Jordan guy expect me to head his way any time soon. However, understanding each other's preferences in sound can go a long way toward creating a more tolerant world—in audio, at least.

The Boulder Amplifiers 2150 is a large, extremely heavy amplifier that weighs 220 lbs each and costs $99,000/pair. It's an update of the 2050 mono amplifier, introduced by Boulder more than 17 years ago—a long run for any audio product. Boulder's promotional statements almost make it seem as if the update was done grudgingly, more to "meet the demands of the market" than because the original's sound quality could be improved.

The changes, more evolutionary than revolutionary, include the use of Boulder's proprietary 99H2 gain stage—a fully discrete, surface-mount, modular op-amp that, in 2013, replaced their 993 module. The H stands for High voltage.


The differentially balanced 2150 is said to be class-A biased to its full claimed continuous power output—1000W into 8, 4, or 2 ohms—by means of a circuit that continually adjusts the bias current based on voltage output, current draw, and load. Boulder claims that this operation is performed more quickly than the speed of the audio signal itself, which means that the circuit can maintain class-A operation when confronted with a taxing musical transient, after which it gently lowers the bias until it senses another peak (footnote 1). The 2150 can thus provide the positives of full class-A operation that's free of crossover distortion, without the negatives of massive power consumption, excessive generation of heat, or, Boulder claims, the audible "steps" produced by other kinds of active bias-management systems.

Each 2150 has two power supplies. The massive primary supply, for its input and output circuits, includes two toroidal transformers, one for each phase of the output, that are custom-wound in the US to Boulder's specs. A smaller, independent, regulated supply powers the microprocessor control sections. Mains connection is via an enormous 32-amp IEC jack—the kind used in shipyards and factories. On the other end of this cord is a standard AC plug: no need to call the electrician. The 32A IEC jack isn't just for show: the 2150 can draw nearly 30 amps of peak current, for high power outputs into low impedances. In my room, with my sensitive speakers, however, the Boulder was never asked to deliver anything close to that.


The 2150's input stage is a fully balanced, differential, high-impedance, servo-controlled, direct-coupled circuit using bipolar devices. Its 26dB voltage gain stage uses the encapsulated 99H2 op-amps in two stages, each 99H2 providing input buffering and voltage gain with a high slew rate, wide bandwidth, high current output, low distortion, and low output impedance.

Boulder proudly uses feedback where others dare not, claiming that appropriate levels of carefully implemented feedback can achieve ideal operating parameters and constant group delay across the entire audioband, for maximizing linear phase response. In fact, Boulder has a take-no-prisoners approach to feedback, claiming that those who don't understand its proper implementation—and so don't use it—have a philosophy that "still exists in a number of hobbyist workshops today." Ouch!

The 2150's output section has a specified nonreactive output impedance of 0 ohm (!), meaning it can drive any speaker to realistic playback levels, regardless of the speaker's sensitivity or impedance. The output section features 80 (count 'em—80!) bipolar devices, 40 each for the waveform's positive and negative halves. Rather than being screwed in, these output devices are clamped, by a CNC-machined bar, to a nonresonant heatsink cut from an 80-lb billet of solid aluminum. Boulder claims that this increases reliability and reduces mechanical resonances—two things that the company takes very seriously.

Boulder says it avoids standalone heatsink fins, which can resonate and ring. (Speaking of which, if your amp has separate fins, try listening to it, then wrapping the fins in adhesive tape and listening to it again.) Of course, the 2150's protection circuit is as heroically designed and built as everything else about it.


Boulder boldly states that "at no point is the 2150 'voiced' or tuned for a specific sound or type of loudspeaker." At this point, the low-watt, single-ended-triode guys (those few who've read this far) have their hair on fire—all of their belief systems have been challenged, crushed, or at least dented, despite the stiff resistance they've put up getting this far into the review.

Inside and out, the 2150's build quality meets and in most ways surpasses that of every other amplifier I've reviewed. Boulder is one of the few electronics manufacturers that does its own CNC machining in-house. I visited their previous facility, and it was mighty impressive. The new one is said to be even more so. Other than the toroidal transformers and individual components on the boards, Boulder designs, manufactures, and assembles everything in-house, including the circuit boards. All metal parts are fabricated in-house from solid stock—no sheet metal is used anywhere.

Hernia-Inducing Setup, Ball-Breaking Sound
Although Boulder recommends that four people move and install each 2150, it was just two diminutive guys—me and Rich Maez, Boulder's director of sales and marketing. We managed.

I ran the amps balanced. Boulder's wingnut-type speaker terminals are the best I've used, especially since the two pairs per amp are located on an "open field" rear panel—only spade lugs need apply. I began with Boulder's stock power cords. Then, to hear if power cords could affect the sound of an amplifier overbuilt to such demanding specs, I asked AudioQuest if they'd send over two of their 6' Hurricane cords ($1495 each), with Boulder supplying the amplifier-end plugs to match the 2150's 32A IEC jacks.


How does a measurement-derived amplifier sound? Essentially, especially in terms of tonality, it doesn't. That may sound like a reviewer cop-out, but the Boulder 2150 didn't really "sound," tonally. It was about as tonally neutral a piece of electronics as I've heard here, neither warm nor cool—unlike, say, the SAE HP2, which I thought was on the cool side.

Overall, my first listen immediately produced a reaction similar to when I first heard Boulder's 2110 preamplifier, which I reviewed two years ago—but to an even greater degree. The 2150s gripped the woofers of my Wilson Audio Specialties Alexandria XLFs (and, later, the Wilson Alexxes; review in the works) with greater clamping force than did my reference darTZeel NHB-458 monoblocks. I can't say the bass extension went further down or that bass definition increased (the NHB-458s perform really well there). But the punch and speed, the starting and stopping power, were definitely accelerated.

As with that 2110 preamp, everything, from top to bottom, tightened up through the 2150s—another good thing, because now the system spoke from one sonic perspective: with greater speed, transparency, and, to a lesser degree, resolution of detail than with my reference darTZeels.

Footnote 1: Unlike an amplifier with a traditional class-A output stage, the Boulder 2150 runs relatively cool.—John Atkinson
Boulder Amplifiers
255 S. Taylor Avenue
Boulder, CO 80027
(303) 449-8220

wineandwires's picture

When do we get to read your comparison of the AudioQuest Hurricane (high-current) and Shunyata Research Zi-Tron Sigma HC?

supamark's picture

'"I'm a recording engineer, so I value accuracy," said a panelist in a discussion—titled "How to Read Between the Lines of Audio Advertising"'

Was your response to this statement deliberate trolling or are you really that unfamiliar w/ what recording engineers do? He meant, and I can't believe I have to explain this to you, that what comes out of the amp should be the same as what went in, but amplified. Jeez.

It's like your silly trolling of people (some of whom are also trolling) who say they prefer digital to analog. The correct answer is neither is better than the other, they each do different things well and really it's a matter of which distortions one prefers.

Note, I worked as a recording engineer in the early 90's - classical, jazz, rock, and pop; I did it all.

mallred's picture

I've wasted a lot of money in this silly hobby thinking if I moved up to the next level, I'd get better results. Experience and approx. $80k (over the years) I know better... Of course, maybe if I had of pushed up even further to Boulder/etc., I would've finally gotten "there"...

supamark's picture

Not sure what this has to do with what I wrote... but if you can't hear the difference between, say, a well designed Class A amp and a well designed class AB or D amp then the problem may simply be that your hearing (or ability to discern fine grain differences in sound) is not up to snuff. Most wine, to me, tastes like grape juice mixed with vodka but I don't doubt that's not the case with most people who actually like wine (I obviously don't), and an experienced wine taster can glean a lot of information from shades of flavor most of us cant discern.

I spent about 5 years where my job was, essentially, to listen carefully (recording engineer). I could hear the distinct differences between two Steinway 9' grands we had my at first job at UT Austin (the Hamburg had more scintillating high harmonics, the NY had fuller low mids) and a Baldwin 9' grand we had (less high end "air" than either Steinway, more pronounced mids than either - no wonder 70's rock bands liked 'em - better at cutting through a mix). I could tell musicians the exact point where they stopped just playing and started thinking about playing (nothing kills feel like thinking about it). Clear differences between different compressors, different EQ's, different reverbs, etc. Audiophiles like to rag on rock/pop recording engineers, but the truth is we pay almost fetishistic attention to the sound but it's the artist and label (the boss) who determine what the final product sounds like (i.e. hard limited, clipped no dynamic range EQ'd to hell and back ear torture of the "loudness wars").

That's something that I like about Stereophile - many of their reviewers have either played or recorded music professionally (or both) so I know they've done a LOT of listening. Also, unlike, say, TAS, they publish measurements of the gear they listen to.

es347's picture


A. Hourst's picture

“Can you measure "dry"? Probably not—but you can hear it.”

Of course you can measure dryness. It’s the lack of harmonic distortion or reverberation in space to which our ears are continually exposed and become accustomed to. You feel something is missing, and you call that dryness. You can say the same of a music hall which absorb an unusually high level of reflections. We call that “dry” because we don’t hear the multiple layers of sound that we hear otherwise.

Anyway, I just wanted to correct this unfounded belief that you can’t measure everything you can hear. Sean Olive even demonstrated that you can measure “good sound”, despite the very subjective nature of this concept. There is consistency in nature. A very few things are arbitrary, even if you’ll find some aberrant data and some people who have developed a dislike for accuracy (and will advance the hypothesis that it doesn’t even exist).

kevon27's picture

Mr. A. Hourst, you should be employed by Stereophile as a reviewer.
Reviewers use words like cold, warm, dry, laid-back, etc, etc. No one ever explains what those words mean or in terms describing sound.
I always believe if you can hear sound, you can measure it.

mrkaic's picture

I buy and read Stereophile because they publish measurements; I would not buy/read it otherwise. I read the measurements section almost exclusively, since I think that everything else in Stereophile (and especially subjective reviews) has very little value.

But the measurements are worth the price of subscription!

A. Hourst's picture

"everything else in Stereophile (and especially subjective reviews) has very little value"

I can't agree more. For example, Mr Fremer, in this review, avoid saying that there exist some "good enough" amplifiers that will sound exactly the same for a fraction of the price. Once you're below a certain threshold of distortion and output impedance, and have enough power, the differences become imperceptible.

Boulder still has a lot of merit for engineering such a stellar measuring amplifier.

Herb Reichert's picture

"Once you're below a certain threshold of distortion and output impedance, and have enough power, the differences become imperceptible"

Imperceptible ? to whom? Sir, I have been listening critically to amplifiers driving loudspeakers since 1966. Many of my best friends (even back then) were (and still are) brilliant amp designers. Not one of them, nor I, would make that statement. Have you personally experienced this phenomenon ? Also....Neither they nor I believe a quality amplifer is easy or inexpensive to make.

I am certain MF is one of the most experienced audio listener/observers on the planet, and I personally, place great trust in his observations. (Perhaps we should compare an vintage Hafler DH-500 to this obviously well-engineered Boulder? Do you think you could hear the difference?)

A. Hourst's picture

Mr Reichert,

Distortion and frequency response variations (which are typically very low in any well-conceived amplifier), are the only factors that can alter the sound of an amplifier. There’s no dispersion pattern like in loudspeakers, so it’s pretty much a straightforward 2-D signal very simple to measure. All your typical audiophile discourse about “bright” or “dark”, “rich” or “dry”, “musical” or “analytical” is embedded in those two parameters. Each one of them represents either an alteration of the frequency response or the signal integrity. I mean, how else could it be? People who have problems with this assertion invariably fail to 1) measure, 2) match the level within 0.1 dB or 3) listen blind. Usually, they even reach their conclusions based on memory. The problem is you can’t free your mind from its numerous prejudices (a 100 000$ amplifier will surely sound better than a 1000$ one) and aural memory is very fragile, since we tend to listen to something different every time music is played.

I do not question your or your friend’s experience or human qualities. I question the methodology, or lack of, that usually goes into the evaluation of most electronic devices.

And since you make this very personal, no I don’t place Mr Framer as an audio authority.

supamark's picture

You're completely ignoring that the amplifier and speaker(s) are a system that interact quite a lot... and a lot of other things. I'm sorry you don't have the ability to hear a lot of these things, maybe spend more time listening - it's a fact that the more time you spend doing something, the more neurons your brain will devote to it. Just as an experienced wine taster can discern shadings of flavor you can't, so can an experienced listener.

There is a lot of sillyness in high end audio (like claiming a power cord coming directly from a wall outlet having an effect on the sound ignores 100 feet of Romex upstream in the wall - assuming both cords are of the same gauge of pure copper wire there won't be a difference) but this isn't one of those things.

Fun story - I once was auditioning a Tandberg preamp (used) to replace my old NAD preamp circa 1990. The sound was significantly different (it sounded like George Massenberg gear - he invented parametric EQ btw) and excellent but I ended up not buying because the RCA jack was poorly designed and the sleeve came off when removing an interconnect. That was just a preamp... if only the build quality had been better I'd have bought it - Tandberg gear was top notch.

A. Hourst's picture

Did you match the level to 0.1 dB?
That's a major "difference maker".
Those interactions you're talking about are the result of high output impedance.
My experience is that the sound of electronics is extremely overrated.

supamark's picture

good to know that you're just talking out your ass. If you want to know what George Massenburg's equipment sounds like, compare 2 Flim & the BB's albums, anything from DMP (Tom Jung engineered) vs "New Pants" on Warner Bros. If you cannot hear the difference (no level matching required), you are not qualified to comment on audio, at all.

A. Hourst's picture

I understand you don't like what I'm saying...

supamark's picture

I don't respect what you're saying, because you don't actually know what you're talking about.

Johan Bottema's picture

Amp Voltage output capabilty as a function of signal phase change and through changing impedance is measured by other Audio mags. The power cube as the Germans call it, Yes there are "Music" signal loads that test the ability of the amp dealing with the speaker. So your story is a bit shortsighted. It just is not that simple and the amp should be tested with several speakers with different drivers and filters.

ChrisS's picture

Many, many fewer regard you A. Hourst as an authority of anything...

ChrisS's picture

Why are you here?

IgAK's picture

Interesting mention of Boulder's avoiding resonating fins, a legitimate concern if a fine point. That is why one of my amps with large fins has dampers inserted between every fin. Running a hand across the fins before that produced obvious ringing, yes. Even after the dampers they try to a little but I did not want to compromise the cooling too much so I stopped where I did.

Did I hear much difference after the dampers? Not a lot but enough to say that there was an improvement and the cost-free tweak was worth spending a few minutes on and something anyone can take care of themselves. The amp is not overtaxed so it does not run super hot, still, after many years the foam dampers are still in fine shape. It should be noted that that amp was being used on only the woofers in a triamped system so additional harmonics would have been cut off by that factor or I may have heard even more of a change. But that this matters became provably true and Boulder's attention to that detail is appropriate to a product costing that much and admirable.