Solid State Power Amp Reviews

I borrowed two 500AEs for my review, which allowed me to audition one amp in straight stereo or both in strapped mono. The fact that both amps worked flawlessly right out of the box and have continued to work for several months was my second clue that they might not be what most of us think of as "high-end" audio products. My first clue was when Boulder prez Jeff Nelson assured me that the 500AE requires no warmup prior to listening. I beg to disagree. To me, the amplifier sounds veiled, edgy, and flat on turn-on. It starts to show promise after 30 minutes, but is still nothing to do cartwheels over. Half an hour later, it starts to sound like something worthy of a review in these pages. After that, things improve more slowly, but continue to improve for another five hours or so. "No warmup needed," eh?

The supplied specs rate the amp's current drain at 1200W, but this is misleading; it only draws that much at full power output. At idle, power consumption is only 250W, which means it could be left on all the time without putting too much of a crimp in your electric bill, and that's what I recommend doing.

As I explained at tedious length in a rabble-rousing "As We See It" several years ago (Vol.11 No.10, October 1988), there is really no way of knowing for sure what a loudspeaker or an amplifier sounds like, because you cannot listen to one without the other. David Hafler used to contend that the ideal amplifier should not "sound like" anything, and I concur, but the truth is that even a "perfect" amplifier will sound like the loudspeakers it is driving. If the speakers too are perfect—even more unlikely—then the resulting sound should be just like that of the signal source, which depends on the degree of perfection of everything else in the recording and reproducing chain. As you can see, this line of reasoning will get us nowhere.

So, like most of Stereophile's reviewers, I tend to take certain things for granted—such as the unquestionable perfection of the signal my preamp and its signal sources are feeding to the power amp. True, it's not a very solid assumption, but it's all any reviewer has, or can have, to go on. I do take great pains to use associated equipment that seems, according to my judgment, to be more nearly perfect than most, but I freely acknowledge that the best I can do when reviewing most products is to make guesses about what they sound like, based on what I hear. The guesses are educated ones, and probably more valid than the judgments of those who have had less listening experience than I and have auditioned fewer products in their search for perfection, but they are guesses nonetheless.

But unlike most of Stereophile's reviewers, I do not trust program sources that I know little or nothing about. First and foremost, I trust my own recordings, because I know more about them than any others. I know how they were miked, what the mikes sounded like, what the original sources sounded like, and what the shortcomings of the recording equipment were. (I modestly confess to having a very good aural memory.) This does not mean I believe they are the "best" recordings available to me, just that I know what their assets and their liabilities are. Second, I trust some of Sheffield Labs' recordings, because I have brain-picked Doug Sax about how they ought to sound. Third, I tend to trust Delos's recordings, because their recording engineer, John Eargle, has been very upfront about how he works, and because his later tapes sound much like what I would be aiming for if I was still doing live recording. But I do not trust any other program sources. Period.

Perhaps I should also add at this point that I do not subscribe to the popular view that "good" is the sole criterion for evaluating reproduced sound. Sure, "good" is important, because live acoustical music sounds good, but it isn't enough. To me, the reproduction must also bear more than a passing resemblance to the sound of the original; but more about that later.

As far as loudspeakers are concerned, I have put my faith during the past several years in products made by Sound-Lab, not because the Mormon owner and corporate head doesn't smoke, drink, or use harsh language, but because S-L's loudspeakers have consistently sounded more realistic to me (with sources in which I trust) than anyone else's I've found to date.

The sound
Boulder makes two similar 150Wpc stereo amplifiers, the 500 and the 500AE. The 500, reviewed by DO in 1986 (Vol.9 No.5), has balanced inputs, input level controls, and front-panel LED displays for output level and protection-circuitry status. The 500AE that I tested lacks the input controls and the LED displays, but is otherwise identical to the 500. In fact, Jeff Nelson tells me they are functionally identical too; neither amplifier has been "upgraded" since they were first released.

Now, this does not sound like the high-end spirit. "High-end" implies an unending search for perfection—the continuing refinement of a product over time, to make it more and more nearly perfect. Jeff does not work that way. As a designer, he is more of a meter-man than a listener. He designs on paper and by computer, rather than by listening, and admits he does not have as good an ear as some of the golden ears in the recording profession who listen every day. So how can a product designed this way possibly be of interest to a Stereophile reader? Because the sound of the 500AE suggests that he may be onto something, that's why.

With my A-3s, a single Boulder 500AE—warmed up for five days—sounded as neutral as any amplifier I have ever listened to. By "neutral," what I mean is that recordings I know about sounded the way they ought to. They were neither closer than they should be nor more laid-back, they were correct both spectrally and timbrally (balance-wise and overtone-wise), and they gave the A-3s the deepest and most detailed low end I have ever heard from a full-range electrostatic loudspeaker. This is supposedly what high fidelity is all about—accuracy über alles—but it may not be what many audiophiles are really looking for in an amplifier.

The 500AE is not a "gorgeous-sounding" amp. It is not rich or warm or sweet or liquid or airy or mellifluous or crisp or spacious; it is just not there. It does not make reproduced music sound beautiful or magical, it just makes it sound as much like the real thing as anything I have ever driven the A-3s with. I'm talking here about the things I hear in live music that I rarely hear reproduced properly: the intense spikes in the sound of massed violin fortissimos, without steeliness; the power and the pitch delineation of double basses; the luminous roundness and flatulent vulgarity of large brass instruments.

There are several kinds of recordings I use for judging LF balance and extension. My own, of course, although relatively little of what I recorded ever went below 41.2Hz, which is the frequency of a 4-string double bass's open E string and the lowest frequency you will normally hear with any power from a symphony orchestra. (A contrabassoon extends to 32Hz, but most of the energy is overtones and reed sounds. An oversized bass drum may be tuned as low as 35Hz.) On the other hand, I have some nice tapes of large pipe organs in large churches, and I hear deeper and more authoritative bass from these than I thought the A-3s were capable of.

Then there are more recent recordings of huge pipe organs, like Jean Guillou's celebrated Pictures at an Exhibition transcription (Dorian DOR-90117), in which 40Hz is only the right-foot end of the pedal scale, and there are many orchestral recordings where the hall itself seems to give rise to very deep subharmonics which are normally not audible but, when they are, produce a subtle but definitely perceptible increase in the feeling of realism from the sound. I hear these things happening with the 500AE and the Sound-Labs.

Other bass material, from CDs, analog open-reel and PCM tapes, and laserdiscs, had noticeably greater detail and impact than I have ever heard from any large-screen electrostatic speaker, and it was this plus the exquisite high end that has kept me on a listening binge ever since I got the Boulder, dragging out one after another of my old (and newer) favorite recordings just to savor the sound.

But has the Boulder 500AE surmounted the major problems with op-amp circuitry? It sure sounds as if it has. At no time did I or any of my bat-eared friends hear anything that sounded like TID even from material that should have caused textbook examples of it (footnote 2). In fact, high-end quality turned out to be one of the 500AE's strongest points. There is no way you can conceive of how good op-amp circuitry can sound until you hear this! It is silky-smooth when the music calls for it, and spiky-sharp when it calls for that too, as when the violin section is digging in hard or someone at the back of the orchestra is whomping woodblocks or gently stroking a triangle.

There's nothing reticent about the 500AE's top. It has none of the sweetness or softness of a good tube amplifier, but there's loads of air at the top, which just seems to keep going out to well beyond the loudspeakers' (or listener's, if you're my age) upper limit. What continues to amaze me, as well as other people who have heard it, is the quality of that high end. Few amplifiers I have heard are able to give acoustic music its proper edginess and detail without crossing the line into steely hardness. The 500AE manages to do it. One result is that non-audiophile women, whose unforgiving sensitivity to HF crud makes them very intolerant of high listening levels from many "high-end" amplifiers, can enjoy listening to these at volume levels they would never normally endure without running for cover.

The 500AE's midrange detailing is nothing short of remarkable. It does not have that etched, sharply outlined quality I have heard from some of the best tube amplifiers, like the VTL 300, which can give the almost spooky impression that every instrument is outlined by its own lateral and front-to-back space. But the 500AE does everything else just a shade better than the VTL 300s.

Extreme highs sound more realistic, albeit not as sweetly soft, lower middles are a bit more gutsy, and there is none of the VTL 300's slight hardness with the A-3s. And the low end is much deeper and better controlled.

How about soundstaging? I mention this last because, while I believe I am as attuned to this important aspect of reproduced sound as anyone, I do not consider it to be nearly as high a priority as do some of my associates. But it is in the area of soundstaging where I have some reservations about the 500AE, although not many.

It is the 500AE's soundstaging which improves the most with extended warmup. After some days of inactivity (footnote 3), the 500AE sounds very flat when first turned on, and even after several hours' warmup, it does not produce as definite or as deep a soundstage as some other amplifiers I've heard, notably the aforementioned VTL 300s and the Manley 350s. But is that necessarily bad? Could it be that the Boulder reproduces the performing space accurately while some others exaggerate it? After all, bass energy is an asset in any system, but it's possible for there to be too much of it. Why, then, can there not be too much apparent depth and spaciousness?

I believe it is foolhardy to try to judge depth from most commercial recordings, because there is really no way of telling how much there should be. Yes, we may be able to guess the width and depth of the stage if in fact the recording was made on a stage, but statistically, very few have been. Even when recording in legendarily superb spaces like Boston Symphony Hall, many engineers eschew stage placement altogether and range the musicians out on the floor of the hall, which offers the possibility (sometimes accepted) of giving the orchestra more dimensional depth than it would normally have on-stage. This, plus the flattening effect of spot mikes, means you can expect the commercial product to have anything from too much to too little depth.

The situation is further muddied by amplifiers or loudspeakers that tend to back things off, leaving the reviewer little choice but to pretend there is no such thing as a "correct" rendering of depth, and just wing it by assuming that the more apparent depth, the better. (A mistake, in my judgment.)

In other words, while I won't say there are no commercial orchestral recordings that portray depth honestly, I will say that they are very few and far between, and that it's hard to tell which they are. To my knowledge, the only ones which are consistently more or less truthful are Telarcs, early RCA stereos, and the Mercury Living Presence recordings. Every other commercial recording I know of has been polluted, at least to some extent, by "creative" fudgings such as false instrumental placements, the use of many spot microphones, or both. Even "purist" engineers like Reference Recordings' Keith Johnson and Delos's John Eargle are not constrained by purist idealism from augmenting their basic mike pair with additional ones. They always have what seem like persuasive reasons for doing so, but the result is always recordings which sound different from the "raw" sound of an orchestra.

This is why I feel most comfortable using my own recordings for judging soundstaging. But not all of them. Because I didn't even think about soundstaging when I was making recordings 20 years ago, I never paid all that much attention to it on those recordings or in the venues where they were made. So my recollection of exactly what those recording spaces sounded like is less than reliable. Later recordings made in Santa Fe, when I had learned (largely in self defense) to become more aware of soundstaging, sound remarkably like the halls where they were recorded when heard through the Boulders and Sound-Labs: The warmly spacious, moderately live acoustics of the Lensic Theater, the drier but rather echoey space of Sweeney Center Auditorium, and the intimate, thin, and swimmingly reverberant sound of the Loretto Chapel are all clearly audible with the 500AE and the Sound-Labs.

I can also clearly hear through them the spatial problems that Doug Sax encountered during Sheffield's Moscow Sessions, where the side walls of the performing hall were barely wider than the orchestra, as well as the subtle center-stage flattening that John Eargle's woodwind spotmikes cause in many (mostly earlier) Delos Recordings. In other words, even though this amp does not produce as much depth or spaciousness as some, I have reason to believe it reproduces those things quite accurately.

Thus far, my comments have related to a single 500AE in stereo mode. What's to be gained by using a pair of them, each strapped for monoblock operation? Whether it's the push-pull operation (which I doubt) or a lot of other things—including, in particular, the power and current-capability increase (which I believe)—there is a definite across-the-board sonic improvement. Most aspects of the sound are only slightly better, but they add up to a significant gain in quality.

The most noticeable improvement is in the low end. With one amp in stereo mode, the low end is only excellent: tight, detailed, and very deep. With two strapped 500AEs, the low end is little short of extraordinary, at least on my pair of Sound-Labs. Forget rich, warm, full, and so on. With this combination, the bass is not something you demonstrate to wide-eyed, slack-jawed guests who marvel at how much the room vibrates; it's just there, the way it ought to be. When there are no bass instruments playing, the sound is almost lean. When there are, the low-end weight is exactly right and the bass detail is, if not awesome, at least good enough that people unfamiliar with the system will occasionally slide up in their seats and say "Whatthehellwasthat?"

---

Footnote 2: While my elderly ears are still good out to about 12kHz, they're missing enough up there that I use surrogate listeners to tell me what's going on above that.—J. Gordon Holt

Footnote 3: I confess, there are times when I don't listen for three or four days in a row.—J. Gordon Holt