Krell FPB 600 Stereo Power Amplifier (945W/8Ohm)

• One Owner from new

• The Krell FPB 700 is virtually the SAME amplifier except for cosmetics and “specification” of 700W – It also reached exactly 945W when tested 🙂

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Latest and largest in Krell’s current range of power amplifiers, the 600Wpc, $12,500 Full Power Balanced 600 joins the 300Wpc FPB 300 ($9000) and the 200Wpc (originally 150Wpc) FPB 200 ($5900). All are single-box stereo chassis and are specified as “Full Power Balanced”—I think to distinguish the essence of these designs from ordinary stereo amplifiers operated in balanced-bridged mode, usually with impaired performance. The FPB 600’s speaker output is balanced; ie, neither “positive” or negative” terminals are connected to ground or the amplifier chassis. (Note that no speaker switches or headphone adaptors, which often have joined channel grounds, may be used, as they will short the outputs.) The output terminals are electrically at 0V, but float above the chassis ground.

Only a serious weightlifter would want to pick up a ‘600—even without its shipping carton, it’s a serious two-man lift. Once it’s in place, you won’t move it in a hurry. Can a 600Wpc amplifier be relevant to domestic high fidelity? From an academic viewpoint, it’s hard to justify, even if price is not considered.

With speakers of average 88dBW sensitivity working in average rooms, it’s surprising just how much music can be replayed at a pleasantly loud peak level with a 30Wpc amplifier. As you can see from MK’s and JA’s comments on this issue’s review of the 30Wpc Pass Aleph 3, this will be loud enough, in practice, for all listeners other than headbanging rockers. With the right speaker match, a little more sensitivity, and a personal sense of proportion and restraint, satisfyingly loud sounds can be achieved with 20Wpc or even 10Wpc SE tube amplifiers. Even here, the peak level is rarely used; except for heavily compressed pop material, most music will cruise along at between 0.1W and 1W.

Conversely, the generous dynamic range of a big domestic audio system can be crucial in allowing full expression of music’s potential, especially in larger rooms. Power alone, if uncompromised in absolute fidelity, is well recognized as a positive factor in sound quality.

It all has to do with a sense of proportion, a scaling of expectation and use. Realistic reproduction of powerful rock music is no easy feat. In the right room, with speakers of greater than 94dBW sensitivity and more than 300Wpc available, you can come close to the sound levels where a stadium rig can still play cleanly. However, when all hell is let loose, a live band winds up to a performance crescendo, and the balance engineer takes the PA rig to its flat-out, 20% compressed limit of sound level, no domestic system can match it.

No simple amplifier!
Krell’s FPB 600 is no simple amplifier. A number of philosophies and technologies have been brought together in its design. On the assumption that engineering has the potential to make a better-sounding product, consider the following:

The output stages of the FPB 600 have fully regulated power supplies—probably the world’s first at this high a power level. Regulation means that there is an active electronic filter/stabilizer isolating the power amplifier proper from the raw, rectified, and capacitor-smoothed power lines. The latter, in addition to suffering from a series of ripple harmonics, carry a proportion of wideband electrical noise from the wall socket supply—hence, undeniably, the wide world outside—plus internal switching noise from the rectifiers.

Even under static conditions, an unregulated power supply has some degree of low-frequency wavering due to the demands made by other users. While amplifier circuits are generally designed to be relatively immune to these interferences, some still make it through. A standard unregulated supply will also vary in voltage according to the current demand imposed on the amplifier by the loudspeaker, this property called “regulation.” Even with a massive transformer and huge reservoir capacitors, there will be significant loss of regulation. The resulting bounce on the internal power rails can induce additional signal-related, correlated modulations which generally impair subjective aspects such as dynamics and rhythm. In my opinion, it’s no coincidence that some of the most rhythmically convincing smaller amplifiers—for example, the Naim NAP250 and its bigger brother, the monoblock NAP135—have fully regulated output stages

Adding full regulation to a power amplifier requires almost a doubling of complexity, since the supply regulator needs to be as powerful, as fast, and as “good-sounding” as the amplifier circuit it serves. In a good amplifier, supply regulation has the ability to stabilize all the operating points, and to remove most self-induced noises from the supply and from the other amplifier channel (if present). Ideally, this benefits dynamic truthfulness, clarity, low-level silence, purity, rhythm, and bass neutrality. The amplifier’s rated performance will also be maintained regardless of local voltage and frequency variations.

Krell’s regulation feature is still more impressive when you take into account Krell’s “Sustained Plateau Biasing,” a patented Krell technique that provides an effective equivalent to class-A biasing—without a long-term power dissipation penalty—by anticipating the size of any and every musical event (footnote 1). It uses high-speed current-feedback circuitry to do this, then holds these required levels in a static condition for tens of seconds after the event is over. This minimizes any possible dynamic interaction of bias level with sound quality. The FPB 600’s seven stages of bias represents the highest evolution yet of this technique.

Driving this powerhouse are two massive Avel Lindberg toroidal transformers, wired in parallel to give maximum power to the regulated supplies. The gain in dynamic performance from such paralleling outweighs considerations of dual-mono working with a potential for better channel separation. In any case, the regulators ensure high channel separation.

The 600’s output stage is run without overall loop negative-feedback, something that can confer excellent high-frequency stability and low coloration. Such output stages tend to have more consistent lower-order harmonic distortion, and, if well executed, have the potential for more natural dynamic expression, the latter often impaired to some degree in conventional high-loop-feedback designs.

More problematical is the design and specification of output devices to provide a sufficiently low output impedance and moderate distortion, lacking the benefits in those areas that are generally offered by conventional loop feedback. This is solved by paralleling a large number of output devices within a short local loop. According to Krell, the 600 is wholly bipolar transistor, with not a FET in sight. The monster arrays of metal-can TO3 output devices are made to Krell’s own specification by Motorola, acknowledged leaders in the field, and are specifically graded 25A devices. The ‘600 uses a total of 120 of these, both for output and regulation duties.

The final factor—one that, a priori, requires an amplifier with a sound heart to deliver its full benefit—is sheer power. If the intrinsic performance of a power amplifier can be kept uncompromised when the design is scaled up, the bigger amplifier should sound better than the smaller one, even at the same operating level. This is because the larger amplifier will be proportionately less stressed.

Compare the FPB 600 with the ‘300, which shares a very high percentage of engineering technology with the big design while possessing half the output current capability. Thus at 100W peak, a comfortable level for both models, proportionately less of the ‘600’s current potential will be used, so providing greater linearity and clarity. This can be confirmed on the test bench.

Circuitry
The FPB 600 is DC-coupled and complementary push-pull, with a jumper to allow inclusion of an input rolloff at low frequencies, for use where a signal source (eg, an incompatible tube preamp), might provide DC or excessive LF. This would otherwise trip the protection, of which the final element is the circuit breaker on the rear panel.

The circuit is configured as a true balanced design from input to output. When driven unbalanced, the negative input is shorted to signal ground (with a supplied link), while the internal cross-coupling ensures that balanced integrity is obtained in successive stages. The all-discrete input and driver circuitry is run in class-A throughout, with high current ensuring a very wide bandwidth and near zero phase-shift in the audio bandwidth.

Seriously thick, plated-copper bus bars link the amplifier boards to the paralleled pairs of output terminals, and there is no relay or output inductance in the output path—there’s nothing in the way. Four “small” sub-reservoir sections (“small” is relative: each is a block of four capacitors, each of these rated at 6800µF, 100V) serve each regulator rail, independent plus and minus for left and right channels, these bypassed by still smaller and faster capacitors. With all of these capacitors integrated with the amplifier’s printed circuit boards, supply link inductance is held to a low value. The detachable power cord is a heavy-duty 30A type.

There is a microprocessor support in each power channel for protection, bias compensation, thermal control, startup, and to decode infrared commands (from a remote control).

Preliminaries
As I wasn’t willing to sacrifice my back for a review product—not even a Krell—two strong men were brought in to elevate the ‘600 to my main listening room. The amplifier was installed between and well behind the main speaker positions in order to avoid spurious acoustic reflections that could have marred the stereo image.

The FPB 600 was factory-fresh, as was the FPB 300 I reviewed for Hi-Fi News & Record Review in December 1996. That model sounded quite unremarkable when auditioned at first plug-in; while the acceleration in quality was impressive as the early tens of minutes went by, it was a week before a close approach to its final performance plateau had been achieved.

Forewarned by that experience, I broke the ‘600 in on a diet of stereo music for a week. Thereafter, matters were fairly stable. The FPB 600 can, of course, be left in standby for longer periods of inactivity, and switched off altogether for a vacation, but for regular use I tended never to switch it off, being generally unwilling to sit out the 15-minute climb in sound quality from “standby” to fully warm.

If you really want to hear what an FPB 600 can do, make sure that the demonstration unit has been broken-in before you begin listening. A fellow listener questioned the need for this procedure: “Does it mean that an amplifier design is flawed?” I opined that it was very likely that all audio electronics show some improvement when run-in, but that this is simply less obvious with poorer-sounding product of inferior transparency.

Power—and then some!
Zero to 60 in 4.2 seconds! So kick down the volume pedal at last! The resulting impact was colossal, unexpected, unprecedented, totally in control, easy, hugely loud yet unfatiguing, exciting, involving, thunderous, and effortless. Here was the audio equivalent of a Porsche or a Ferrari—no excuses, no practical limits. In musical terms, the ‘600 suggested an impact comparable to that of such high-performance automobiles—and for a whole lot less money!

And while a Porsche or a Ferrari will demand to be exercised—to be stretched, revs soaring—this amplifier was also happy to glide along like a Lexus when fed undemanding, low-level program. It could do both. Mighty impressive.

My preferred speakers have never sounded more themselves than in the company of the FPB 600. I learned to highly value that innate truthfulness during the review period. That particular quality of the FPB 600, and its overall performance level, made comparisons with SE tube designs largely redundant—even when the Krell’s volume was contained within the power compass of the smaller amplifiers. Even though the best SE tube designs sound sharper and meaner than the FPB 600 (due in part to loudspeaker interactions), with an expression of microdynamics that the Krell didn’t quite capture, no SE can deliver the top-to-bottom tonal balance and neutrality of this solid-state design.

In the bass the FPB 600 was simply awesome, interacting powerfully with the Wilson X-1/Grand SLAMMs to deliver previously unheard bass extension, while superb articulation and slam were both evident. Powerful bass notes stopped and started with amazing control, percussive bass was very realistically “present,” and complex bass lines were played tunefully and with very fine rhythm. The FPB 600 felt as if it could bodily pick up those X-1 juggernauts and run with them. With such a display of raw power, this amplifier seemed equally adept at conveying fine texture and tonal color on acoustic string bass. The title track of Airto Moreira’s Killer Bees (B&W BW041), for example, sounded awesome, a huge soundstage building to superbly developed dynamics and great jazz rhythms.

Solo grand piano verged on the revelatory. Somehow John Atkinson’s full intention as recording engineer seemed to be mightily expressed by the Krell/Wilson combination. Truly dynamic, wholly realistic sound levels and quality were attained on his excellent recording of Robert Silverman playing piano works by Liszt (Sonata, Stereophile STPH008-2). I have never heard a Steinway Grand reproduced with such palpable reality as this. Powered by the ‘600, the smaller Wilson WITTs didn’t do such a bad job either on this recording.

Without fail, every time I returned to the ‘600 I heard an unmistakable sense of grip, a close embrace of speaker and amplifier, always in step. Its sound was beautifully poised and superbly founded, its source deep, even immovable—unshakeable. With the larger loudspeakers, the effect was comparable with moving my system from a room with a suspended timber floor to one with 2’ of reinforced concrete under the speakers.

I rate the FPB 600 as the “fastest” of the big superamps in terms of rhythm and timing. When downscaled into a small, “fast” UK audio system, the big Krell remained graceful and capable, while classy amplifiers—albeit of more limited bandwidth and power, such as the Naim NAP250—still had the edge in terms of absolute pace and timing. However, even here the gap has now been narrowed. Only the most critical rhythm fans will find the Krell wanting.

Imaging, transparency…
Soundstages were very stable, very deep, very wide, and excellently focused. Layering of perspective was most convincing. The big Krell presented the primary image just right, neither too close nor too distant. There was no detectable shift in perspective or depth with changes in loudness or program complexity. Perhaps as a result, the FPB 600 somehow induced the speakers to perform with greater smoothness, showing unexpected gains in driver integration and the sense of musical coherence. Stereo images floated free of their reproducing acoustic sources and were bathed in a luminous, ambient soundfield. With the best program quality, images reached far back into the imagined soundfield.

Traditionally, transparency has not been the strongest territory for big solid-state amplifiers. Audition a well-warmed FPB 600, however, and discover that it ain’t necessarily so. In both the mid and treble, this amplifier delivered transparency at a sufficient level to fend off competition from the finest tubed creations, SE or otherwise. Then the fun started. Drifting down from the midrange, it was in the lower mid, the upper bass, and the main bass where the ‘600 also kept its promise. In context, it showed revelatory transparency in these areas, far better than that of any other design I have auditioned.

In some systems it’s possible to achieve a feeling of transparency and fine depth yet lack for resolution of fine detail, the latter an area in which Audio Research’s tubed designs have traditionally performed well. This behemoth has bridged the gap between solid-state and such tube designs. The FPB 600 presents the listener with a wealth of fine detail: finely etched, sparkling, immediate, and locked in. This is high resolution in spades.

Classic descriptions serve best here. Take a large choir: Working from previous experience, I expected fine reproduction of solo passages and small group work, but anticipated a noticeable degree of blurring and congestion when the full ensemble opened up. With the big Krell, these same passages were presented in a new light. Yes, the simpler sections were as good as ever, arguably even better than before, but the amplifier’s ability to hold focus, detail, and choral registration in the full-bore sections was little short of astonishing. Now the sopranos didn’t overwhelm the tenors, nor did the tenors overwhelm the basses. All were accorded equal clarity, their chosen musical weights preserved. Equally, on large orchestral works the approaches to big climaxes weren’t dodged—the FPB 600 didn’t crunch up.

Neither did it exaggerate highlights in the scoring. In live performances the brass can typically appear to be scored outrageously loud, yet the rest of the band can still make itself heard over such an apparent imbalance. When recorded in purist fashion and reproduced by a hi-fi system, such scoring may not sound so effective. Those same brass sections may overwhelm the presentation, this often signified by a quick reach for the volume control.

With the FPB 600, I found that a number of previously troublesome recordings no longer presented this problem: I could let the system go. All remained in place, the composer’s intentions dynamically unsullied. The big Krell got significantly closer to live sound in this respect.

As regards character or coloration, this proved very hard to pin down—of the various technologies employed by Krell in the FPB 600, I really couldn’t hear any “at work.” The dynamic slide through the sequence of class-A plateaus was aurally invisible; indeed, at moderate volume levels, the cool, stable running of the heatsinks told me that, under these conditions, I wasn’t even igniting the second step of class-A operation.

Conversely, if I whacked the FPB 600 hard with loud, percussive rock, it warmed up, quickly growing too hot for more than a momentary touch of the fingertips. Essentially, it drew power only as I needed it, successfully combining the efficiency of class-B with the sound (if there is one) of class-A.

Once it was well run-in, the FPB 600’s sound was as close to neutral as I have yet heard from any power amplifier. Somehow the center of gravity was restored with the FPB 600—that common tendency for hi-fi to sound thin and forced was wholly avoided here. This amp was pitched very naturally, with just the right sense of richness and weight. No tone controls are involved here—honest.

Sundries
Comparing the FPB 600’s balanced and normal-input operation, I found that, in absolute terms, the sound quality was remarkably similar. Still, there were minor, subtle differences: Balanced drive favored a slightly deeper and more impactful bass, a tad more air and ambient glow in the soundstage, with a touch more sweetness in the upper treble. Single-ended connection had a marginal advantage in coherence and rhythm. The bass was slightly tauter and more focused, while the upper registers were a little more dynamic and expressive. I used both with equanimity, finding that cable differences, even at the top of the tree, were larger.

Aside from the choice of cables (not easy, in view of the performance potential), two further areas remain of interest: physical support and mains power supply. Like the FPB 300, the ‘600 benefits from something more than carpet under it. In any case, if the carpet is too deep, air flow will be restricted. I tried spiked marble slabs to good effect, and a pair of Mana platforms back to back—unwieldy and probably suboptimal, but nonetheless revealing. The floorspikes gave an impression of a deeper, clearer bass and a more solid mid. The platforms gave faster, more tuneful bass with a clearer mid. More work here would prove rewarding.

Conventional wisdom tells us that a fully regulated amplifier such as the FPB 600 will have superior isolation from line imperfections and noise emanating from the supply side of its transformer. Thus it should be relatively uncritical of supply quality. Though this may well be true on the usual basis of judgment, it remains a fact that both the ‘300 and the ‘600 were sensitive to supply quality. I experimented using conventional socket strips, standard wall sockets, and dedicated 45A, 240V supplies (equivalent to 90A on US 120V).

Easily reading these differences, the FPB 600 thrived on the dedicated facility, the sound improving still further when the end of the mains cable was hard-wired into the terminal strip of the supply outlet (consult a qualified technician on these matters). I wonder whether the detachable high-power cable connector Krell has adopted for this amplifier really gets the very best out of it. I also found that it sounded best with the supply ground installed as instructed. (With some amplifiers, floating the ground may improve performance.)

Comparisons
Working with my personal ranking order for power amplifiers, until now I’ve placed (with minor caveats) the Conrad-Johnson Premier Eight A monoblock (non-triode) at the top of the tree, especially for its vibrant tonality and lush transparency in the mid-treble, in addition to its great dynamic range, average-to-good load tolerance, and fine, tuneful bass. Once the Premier Eight A has been experienced in an appropriate setting, one is forced to question the fuss over little SE amplifiers.

Perceptibly close are both the Krell KAS-2 and Audio Research VT150SE. At the time of review, the KAS-2 was considered the best solid-state design yet, substantially improving on Krell’s discontinued KSA-S series, but at a far higher price. Albeit with somewhat less power and bass slam, the VT150SE was and is head and shoulders above the rest of the Audio Research range, and often sounds very close to the KAS-2 in sonic merit while providing a still faster and better-resolved sound. Account has also been taken of other contenders: the Audio Research Reference 600, VT130SE, and VT100, and the Mark Levinson No.333.

Then Krell introduced the FPB 300. I finally rated this model above the C-J Premier Eight A for overall attainment, but still conceded that the “Eight A provides superior purity, delicacy, and ‘air’ in the mid and treble” (HFN/RR, December 1996). But now I have no hesitation whatsoever in rating the FPB 600 a full 20% better than the ‘300, and this in territory where improvements of just 5 or 10% are hard won, greatly valued, and willingly paid for. Yes, the ‘600 is quite expensive, but in real terms its price is actually lower than the competition’s. (For readers who follow my personal numeric ratings for sound quality, as published in HFN/RR, the FPB 600 scored an industry record of 42 points.)

It would be too easy to mark the FPB 600’s success with a torrent of superlatives. Indeed, I think I’ve strayed some distance down that path already—but what else can I do?

Take the comparison with Conrad-Johnson’s Premier Eight A: Though the C-J still sounded a trace more liquid, the ‘600 reached right into this amplifier’s previously undisputed territory. Precision, control, subtlety, superb detail, and state-of-the-art depth and transparency were all fully described by the new Krell. Detail was extraordinarily focused, right into the high treble. Once again in a review I must report that complex treble now sounded even more natural and even better differentiated than before, and was achieved with a greater sense of life, producing a sparklingly clear sound.

The broad midrange breathed tubelike tonality, the FPB 600 now distancing itself from many established solid-state contenders, cruelly exposing their old-fashioned hardness and glare—that “strained” coloration which can be mistaken for superior dynamics and precise transient edges, but which ultimately fatigues the ear. Such a compromised sound is also found to compress image scale and depth, and marginalize true dynamics.

And compared with the FPB 300? All too often, larger amplifiers concede quality for quantity—the treble roughens, and the more powerful amplifier may sound less well integrated, even lacking in clarity. Of course, a big amp can also sound more gracious and bighearted, if at the same time slower, less agile, and perceptibly weaker in terms of rhythmic drive.

Not so the FPB 600. It conceded absolutely nothing to its smaller sibling. I noted a more open, more naturally explosive and better-balanced sound, particularly in the upper mids and treble. Looking back at my notes for the ‘300, I wondered how this could be possible—yet it really was the case!

Would such an improvement in transient speed be reflected in a loss of purity and tonal balance through the midrange? Not a bit of it. Indeed, the ‘600 built on the ‘300’s strengths, significantly improving on its mid character and sounding yet more positive, still more naturally “sweet.”

At first the ‘600 sounded a little less “concentrated” in the bass than the ‘300—until I realized that the ‘600’s bass breathed more easily, reached deeper, and had a “sweetness” which was, in its way, equivalent to that same quality in the mids that is usually expressed as a lack of “hardness” or “glare.”

Too many superlatives?
Regular readers know that I am not prone to effusive language. A traceable continuity of opinion matters to me; I don’t want to reread a review in six months or a year, only to regret that I’d overplayed my hand.

Nonetheless, I am driven to say that the Krell FPB 600 power amplifier, when properly installed and optimally matched and connected, sounds simply stunning. From a wholly musical point of view, this amplifier reigns supreme in its power to better convey the impact, meaning, emotion, and involvement of great music. So vestigial are its faults, and so great the sum of its better parts, that it’s almost impossible to put the former into any valid perspective.

When I evaluated the Wilson Audio X-1/Grand SLAMM (Stereophile, December 1994, Vol.17 No.12, p.115), I felt that, as an industry, we were some distance away from realizing that loudspeaker’s full potential; that it would continue to sound better with better systems. Krell’s new amplifier is proof of this: The big Wilson went substantially further with the FPB 600. Conversely, the X-1 helped to confirm the very high performance achieved by the FPB 600. What a magnificent combination. Here, a closer approach to truly realistic rock-concert levels was possible in the home. (I can write this with confidence; as this section is penned—literally, with a gold-nibbed fountain pen—I can vividly recall Sting in concert last night at London’s Royal Albert Hall. Sporting a Bruce Willis hairdo, he was in good form.)

Conversely, with lesser speakers the FPB 600’s quality remained at the forefront; for example, it shoved the smaller WITT significantly up the performance band. You don’t need a pair of Grand SLAMMs to hear what the FPB 600 can do for you. Even a pair of Epos ES12s showed what it could do.

Conclusion
I suspect that even the Krell team was surprised when the Full Power Balanced 600 began to take shape. On paper, it was simply a power-doubled FPB 300 delivered at a moderate and appropriate hike in price. In practice, the FPB 600 has gone much further. Great as the ‘300 undoubtedly is, the ‘600 holds all the aces, with a potential to severely embarrass a wide sampling of top-brand power-amplifier references. I’m quite certain that it redefines the art. It will certainly dictate a vigorous shakeup of the current stack of Class A power amplifiers in Stereophile‘s “Recommended Components.” Since a Class A rating means “the best we know,” I feel that, in the light of this design achievement, the rest will have to be re-classed (footnote 2).

The FPB 600 advances substantially on the FPB 300 in particular, and on the art of the power amp in general. In fact, I suspect that the Krell designers are now acutely embarrassed at the audible shortfall in matching preamplifier performance. With no disrespect intended to the KRC-HR (Stereophile, October 1996, Vol.19 No.10), just use one in conjunction with the KPS-20i/l and the FPB 600. Then bypass the KRC-HR by simply linking the existing balanced cables input to output, with no other changes whatsoever—and hear the system take off. Such is the ‘600’s transparency that care in system alignment, choice of cable, and quality of connectors all offer substantial payoffs.

The FPB 600’s sound quality was at the leading edge. The Wilson X-1 Grand SLAMM proved as nearly perfect a match as one could wish for. The overall quality and dynamic range available from this combination was both stunning and breathtaking.

Add into the equation its huge, uncompromised peak loudness, the incomparable power delivery confirmed in its excellent laboratory results, and the Krell FPB 600 is something quite special among power amplifiers. And, lest I forget: at the price, and taking everything into account, this big Krell is actually good value for money.

I confidently dub the Full Power Balanced 600 the “Grand Slam” of power amplifiers.

Footnote 2: Given Martin’s enthusiasm for the big Krell amplifier, it looks as though Stereophile will indeed have to reexamine the solid-state rankings in Class A of the magazine’s “Recommended Components” listing. Accordingly, Wes Phillips will be comparing the FPB 600 with other leading amplifiers in the next few issues of the magazine.—John Atkinson

Wes Phillips wrote about the FPB 600 in January 1998 (Vol.21 No.1):

How does a pair of Mark Levinson No.33H monoblocks compare to the Krell Full Power Balanced 600, Stereophile‘s joint Amplification Product of the Year for 1997? After all, Martin Colloms went so far in his review last April as to claim that the Krell so rewrote the book on amplification as to require a total reexamination of Class A power amplifiers in Stereophile‘s “Recommended Components.” I’m not sure I’d go that far, but Martin is essentially correct: Compared to the Krell, almost everything else sounds broken.

Directly comparing the FPB 600 and the No.33Hs proved a logistic nightmare. Both amps required extended warmup—ie, music playing through them for several days—before either reached its optimum. The upward climb was far shorter, of course, from powered standby. The problem stemmed from my house’s wiring, which simply wasn’t up to both amps sucking all that current through the same circuit. (That’s it—my New Year’s resolution for 1998 is to rewire the house with beefy audio-only circuits.) So my comparisons are not direct A/Bs of specific passages, but are the results of longer listening sessions separated by the requisite warmup periods. Flawed? Yes, but the best I could do under the circumstances.

Using Dynaudio Contour 3.3 and EgglestonWorks Andra loudspeakers, the differences between the amplifiers were subtle, very subtle. Both were essentially not there in terms of having an effect upon the music. And, while each was capable of kicking some major audio booty, the most impressive thing was that neither sounded like a big amp during the quiet passages. Both were delicate and graceful.

However, the Levinsons seemed to present low-level detail with less “light-against-black” spotlighting than did the Krell. Audiophiles frequently speak of silence as the “blackness” against which sounds are highlighted, but I think that most sounds appearing from silence are far less dramatic than that. In a sense, then, I’m calling the Levinson more natural-sounding for its lack of “added” drama—and I have to put “added” in quotes because that might be merely my preference. Another listener could as easily call the No.33H “duller,” and laud the Krell for its ability to extract excitement.

Allied with that ability to portray low-level detail came a sense of natural ease; I found the Levinson ever-so-slightly more transparent in the midbass and low bass. This is the quality I alluded to earlier when I praised its ability to “float” low tones in the same manner as the highs. While the Krell has superb transparency down to its low midbass, it tends to favor muscle over subtlety in the deepest regions. Yet that seems too harsh a criticism of the FPB 600: it takes charge of the bottom end in a way few other amps ever have, while remaining very attuned to the musical moment.

As did the No.33H. I compared the two amplifiers using the first movement of the Masur/NYP Mahler 9 (Teldec 90882-2), recorded live in Avery Fisher Hall in April 1994 and chosen because of my familiarity with both hall and musicians. The Andante comodo begins extremely quietly, with a three-note syncopated rhythm in the cellos and horn (Myers again, sounding forth brassily). This must have had a deep meaning for Mahler—at the movement’s climax he brings it back marked fff, h;dochste Kraft (“with the utmost force”). Mahler’s friend, Alban Berg, called the riff “Death itself.” God knows, it’s forceful enough to qualify for such a frightening description.

Both amps handled the climax with ease. Both did superb jobs of conveying the emotion implicit in the intense wash of sound. But I felt the Levinson’s clarity in the bass allowed me to hear the NYP double basses as distinct entities separate from, but still members of, the ensemble. The Krell certainly conveyed their power as members of the whole orchestra, but through the Levinsons I felt as though I could actually make out Eugene Levinson, Jon Deak, et al as individual players surrounded by air and anchored to the floor of the hall I know so well, playing in unison among another 100-odd players—all blowing, bowing, and banging furiously.

Comparing the two best amplifiers I’ve ever heard to one another, I reckon one has to be better. But if I seem uncomfortable in proclaiming the Levinson to be better than the Krell, I am—until I heard the No.33H, I never would have guessed the FPB 600 had an equal. These two amps are so close in character that another listener could very easily call it the other way. I can’t imagine anyone being less than satisfied with either. Yet to my ears, no matter how slightly, the No.33H sounded like the better amp.—Wes Phillips

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Preliminaries

I have had the Krell FPB600 in my listening room for about 18 months, certainly enough time to become familiar with it and recognize its strengths and weaknesses. At the time of purchase, the Krell to my ears easily beat two competing solid state amps and two tube amplifiers that are highly reputed. I recently obtained the KR Enterprise VT8000MK amplifiers ($24,400 retail). These are an all tube, zero feedback, push/pull design and put out a maximum of 300 watts per monoblock into 2, 4 or 8 ohms. The KR amps match or better other tube amps in important areas such as imaging and holography and are killer in the mid/treble frequencies. As such, they afforded an excellent and rigorous comparison with the Krell FPB600 solid-state amplifier.

Krell is one of the most familiar and highly regarded names in high end audio—it is sort of the BMW of the high end—performance comes first and this has led to a cache whereby other products frequently are compared to Krell. This is of some importance, given that when you buy a high-end component you are also buying a high-end company. Krell’s reputation is to build extremely durable, reliable and well engineered products (as with BMW, some would say over-engineered!). Krell is accordingly able to offer 5-year warranties on their products and the warranties are transferable to other buyers. I have owned four Krell products and had a problem with only one of them, a KSA200 amplifier that had its ground shaken loose by UPS gorillas several years ago. Krell fixed this promptly; the amp was back within 10 days and worked ever after.

The Krell FPB 600 has performed flawlessly from the time it came out of the shipping box. Krell is also an innovative company. It started the production of high-end single box CD players with the KPS products, a trend that others have now adopted. Similarly, it led the move of top end companies into ‘affordable’ products such as the 300 series of amplifiers and CD players, a trend that has also been copied by several others. Krell is usually at the forefront of engineering, as I will discuss in more detail for the FPB600 amp. These are all factors contributing to the traditionally high re-sale value of Krell products.

The FPB 600 amplifier is a stereo device. The amp contains two huge toroidal transformers with a total of 8 kilowatts capacity, some sort of record I would say at the price point! This also means that the amp is heavy, about 180 pounds, and long, about 26 inches front to back, and 10 inches high. The amplifier is pure class A and direct coupled and also has fully regulated input and output sections, unusual for a high powered amplifier. All of these features, however, would be expected to contribute to good sound.

The arrays of specially selected Motorola bipolar transistors collectively put out a claimed 600 watts/channel at 8 ohms impedance, doubling as impedance is halved down to at least two ohms. The definitive review on the Krell 600 in Stereophile by Martin Colloms measured output as an actual 935 watts/ch into 8 ohms before clipping! Thus the amp is a powerhouse. Two sets of high quality speaker terminals with thumbscrews are provided as outputs and both balanced and unbalanced inputs are available. Krell, however, maintains that the amp performs much better with balanced inputs.

Krell also uses a clever strategy to permit full class A operation without burning your house down. Their patented variable biasing system allows the amp to run in 7 different bias levels depending upon input demand. Thus, at idle or during low volume material, the amp loafs along at something like 30 watts/ ch maximum. However, if all hell breaks loose on the musical program, the amp has an anticipator circuit that detects this and rapidly ramps up the biasing to the required level. If input again decreases, the bias level also decreases—clever! Neither I nor anyone else I know has audibly detected this technology at work, but it does have the major advantage of allowing a full blown class A amp to run relatively cool and save on your power bill.

Krell amps have stereotypically been able to handle any speakers or program material and to provide incredible extension and resolution of bass frequencies. Their Achilles’ heel has been in the mid and high frequencies, where historically they could elicit visions of mid-fi cat howling, largely I suppose due to odd order distortions from the bipolar output transistors. The company has been working on that and it is generally agreed that the KSA series of amps provided improvement and further improvement has also been claimed for the FPB series of amps. Let’s find out, using the KR VT8000MK tube amps as a very high bar for comparison!

The System

Von Schweikert VR-8 speakers with the silver internal wire option were used throughout. These extremely revealing speakers were set up in a large room and placed about 6 feet from one side wall and more than 15 feet from the other. Only 26 inches from the front wall proved the best bass compromise in an irregular California style split level living area of greater than 25,000 cubic feet. The back wall was not parallel, with the furthest point about 45 feet away. The floor has a moderately thick carpet and the ceiling is open wood-beamed cathedral, angling up and away from the speaker position. The speakers sound good in this location without room treatment, almost a necessity due to Wife Acceptance Factor (WAF) (not to say that the usual tricks wouldn’t improve things). The speakers were separated by about 12 feet center to center and the listening position was about 11 feet from the speaker centerline. Only slight toe-in proved the best compromise for imaging and sound staging since the speakers are not particularly directional.

The speakers were coupled to the concrete slab floor with the high quality brass spikes supplied by VSR. These produced the greatest increase in focus and sound staging of any speaker I have ever heard relative to performance unspiked and floating on the carpet! CDs only were used as source material through a Krell KPS-20iL CD player run directly into the amplifiers through Harmonic Technology Truth Link unbalanced interconnects. For some auditioning, Audio-Magic Sorcerer balanced interconnects were used with the Krell amplifier. Performance of the excellent 20-iL CD player was elevated another few notches by an ElectraGlide ReferenceGlide power cord. My reference speaker cables have recently become the top line Harmonic Technology pro-9 copper cables bi-wired to the mid/tweeter and bass modules of the VR8es. These cables afford new levels of detail and resolution and fully deserve the accolades many reviewers have given them.

I also recently obtained the highly rated Arcici Suspense equipment rack (see review in this issue), which was used for all amplifiers (a total of about 360 pounds!). The CD player was similarly placed on a BDR Shelf for the Source with #3 cones and set onto the top shelf of the Arcici. I found that a mixture of #1 and #2 Vibrapods between the shelf and the top of the Arcici rack also provided an improvement in tonality. Components were plugged directly into a Magnan Signature power cord with strip, in turn plugged into a dedicated 30 amp circuit. CDs were liberally tweaked with Compact Dynamics Optrix spray on the playing surface and CD Upgrade discs, as well as Eco-3 antistatic formulation sprayed on the label side. CDs were also routinely demagnetized with the Bedini Ultraclarifier before play and the Audio Prism CD Blacklight was used. Yes, green paint on the edges of CDs made an improvement, and I have evolved to the product from LAT, that does not smear when discs are treated with Optrix. All of these tweaks gave audible improvements that varied somewhat in degree, depending on source material. The system was occasionally treated with the latest Purist Audio break-in disc. Finally, monthly cleaning of all terminals on cables/interconnects/power cords with ‘Pro Gold’ was beneficial.

Evaluations

Suffice it to say, the Krell 600 is magnificent in the bass and mid-bass frequencies, probably better than any amplifier at or below its price point for providing extended but fast and tight bass. In addition, the amp has incredible presence–the ability to make you think you are there—no quarter mile away music or struggling to handle the next transient here! Some might call this effortless—the amp just spits out whatever comes in, regardless of how raucous it might be.

Soundstaging, both vertical and horizontal is immense, and imaging is excellent, regardless of complexity of material or pyrotechnics. With 1200 watts per channel available at the nominal 4 ohms impedance and 96 dB/w/m of the VR-8 speakers, headroom is not an issue with the Krell. Indeed I have never managed to make it clip, and to do so would I suppose require being outside the house or having a set of very good earplugs. The Krell will also beat most solid state amplifiers and many tube amps for mid and treble purity and transparency, but I will say more about that momentarily.

The Bad News

I have run interesting comparisons of the Krell 600 with the KR Enterprise VT8000MK amplifiers in my system and, suffice it to say, the two are very different. In some respects, the KR amps clearly beat the Krell, which they perhaps should, retailing for twice as much. The secret of the KR amps is outstanding house-built input and output tubes, the latter actually producing significant current – unheard of with conventional tubes. The KR amps are accordingly very linear and excel in the mid and treble frequencies and to my ears sound as good in this regard as any amps I have ever heard. The 8000es can be characterized by the three ‘Ts, tone, timber and transparency, all of which the amps have in spades, easily beating the Krell on most material.

The KR amps detail well and also provide wonderful holographic realism—those indefinable cues that we all perceive in real music, and which create the illusion for a sound system that you are in the concert hall and the music is live. These characters made the KR amps excel and easily better the Krell on ‘small scale’ music such as jazz, blues, small ensembles, soloists, small singing groups, solo piano etc. Reproduction of cymbals, bells and such was marvelous.

For instance, the Golden String All Star Percussion Ensemble CD (GSCD 005) has lots of bells, drums and other percussion sounds that generally are not as well reproduced by the Krell 600. Listening to tracks from this CD through the 8000es was a revelation for clarity and rendition (and made me think that 16/44.1 is not as bad in the treble as we’ve been led to believe—amps have clearly been part of the problem!). Similarly, vocals such as on Muddy Waters’ famous ‘Folk Singer’ album (Mo Fi MFSL UDCD 593), Patricia Barber on ‘Café Blue’ (Premonition 737-2) and Margo Timmins’ smokey voice on the Cowboy Junkies ‘Trinity Sessions’ (BMG 8568-2-R) were all rendered with incredible clarity, timber, emotion and presence.

Instrumentals were also treated well by the KR8000 amps. Dean Peer’s ‘Ucross’ recording (Redstone RR91012) has never really sounded quite right through the Krell 600, with lots of boominess and poor string harmonics. I had erroneously chalked this up to the recording, but the 8000es did a credible job of reproducing Peer’s bass, including excellent articulation of harmonics in the lower registers. Similarly, Gino D’Auri on Flamenco Mystico (Golden Strings GSCD 016) was reproduced by the 8000 amps with stunning timber, realism and detail on the strings of his classical guitars, but the Krell 600 missed much of the harmonics and “you are there” holographic nature of the tube amps. “Difficult to do right” instruments such as the saxophone (try cuts from Jeremy Cohen on A Taste of Violin, Clarity CCD-1012), trombone (Shades of Brass, Mapleshade 03932) and piano (Schumann, track 7, DG Originals 447 451-2) were reproduced as well through the 8000es as I’ve ever heard. Accolades are also in order to the VR-8es for correctly reproducing the wonderful sounds of these bell weather instruments.

The Good News

Now for the counterpoint, or “don’t conclude that I have thrown out ss amplication”! When one starts to encounter music with anything resembling a ‘big’ character such as most pop groups or large orchestral compositions, especially involving deep bass with organs or synthesizers, the 8000es largely abandoned the scene and the big Krell came into its own.

It’s not that the 8000es don’t make bass. Its just that for “big” music with lots of instruments and dynamic passages, the 8000es fails to match the Krell’s strengths of producing huge dynamic jolts while providing great weight, presence, and the ability to keep instruments where they’re supposed to be in the soundstage. A lot of this probably revolves around the power and the great speed of the Krell amp, the latter an area where it excels and one in which no conventional tube amp with output transformers can hope to compete well.

Even on some vocals such as the new Gram Parsons GP/Grievous Angel CD (Reprise 9 26108-2), the KR amps certainly sounded detailed, but showed a rather sterile, analytical character in the mid-bass that was lacking in warmth relative to the solid states amp. The Krell amp was more musical to my ears on this CD, better portraying sound staging and the presence and magnitude of the music. When one gets to bigger music, such as the excellent Lost World CD from Michael Stearns (Hearts of Space, 11054-2), it is no contest. The KR amps did simply not match the major league drum whacks, synthesizer jolts, and room wide soundstaging afforded by the Krell amp. The same held for the excellent Calvary Grand Organ Dedication CD (no number listed), with the Bach piece from track 4 produced with pace, power, and immense sound staging by the Krell. This track has a large dynamic range, in excess of 40 dB I would say, easily leading the VT8000MK amps into clipping (and pretty harsh clipping at that, surprising for a tube amp!). On the other hand, the Krell, with its greater than 6 dB of extra headroom, fully regulated power supplies and 8 kw power transformers, zoomed through any and all of the pyrotechnics and in-room peaks of 100 dB or more with no complaint.

Finally, it should be emphasized that even on smaller scale music, the Krell’s superior vertical and horizontal sound staging, weight, immediacy and presence have great appeal. For example, on the Love Over Gold album of Dire Straits (Warner 9 23728-2), “Telegraph Road” was reproduced with much greater power and palpability by the Krell 600 than the KR amps. On big orchestral music such as tracks 15/16 of the outstanding Reference Recordings Tutti CD (RR906CD), the 8000 amps did a wonderful job on timber and inner detailing. However, when things get hot and the big drums come out along with massed instruments, the KR amps fail to convey the power and emotion of the music as well as the Krell amp.

Yes, maybe some of those massive bass drum and tympani whacks are overdone by the Krell and not entirely life-like, but boy they surely convey the emotion that up front seating at live music has, but the KRs don’t! On heavy rock music, it was no contest, perhaps not surprisingly. For example, Journey’s “Faithfully” (Greatest Hits, Columbia, CK44493) was reproduced via Krell with the power and emotion that I remember from hearing this piece performed live years ago. The KR amp was simply out of its league–nice but not convincing.

Bottom line

The Krell 600 earns the accolades it has received from all reviewers lucky enough to receive review samples. On music with complexity pace, rhythm and/or heavy bass/midbass, the Krell is hard to beat. On the other hand, with ‘small music’, the KR amps clearly prevail, so we have somewhat of a conundrum here–the Krell FPB 600 and KR Enterprise VT8000MK tube monoblocks are both great amplifiers when used with the right music.

In some ways the KRs are the best tube amps I have ever heard, retaining the conventional strengths of tubes with at least a modicum of bass power and definition, dynamics and pace. But if, in addition to jazz, blues, and vocals, you also like “big” music with lots of dynamics, massed instruments, and low bass frequencies, the 8000 amps will not perform as well as a top quality solid state amp such as the Krell 600.

For the overall eclectic mix of music I like, the Krell is the better single choice, but boy the KRs are killer on the stuff they do well. What then to do? I am keeping both amps and switching speaker cables as the mood hits me. While it is too early to say yet whether it will work, I have also been experimenting with what could be nirvana–passive bi-amping! Yes, with the KR amps (fortunately they have variable input sensitivity) feeding the mid/tweeter modules of the VR-8 speakers and the Krell 600 pushing the bass modules. There is promise here for extracting the best properties from both of these outstanding amplifiers.