Solid State Power Amp Reviews

It's somewhat unusual for two reviews of a product, by different reviewers, to appear in the same issue. However, JA asked me to relate my impressions of the Krell KSA-250 for several reasons: I'd been listening to the KSA-250 as a basis for comparison with the comparably priced Threshold S/550e amplifier (see review this issue), and I'd spent some time with the KSA-250's predecessor, the KSA-200, in the same system and room. The KSA-250's performance could thus be put in perspective with the older unit, as well as with a similarly priced competitor. In addition, this second review could include measurements of the KSA-250's bench performance. Since Lew Lipnick has already described the amplifier's technical details, let's get right to my impressions of the KSA-250's sonic character.

Listening: I auditioned the KSA-250 over the course of several weeks in my usual reference system, as well as driving the two pairs of inexpensive loudspeakers reviewed this issue. The KSA-250 was compared with the Threshold S/550e and to my long-term reference amplifiers, VTL 225W Deluxe monoblocks. The primary loudspeakers were Hales System Two Signatures, connected with 3' bi-wired runs of AudioQuest Clear cable. CDs were played on an Esoteric P-2 transport and decoded by the VTL Digital to Analogue Converter through an Aural Symphonics Digital Standard coaxial cable.

LP playback was via a Well-Tempered Turntable with a Lary Pederson–modified Well-Tempered Arm. Phono cartridges included a Sumiko Virtuoso Boron and an Audio-Technica AT-OC9, stepped up with an Expressive Technologies SU-1 transformer, driving an Audio Research SP11 Mk.II, used for its phono section. An Electronic Visionary Systems Stepped Attenuator provided level control. Interconnects were Expressive Technologies IC-1 and AudioQuest Lapis. AC power to the system was conditioned by a Tice Power Block and Titan, except to the power amplifiers, which were fed directly from the wall socket. The dedicated listening room has optimum dimensional ratios for room-mode distribution.

Immediately upon hearing the KSA-250 for the first time, one knows that this is an extraordinary power amplifier. The sheer power in the bass, effortless delivery, and unaggressive presentation all indicated that the KSA-250 is something special.

Starting with the bass, the KSA-250 had the deepest, tightest, most effortless low-frequency presentation of any amplifier I've heard. The only other amplifier that could compare in bass performance was the Threshold S/550e. I had been used to the VTLs, which have outstanding bass for a tube amplifier, but they were far outperformed by the KSA-250's depth and control. The KSA-250 seemed to add an octave of LF extension to the somewhat lean Hales System Two Signatures. The sense of weight and authority in the low end was stunning. LF dynamics were effortless, punchy, and quick. The lowermost component of bass drum, the rhythmic foundation of much music, had a dynamic impact that greatly enhanced the music's rhythmic drive. Bass lines were easily resolvable, with precise articulation and pitch definition. The entire low-frequency presentation—dynamics, pitch definition, control, weight, effortlessness—was unparalleled.

I felt as though the KSA-250 had an inexhaustible power reserve that was barely tapped, even at high levels. Although the Threshold S/550e had very similar qualities in the bass, the KSA-250 had just a bit more punch and impact, especially in the lowermost frequencies. Both amplifiers, however, are exceptional in this regard. I think there can be no debate as to the KSA-250's extraordinary abilities in reproducing music's low frequencies.

Moving to the mids, I found the KSA-250 to be remarkably smooth and liquid. The amplifier had a laid-back quality through the midrange I found particularly musical and inviting. There was not that dry, up-front aggressiveness I dislike in some solid-state amplifiers. In terms of natural textures, lack of grain, and general sense of ease to the music, the KSA-250 easily surpassed the KSA-200. In comparison with the Threshold S/550e, the KSA-250's more laid-back and less vivid rendering was more to my taste and perhaps associated components. Some may prefer the S/550's more palpable, immediate presentation, however; I advise potential purchasers of either amplifier to audition both, preferably with their own loudspeakers.

I generally prefer good tube amplifiers for their more realistic instrumental textures and subtler rendering, but the KSA-250 went a long way toward achieving tube liquidity and musicality through the midrange. Instruments and voices had a round, rich character, the antithesis of cardboard sterility. Textures were finely woven, accurately conveying the essence of an instrument's character. I never felt the KSA-250 imposed an artificiality to tonal shadings that can be a constant reminder of the playback system intervening between music and listener. In this regard, the KSA-250 is vastly superior to the KSA-200, which I felt to be a bit dry and less than thoroughly involving. Despite the KSA-250's extraordinary portrayal of natural textures, I still consider the VTLs to be the last word in timbral accuracy and harmonic rightness.

The KSA-250's treble presentation was similarly smooth and laid-back, inviting the listener into the music. I never felt like my ears had been through a threshing machine after a long session with the KSA. The treble was relaxed and unfatiguing, yet still conveyed the transient detail and liveliness that creates a big, open, "up" feeling. I was able to listen at fairly high levels without wanting to turn down the volume, unusual for a solid-state amplifier. Detail was not hyped or overblown in a way that assaulted the listener. Musical subtleties were naturally portrayed, without being etched or aggressive. Again, the KSA-250 is far superior to the KSA-200 in this regard. The older amplifier tended to be a bit dry and sterile through the treble, rather than warm and involving. This is, in my opinion, the biggest improvement in the newer model.

The KSA-250 had a nice spatial presentation, with good soundstage depth, feeling of air around instruments, and ability to convey hall size. On Dorian's The English Lute Song CD (DOR-90109), Julianne Baird's voice was enveloped by the superb acoustics of the Troy Savings Bank Music Hall, yet was palpable and immediate. The hall's depth and acoustic character were clearly resolved by the KSA-250. Instruments never assumed a too-prominent position in the presentation. Instead, the amplifier presented images in their correct spatial contexts. There was a distinct feeling of three-dimensionality that I find important if the playback system is to transport the listener outside the confines of his relatively tiny room. In this respect, the KSA-250 was superb, yet did not quite convey the hall depth and a sense of distance that I hear through the VTLs. Lew raised the point that some amplifiers add an unnatural sense of depth at the expense of musical accuracy. This is an unresolved question in my mind, but I still preferred the VTLs, especially with a digital source which tends to truncate the recorded acoustic and present a drier, smaller rendering. The ability to portray depth and space, as well as the listener's apparent distance from the presentation, are perhaps the biggest differences between the S/550e and KSA-250: the S/550e puts you in row 5, the KSA-250 in row 25.

The KSA-250's soundstage had excellent transparency, allowing the listener to see into the very back of the presentation. Instrumental outlines were clearly defined and precise. However, in these areas, the KSA-250 was surpassed by the S/550e. By comparison, the KSA-250 didn't have the utter soundstage transparency, complete lack of veiling, or precise focus of instrumental outlines that characterize the S/550e. This is not to say the KSA-250 was poor in these areas. On the contrary, it was exceptional, but nevertheless came in second to the S/550e. The KSA-250 tended to have a softer focus and less palpable image rendering than the S/550e. The Krell's soundstage also had a bit of congestion that made individual instrumental lines less distinct. This made for a more relaxed presentation through the Krell, yet it lacked the vivid clarity, image tangibility, and pinpoint localization of the S/550e. These impressions perhaps go hand-in-hand with the KSA-250's less immediate and vivid rendering in relation to the S/550e. On this matter, which interpretation one prefers is largely a matter of personal preference and associated equipment.

All things considered, I felt its relatively laid-back perspective, more subtle rendering of detail, greater illusion of soundstage depth and three-dimensional layering, as well as its feeling of ease, make the KSA-250 my first choice in solid-state amplification.

Build Quality: Finally, I would like to comment on the KSA-250's extraordinary build. While it is not as visually striking as the Threshold S/550e, it appears to have beefier construction. This is reflected in the two amplifiers' respective weights: 97 lbs for the S/550e, 140 lbs for the KSA-250. Most of the KSA-250's additional weight is in the massive 4.5kVA transformer, which in itself weighs 85 lbs. One cannot appreciate just how heavy this amplifier is until required to carry it. (Stereophile's test lab is up a flight of stairs!)

I had an opportunity to visit the Krell factory and see KSA-250s being built. The assembly process has been remarkably streamlined by the removal of point-to-point wiring, allowing amplifier sections to be bolted together in modular fashion. This perhaps explains Krell's ability to offer the higher-power KSA-250 for the same price as the more labor-intensive KSA-200.

During the measurements of the KSA-250 and S/550e, JA tried to make a photocopy on the copy machine, only to find he had to wait 30 seconds for its "energy miser" feature to warm up the machine. Five feet away, these two current-hungry amplifiers had been turned on all day!

Conclusion: The Krell KSA-250 sets a new level of performance among the solid-state amplifiers I've heard. Bass reproduction is unparalleled: the KSA-250 imbues music with a solidity and effortlessness that provide a very satisfying musical foundation. In addition to sheer LF quantity, extension, and dynamics, the quality of the low-frequency reproduction was remarkable. It was tight, fast, articulate, yet never dry or sterile. The KSA-250's bass must be heard to be believed. Although the Threshold S/550e had a similarly stunning bass presentation, the KSA-250 had slightly more punch in the lowermost frequencies.

The KSA-250's midrange and treble presentation was surprisingly smooth and liquid. Instrumental textures were finely woven, clean, and free from glare. The treble was remarkably delicate, yet detailed. Musical nuance was presented in a natural way, without becoming etched. The entire presentation had a laid-back, relaxed quality that I found particularly inviting. In comparison with the KSA-200, the newer amplifier clearly surpassed its predecessor in warmth, treble smoothness, and ability to involve the listener in the musical performance. However, I still preferred my reference VTL 225W Deluxe monoblocks for sheer midrange and treble liquidity.

Soundstage depth was equally impressive. The KSA-250 painted a broad and deep sonic landscape, with an exceptional sense of three-dimensional layering. In addition, the KSA-250 could resolve low-level spatial cues that presented a convincing illusion of hall size. I found the KSA-250 portrayed depth and space better than the Threshold S/550e. Instrumental outlines were clearly resolved within the soundstage, yet not to the extent heard through the Threshold S/550e. In addition, the S/550e had a more transparent, less congested rendering. The KSA-250 had a more distant, less focused interpretation.

Beyond these descriptions, I enjoyed music immensely through the KSA-250. It was unfailingly musical, inviting, and intimately involved the listener in the performance rather than the sound. This is the essence of what music-reproduction components should do, and the highest compliment one can bestow on any product. In addition, the KSA-250 is solidly made and exhibits excellent bench performance, especially in its near-perfect voltage source behavior. Considering the KSA-250's superb sonics and bench performance, I can unhesitatingly recommend it for even the most demanding high-end systems. It is hard to imagine anyone not liking this amplifier.

I suspect that Larry Archibald is eagerly awaiting the return of the KSA-250 (Stereophile bought this sample) to his listening room to drive his Thiel CS5s. All I have to do now is think of an excuse to keep the KSA-250 at my house for a while longer.—Robert Harley