Solid State Power Amp Reviews

The maximum output available from an amplifier, and, more pertinently, the maximum clean acoustic output it can achieve with the owner's chosen speakers in a specific room, are controlled by the true speaker loading, the true speaker sensitivity, and the capability of the power amplifier when driving this load.

In theory, an amplifier doesn't need to have a high power reserve, since most loudspeakers are specified as having a 4–8 ohm impedance, typically averaging 6 ohms. In practice, however, loudspeaker impedances can frequently be unexpectedly severe. A nominally 8 ohm loudspeaker may feature impedance minima of as low as 2 ohms; even lower values are not out of the question with rhythmically demanding and complex program.

Loudspeaker sensitivity is often inflated by manufacturers. A once-common method was to specify the acoustic output in the speaker's loudest region rather than the preferred main reference band. In addition, the sensitivity of many "real-world" loudspeakers is given against an "8 ohm" watt: ie, a 2.83V input voltage, which will raise 1W in an 8 ohm resistor. A loudspeaker with an impedance of 4 ohms will draw double the current with test voltage; it will therefore command a specification apparently 3dB greater at the expense of double the real number of amplifier watts!

While a true 8 ohm, high-sensitivity speaker might just be fine if driven by a well-specified tube power amplifier featuring a normally modest adverse load capability, another so-called sensitive "8 ohm" speaker might in practice offer a severe 4 ohm load, which would suck the heart out of such an amplifier. Losses in clean acoustic level of 10dB are quite possible under such conditions—your 100W-into-8-ohms tube amplifier might achieve the equivalent of only 10 clean watts when poorly matched in this way.

Krell's view is simply to make their amplifiers essentially immune to loading. If a difficult speaker demands four or eight times the nominal current expectation, designs such as the KSA-200S remain unperturbed. If nothing else, this helps to provide a consistent level of quality regardless of the speaker choice. Indirectly, you can use this aspect to argue that it makes for a superior amplifier, since it will achieve its performance target under a wider range of conditions.

Listening
Once I had converted my speaker cables to spade terminals, the KSA-200S was put through its paces after first being thoroughly run in. There were no problems as regards loudness: this new design played almost as loud as a pair of the older MDA-300s, and it clips so sweetly that this can pass unnoticed on quite a lot of material, especially heavy rock. This is a big amplifier; its generous margin of headroom and low-end grunt was clearly evident when compared with my reference amplifiers.

The KSA-200S was not just louder than its '100S sibling; it also sounded more confident, more relaxed. Yet it could still produce the necessary thunder when called upon to deliver it. I can well believe that the near-doubled toroid rating employed in the KSA-300S can do the same trick at a still higher level.

Throughout the listening period, I was never aware that the plateau bias system was operating. There were no detectable changes of pace or character, and no spurious noises. Krell's literature promised a quick warmup, and it certainly sounded pretty good from cold, proportionately better in the first few minutes than many other designs achieve in the first 20. Nevertheless, used from the idle state, it clearly sounded better over the first few minutes than from a cold start. For the perfectionist, a quick blasting with high-level program seems to help the amplifier clear its throat and quickly reach peak operating condition.

The plateau idea begins to make sense when you observe the LED bias display in action. Once the music begins to cook, the KSA-200S instantly picks an appropriate bias level and settles into that class-A mode for the duration, by which I mean as long as the music plays that loud, plus half a minute for safety. It was obvious that the KSA-200S did not spend much time changing bias. On some rock tracks—from Genesis's We Can't Dance, for example—it never changed at all. The more I used the KSA-200S, the more confident I became that it was setting its bias appropriately. Eventually, I turned the display off, leaving the amplifier to fend for itself.

Krell's bigger amplifiers weave their own magic; as their recent designs sound more subtle than their predecessors, so their virtues may take a little longer to appreciate. For example, I came to realize that the '200S had considerable transparency, if not quite in the picture-window category. Initially, however, its lack of emphasis, edge, hardness, or roughness suggested the reverse criticism: that it sounds too soft and lacks clarity. Soon, however, I began to realize that the amplifier's subtle, laid-back transparency was helping to portray very good levels of detail, with natural, well-layered depth perspectives. The sound was not thrown unnaturally forward, and there was a "low-feedback" grace to the presentation. Stage width was excellent, and image focus was rated very good to excellent.

Through the treble, the '200S sounded unobtrusive—nicely balanced and well-detailed. At the limit, it still had a mild shortfall in delicacy and air; this could be heard as a hint of "dulling" on finely graded treble harmonics. The midrange was neutral, finely defined, and with powerful-sounding peak dynamics. High sound levels reproduced cleanly, without tonal shifts or compression.

The amplifier was a powerhouse in the bass. While not sounding as crisp or as sharply defined as some competitors, it had terrific slam and reach. Very low frequencies were effortlessly generated at considerable power, the amplifier never missing a beat. On the right system—notably the Apogee Mini Grand and the Wilson WATTs and Puppies—the Krell displayed good pace and rhythm. It was fascinating to hear the lovely but often earthbound Apogee Stage taking off in the actively crossed-over Mini Grand system (footnote 2). It could produce topflight, wide-range bass, mid-, and treble with the '200S as motive power.

The often-opposed parameters of strength and inner balance could well define the sound of the KSA-200S. It sounded equally at home on orchestral or rock music; it was in its element on complex, wide-range material, where its open and authoritative high-level performance really counted. The KSA-200S scored surprisingly close to my reference dual-mono Krell MDA-300 combo. It also (just) beat Krell's earlier KSA-250. The new design simply sounds more musical, more natural in the longer term, than the old. Given its price and quality, it rates in the top group of today's amplifiers.

Conclusions
A careful conclusion to a responsible review must uncover the true balance presented by the product under test. While all manufacturers wish for rave reviews—in fact, they often insist that anything less is of no marketing use—in the real world, products worthy of raves are few and far between. And, surely, that is how it should be—a surfeit of raves devalues the entire reviewing structure.

Manufacturers should give Stereophile's readers and their potential customers a little more credit for their intelligence and discrimination. A favorable review can be less than the ravings of a lovesick reviewer, yet still do the product justice by setting it in its context and supporting a sensible purchasing judgment.

So it is with the Krell KSA-200S. It is an immensely capable power amplifier, yet it doesn't get an uncritical rave review. Krell's new KSA-series amplifiers are better than their predecessors, but only incrementally so. A rave review would require perhaps a sound radically better than (or perhaps just different from) anything that has gone before. Nevertheless, after reading the following summary, you should be left in no doubt as to my positive opinion; indeed, the facts speak for themselves.

Here's what you get for your $7500:

• a thoroughly up-to-date, energy-efficient, and good-looking solid-state stereo design;

• a performance closely approximating genuine class-A output stage operation without the power and heat penalties, and which allows for a relatively compact amplifier chassis for its power rating;

• a conservatively rated, true 200Wpc-into-8-ohms, 1600Wpc-into-1-ohm power output with ample current capacity and the ability to drive any conceivable load to the amplifier's rated level with very little variation in sound quality;

• a sound which varies little, whether driven balanced or unbalanced, thus enhancing the amplifier's versatility;

• a sound quality which builds upon established Krell strengths of image stability, low-end grunt, sense of musical foundation, and higher-than-expected dynamic range. Improvements over earlier Krells concern treble purity, tonal accuracy in the midrange and lower midrange, superior microdynamics, better pace, and a more spacious, more ambient soundstage. The result is both more musical and more satisfying in the longer term; and • a fine finish, a first-rate build quality, system remote facilities, and a five-year transferable warranty which helps maintain the amplifier's value.

Dependable and even-tempered, the Krell KSA-200S gets my recommendation. It may not be exceptional in any one respect, but it is exceptional by virtue of its fine balance of properties. This is as true of the '200S as it is of the matching Krell KRC-2 preamplifier, and positions the Krell at the forefront for amplifiers of its class.

Speculation as to its audiophile status indicates that it would be a Class A contender in Stereophile's "Recommended Components." Its ultimate rating will, of course, depend to some degree on how the performance of the competition shapes up.

OK, it might not be a tube design, but it's damn good nonetheless!

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Footnote 2: See Vol.17 No.3, March 1994, p.101.—John Atkinson