Krell KST-100 power amplifier

There seems to be an inexhaustible supply of amplifiers; it's hard to choose which ones to review. Big-name operators are difficult to ignore, while smaller outfits often complain of neglect. In the case of a new and moderately priced introduction from Krell there's no need to find excuses: it's available, it's likely to be important judging from this company's track record, and we'd all like to see just how well it performs.

This review features the KST-100 stereo power amplifier. Initially this product was differentiated from the more expensive Krell components by having an all-black livery. However, as customers showed a preference for Krell's traditional anthracite finish, the KS series is now also available in this finish.

The KST-100 is quite compact by Krell standards, its heatsinks contained within the overall width of the alloy front panel. Described as class-A to half power (more on this later), it is quite powerful at 100Wpc into 8 ohms (20dBW); ie, 50W 8 ohms class-A, with the specified power doubling into successive halvings of that impedance; with 800W quoted into 1 ohm. Simple removal of the top cover provides access to a mono switch which allows the amplifier to perform in unbalanced bridge mode, working as a monoblock delivering 400W into 8 ohms and 800W into 4 ohms (both equivalent to 26dBW, footnote 1). Given the past history of Krell products being able to drive low-level impedances, these figures are believable.

Balanced and unbalanced working is possible: around 1.5V input is required for full output; only the highest-output CD player/processors, capable of giving 3–5V, will be able to drive it directly without a line amplifier.

On the subject of "class-A," the KST-100 simply does not run hot enough on first inspection for this to apply. In addition, the output stage bias current gives a voltage drop of 80mV over each of the four 0.5 ohm emitter resistors (there are four output transistors per totem); this gives a total standing bias of 160mA x 4 = 0.64A. This equates to an RMS class-A output of approximately 6.5W (8.2dBW); a standing current of 2A is required to comfortably support a class-A power of 50W into 8 ohms (17dBW). Krell quotes a 2A total idle current drawn from the 115V AC supply, which is "230W" before rectification. With a typical overall efficiency of 25% at these low powers, this alone would only support a class-A output of 29Wpc with optimum impedance matching, in this case into loads over 16 ohms. I therefore must refute the claim for the KST-100 to operate in class-A to one-half its rated power into 8 ohms, but I'm also happy to say, "So what!" Absolute proof that class-A is correct for the best sound is hard to come by. All that is required is a sufficiently healthy bias level at standing current to overcome crossover and related output-stage nonlinearities.

Power connection to the KST is via an IEC AC plug, while both phono and XLR input terminals are provided. In unbalanced mode, small shorting pins are provided for the unused XLR signal polarity. Five-way binding posts are provided for speaker connection, their hex heads allowing the use of a wrench for good contact. Two sets are present, to aid bi-wiring arrangements. The only front-panel control is the illuminated on/off rocker, a circuit-breaker type included in the amplifier protection system.

Design Notes
A reduced-size KSA-80 running at a much lower bias level, the KST-100, like the KSA-80, uses a single central toroidal transformer feeding separate rectifiers and reservoir capacitors for each channel. By normal standards, these are generously rated, this necessary in view of the 1-ohm load, 800Wpc rating. Eight high-current, TO3-can power transistors are used per output channel, arranged as complementary pairs, with two more employed as burst-proof emitter-follower drivers.

The design uses a fully complementary balanced circuit, symmetrical throughout, DC-coupled and DC–servo-controlled for output offset. Effective over-current, short-circuit, and overheating protection are provided.

At the input, cascades of differential FET and bipolar stages are used to provide a true balanced input of beneficially high-input impedance. Following the practice of the latest series, the KST-100 is essentially "wireless," with the amplifier cleverly integrated on one full-sized printed circuit board which integrates with the large-screw terminal power-supply reservoirs.

This build quality is first-rate, and a claim to longevity is supported by the 5-year warranty.

First impressions were of a thoroughly up-to-date, low-feedback sound, one of greater delicacy, air, and transparency than the sound traditionally associated with solid-state electronics. The KST-100 did have a good proportion of Krell character: a firm, confident approach, highly stable imaging, and a strong, highly controlled bass. I rate Krell's KSA-150B highly, and found it to have much in common with the (relatively) budget-priced KST-100. In this context, it was a touch more dynamic and a shade clearer than the KSA-80. Given that the '80 came top in its class only a year or so ago, this is high praise indeed for the KST-100.

Taking the stereo performance in more detail, it was close to today's best in image depth and the transmission of hall and stage acoustic. It could help define the performance of top digital replay systems, and sounded genuinely transparent. Direct comparison with the '80 showed the latter to have a mildly cloudy effect with a shortfall in stage depth. Here the KST has moved rather closer to the ARC Classic 120. Specific focus was also close to the best, held both in terms of width and depth. Stage width was virtually to the limit of available program, and only fractionally less than the best monoblock amplifiers in percentage terms.

Possessing a fine tonal balance (to be discussed separately), the KST-100 was also capable of a strong presentation of perspective, with convincing layering of players in large orchestral recordings. I do not believe height is a property of electronics which are inherently neutral; however, certain combinations of room acoustic, phase, and perceived system frequency response, particularly in the 2–8kHz range, can give rise to sensations of image height which may be enhanced by the accurate and transparent reproduction of recorded ambience. In this latter respect, the KST was very capable.

The KST-100 conveyed a dynamic, lively quality well beyond its class, remaining interesting and alive over long listening sessions.

Dynamics do not only relate to inner vigor but to an overall feeling of power, specifically dynamic range. Technically, this can be described as the range in dB between the inherent background noise and the maximum level possible with a given loudspeaker load. Subjectively, this also concerns an amplifier's ability to reproduce low- and moderate-level detail while being exercised by powerful ones, and its ability to sound unchanged and unstrained when crescendi arrive.

The power amplifier passed this test with traditional Krell imperturbability. The much more expensive KSA-150 does have more authority, notably expressed in the bass as an additional measure of slam and extension. I rate the KST highly, but on the bigger speakers it lacked a little in gut-wrenching ability. Nevertheless, the KST's bass was taut, agile, tuneful, and had good extension; I doubt that there is a class rival for bass precision.

The broad midrange was highly neutral, in the KSA-150 mold, and this continued way into the treble register. This formed the foundation for its ability to portray perspectives. The mids sounded easy on the ear, very low in solid-state artifacts. Glare, hardness, etc. were all at very low levels.

However, the story strayed from the true path as we entered the high treble. Here the KST descended somewhat from the heights, showing a lazier, mildly emphasized effect in the last half-octave of audible treble. With classical program, and much CD material reissued from older tapes, this region is rarely entered with any power; here the KST was as good as gold. However, more recent pop material, with Aphexed or similarly doctored, "zingy" vocals, gives the KST a hard time. The resulting effect is akin to hearing a distant wasp's nest, and took a little getting used to.

This is a sensitive area. One's perception of this anomaly depends on many factors, including the treble quality and smoothness of one's chosen speakers, the "cleanness" of the sound of the system cables, and finally the preceding audio chain, both source and control unit. Cables with sweet treble and speakers without any lift in the high treble are advised for use with the KST-100.

Taking all these points into consideration, in absolute terms, the KST-100 matches the performance level of the earlier KSA-80B without sounding the same. Here preference might well play its part.

I found the KST and KSL highly complementary. The best subjective and objective dynamic range was obtained with the balanced link used between them, in this case one of the Dutch Siltech Silver cables, balanced and screened. Interestingly, the KSL also showed a tinge of the same "zippy" quality in the high treble, but overall worked so well with the KST that it would be difficult to suggest an alternative.

In creating the KST-100, Krell has succeeded in trimming the fat from the audiophile KSA series. This recent introduction is definitely not a cheap version of the old KSA-80; rather, it is a cost-effective cousin of the new KSA-150. In absolute terms it equals the sonic performance of the KSA-80 and has a comparable power-output delivery, though it did not sound quite the same. Here you have to balance performance in terms of the bass speed of the KST and the absolute slam of a bigger Krell, or the lively, quick, detailed midrange of the KST vs its mild zippiness in the high treble.

Given a wise choice in the matching system—for example, a clean-sounding analog cartridge or digital source, plus a speaker without any treble fizz (leaning more to Apogee than Magneplanar, for example)—I would rate the KST highly, undeniably a class leader. Where this standard is required at a much higher power level, remember the facility for the use of a pair of KST-100s as 400W/8 ohm monoblocks. These would be leaders in their power class too.

With the option of balanced and unbalanced working, excellent finish, durable build quality, plus the five-year transferrable warranty, the KST-100 represents a worthy audio investment.

Footnote 1: Applying a –3dB correction for the halving of load impedance from the initial 8 ohm reference gives, I feel, a more easily understandable notion of the manner in which a power amplifier's output voltage drops with increased current. In other words, as a perfect amplifier will give the same number of dBW every time the load impedance halves when the –3dB correction is applied, the departure of a real-world amplifier from this paradigm is easily grasped.—John Atkinson
Krell Industries, LLC
45 Connair Road
Orange, CT 06477-3650
(203) 799-9954