Krell KSA-50S power amplifier

In the fall of 1982, I had just become the Editor of the English magazine Hi-Fi News & Record Review. Hi-fi was in a state of flux. The Compact Disc had just made its debut in Japan, but the British and American launches were six months and a year away, respectively. The Linn orthodoxy prevailed about the role of the source in system performance, but there was no agreement about what was and was not important when it came to enhancing the musical experience. "Objectivists" insisted that amplifiers and even loudspeakers had pretty much reached a design plateau where no further improvement was necessary or even desirable, while "subjectivists" were fragmented. All I was aware of was that my system, based on Celestion SL6 loudspeakers, needed more of an undefinable something.

Then I tried a modestly powered, hot-running class-A amplifier, the KSA-50, from a new US company: Krell Industries. (The company was modestly named after the legendary super race in the classic 1956 SF movie, Forbidden Planet: "You fool! To think that your ape-brain can contain the full knowledge of the Krell!" exclaimed the marooned Dr. Morbius when he found one of his human intruders dying after trying the Krell mind amplifier.)

Now that was what I'd been looking for, without knowing it. Yes, the amplifier had tremendous dynamic range, coupled with awesomely deep bass, even on the little Celestions. But it was the KSA-50's imaging capability that opened my eyes—and ears—to something that I had not been particularly aware of: the transformation of a traditional stereo image into a fully dimensioned "soundstage."

As I wrote in my 1983 HFN/RR review (footnote 1) of the Krell KSA-50: "The beginning of Andreas Vollenweider's electronic harp LP (CBS FM 37793), for example, consists of immaculately and naturally recorded woodland birdsong. With the Krell, I was in the wood, with a pheasant squawking some 60' ahead of me. The crapgame in the Decca Porgy and Bess sounded as though the dice were actually rolling across the floor of a large bare room adjoining mine, and at the end of Act One of the Colin Davis Tosca...the wall behind the speakers disappeared, putting me in the church almost close enough to smell the wax on Scarpia's moustache as he twizzled the ends. And as for the cannon from the castle of Sant'Angelo, they were in the next village!...And to play the sadly neglected Checkmates' version of 'Proud Mary' was to discover anew the genius of Phil Spector in the way he had captured, in mono, the atmosphere and acoustic of a large hall revivalist meeting!"

A paradigm shift of the finest kind. I bought the review sample of the amplifier with money I'd been saving for a car.

Fast-forward exactly 12 years into the future from that August 1983 review. Sitting between my B&W Silver Signatures is the subject of this August 1995 review: the Krell KSA-50S.

The "S"
The KSA-50S is the smallest member of Krell's "Sustained Plateau Biasing" range of power amplifiers, launched in 1993. The other members of the family are the $9500, 300Wpc KSA-300S (reviewed by TJN in Vol.17 No.1, p.92); the $7500, 200Wpc KSA-200S (reviewed by MC in Vol.17 No.6, p.113); and the $5500, 100Wpc KSA-100S (reviewed by RD in Vol.17 No.9, p.98). The KSA-50S looks almost identical to the other S-series Krell amplifiers, but is less deep. The amplifier's rear panel has one pair of brass binding posts against the bigger amplifiers' two pairs. Input sockets are XLRs (balanced) and RCAs (unbalanced). When the amp is used unbalanced, Krell recommends shunting the "cold-polarity" XLR pins to ground with the supplied jumpers to avoid excessive noise pickup.

The "Sustained Plateau Biasing" concept is a patent-pending way for Krell to have its class-A output-stage cake and eat it too, without paying the penalty of wasting large amounts of wall power as heat. A fast "anticipator" circuit, one for each channel, looks at the incoming signal and increases the output stage's bias current if the level would be likely to turn off one half of the transistor array on half-cycle peaks. In this manner, when there's little or no signal, the amplifier is not being asked to pass a large standing bias current with the concomitant radiation of heat. If the signal level allows it, the amplifier's output bias progressively drops back to the lower levels after 20 seconds or so. If continuous high powers are required that would lead to overheating, the amplifier reverts to classic class-AB operation.

Whereas the larger Krell S-series amplifiers have five bias levels, the '50S has three, indicated by first a red LED lighting for each channel, then a blue one at the third, highest bias level. A central blue LED remains on all the time to indicate that the amp is powered. These LEDs are carried on the central black vertical styling strip on the dark-gray front panel; the KSA-50S doesn't have the glass window of the more expensive Krells.

The KSA-50S is specified at 50Wpc into 8 ohms, but, like Krell amplifiers since time immemorial, it offers considerably more than its specified power. The original paradigm-shifting KSA-50 cost $1800 in 1983; the '50S costs $3300, which is actually cheaper in real terms than its illustrious ancestor.

The first thing I noticed even before I listened to any music was that, despite its "Sustained Plateau Biasing," the KSA-50S runs quite hot. A couple of hours of operation at the highest bias setting (which came on at a peak sound-pressure level of exactly 90dB at the listening position with the 86.5dB-sensitive B&Ws), and the enclosure became too hot for me to keep my hand on for more than a few seconds.

Footnote 1: The KSA-50 was reviewed in Stereophile by Anthony H. Cordesman in September 1985 (Vol.8 No.5, p.84). He liked it too, though he thought it expensive at nineteen-hundred 1985 dollars.—John Atkinson
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