Krell KSA-300S Power Amplifier:

Great highly desirable Class A Balanced design...

Serial # 34-40235

7/10 condition some scratches on the face hard to see but overall in great condition.

Sounds great.

Stereophile review@
http://www.stereophile.com/audaciousaudio/krell_ksa-300s_power_amplifier/#WVC45QvR6wr8xhzC.97

From Stereophile: The KSA-300S:
Class-A amplifier operation is extremely inefficient, in terms of both build cost and power consumption. While class-A is often put forward as a sonic touchstone, not all experts agree that the gains are worth the trouble and expense. In class-A operation, the output transistors are conducting at all times, the current through them only dropping to zero (one per cycle) at maximum power. This requires large amounts of power dissipation in the amplifier's heatsinks, even with no input.

The class-A amplifier does, however, operate with the maximum linearity of which it is inherently capable, and little feedback is generally required to achieve acceptable distortion levels. Most audio power amplifiers operate in class-AB, in which each half of the push-pull output stage conducts for slightly more than a half cycle. This allows them to operate in class-A up to some small percentage of their total rated power. Even the Krell KSA-250—Krell's top, last-generation stereo amplifier—was subject to this limitation. (See JA's footnote to Lewis Lipnick's and Robert Harley's review of the KSA-250 in Vol.14 No.1, pp.179–180 for more on this.) To operate in full class-A up to 250Wpc output at 8 ohms (the KSA-250's power rating) would require unthinkably large heatsinks. But the earlier Krell amplifiers still operated deeply enough into class-A to run very warm—even hot—at idle. When, just before the model was discontinued, Stereophile's early KSA-250 was exchanged early in 1993 for the most recent version, the new sample ran decidedly hotter than the old—an indication that it was biased more deeply into class-A than the earlier sample—the one we measured for our 1991 review.

When they designed their new Audio Standard monoblock amplifier, Krell realized that a new approach was needed to maintain class-A performance while at the same time increasing efficiency. The so-called sliding bias design—in which the bias fluctuates rapidly up and down in response to the input's requirements—has been available for a number of years. Krell rejected this approach in favor of a related, though unique and perhaps more elegant, concept, which they have dubbed Sustained Plateau Bias. Following its successful implementation in the $32,500/pair Audio Standard amplifier, the technology has been incorporated into Krell's new "standard line," S-Series amplifiers. Each of these, the KSA-300S included, is designed with five bias "plateaus." Circuitry within the amplifier analyzes the input signal and selects an appropriate bias level to keep the amplifier operating in class-A.

The most obvious question here is: How is the amplifier able to respond fast enough to keep it from being "fooled" by the input into setting too low a bias and thereby dropping into class-B? According to Krell, the bias is set by what they call an "Anticipator" circuit, whose fast (1800V/µs) slew-rate is said to be at least ten times faster than the maximum slew-rate of any musical signal being handled by the output stage. By the time the output voltage has risen, therefore, the bias has already been increased to the appropriate level.

While an upward jump to a higher bias plateau can occur at any time the input demands, a drop to a lower level does not occur immediately upon a drop in demand; plateaus are sustained, by predetermined time constants, long enough to ensure that the drop is likely to be prolonged. While a restoration of higher demand will "reset the clock" and maintain the present (or higher) bias, there are bound to be instances—given music's unpredictable nature—in which the bias is dropping just as a new musical transient is coming along. In this case, it would appear that the fast slew-rate of the amplifier will again come into play, reversing the drop and reestablishing the higher bias. Krell's promotional literature states that "downward changes are made only when the next lowest bias level would accommodate the input for approximately 20–30 seconds"—this would clearly be "anticipation" more worthy of a psychic than an electronic circuit. The KSA-300S's owner's manual states—more reasonably, I think—that the plateaus are held for 15–20 seconds unless continuing demand requires otherwise.

Read more at http://www.stereophile.com/audaciousaudio/krell_ksa-300s_power_amplifier/index.html#KWY9ckb1V0jCkSwB.99

Specifications:

Description: Solid-state stereo power amplifier.

Power rating: 300Wpc into 8 ohms, doubles with each halving of load impedance down to 1 ohm (24.8dBW).

Frequency response: 20Hz–20kHz, +0.0dB, –0.1dB; 1Hz–150kHz, +0.0dB, –3dB.

Distortion: <0.1% at 1kHz, <0.3% at 20kHz, full power.

Slew rate: 100V/µs.

Input sensitivity: 2.5V rms.

Gain: 25.5dB.

Damping factor: >120. S/N ratio: 110dB.

Input impedance: 47k ohms.

Power consumption, idle: 130W.

Dimensions: 19" W by 8.5" H by 24" D. Weight: 185 lbs (net).
Price: $8900 (1994); no longer available (2010).

Stereophile's conclusion:

Conclusions:
Sam knew he was in trouble as he watched Jim disappear down the front walk, stuffing a down-payment check into his pocket. How would he explain this to Mary when she returned from visiting her mother? Oh, well, maybe she wouldn't notice the change. Uh-huh.

That new amplifier does look a bit like the old one, though its black, front-panel trim pieces and less art-deco–ish heatsinks sort of give it away. But how will he explain the less-toasty listening room? A mod, that's it! He'd had the old amp modified! Explaining the preamp would be more of a challenge...

My Curmudgeon Club membership has taken a real beating during this review. I should say something negative. Okay, the amplifier is too heavy to be practical. Forget rearranging your system without help. And the front "handles" on the amp, though stylish, have uncomfortably sharp edges. Tube-o-philes will still be hard sells. It's too expensive for most of the rest of us. In normal-sized rooms with normal-sensitivity loudspeakers, those of us with normal-sized listening-level tastes will rarely or never use its full power (or, for that matter, its extreme low-impedance drive capabilities). The less expensive and less powerful KSA-200S or 100S, which use the same technology (but which we have not yet auditioned) might better fit our needs and budgets.

This review will not please those readers who complain that we review too much expensive stuff. (An equal number seem to complain that we review too much inexpensive stuff.) This Krell tandem costs more than I care to think about, and certainly there's plenty of solid-performing, less–budget-busting equipment out there capable of giving a great deal of pleasure. I am as bothered as any reader by the perceived growth in prices at the high end of the High End—a growth heavily fueled by demand for such products in certain overseas markets.

But what can I say? Costly or not, there's no denying the bottom line: These Krell's are the best-sounding preamplifier and power amplifier I have heard in my system.

Read more at:
http://www.stereophile.com/audaciousaudio/krell_ksa-300s_power_amplifier/#WVC45QvR6wr8xhzC.97

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