Krell KSA-200S power amplifier
Ever since I made the acquaintance of my first Krell product—the KSA-50 power amplifier—I've been impressed by this Connecticut-based company's offerings. Krell has managed to maintain its position at or near the leading edge of sound quality since the company was started by Dan D'Agostino and Rondi D'Agostino at the beginning of the 1980s. When it comes to design and build quality, Krell products are first-rate. This month or this year, a Krell product may not always be the very best-sounding in its class, but the chances are that it will come close. What also matters is enduring character and quality—this is where superb build, a durable finish, and a conservative design philosophy pay off.
This review looks at a model from Krell's relatively new amplification range: the KSA-200S stereo power amplifier.
Krell's new KSA-200S is a large stereo chassis, conservatively rated at 200Wpc into 8 ohms (23dBW). The rating continues to double down to a 1 ohm loading, where a 1600Wpc output is specified. Krell's proprietary output-stage bias system delivers dynamic class-A operation up to the maximum 8 ohm power rating, with an increasing proportion of class-AB operation with the higher current associated with lower load impedances. This "Sustained Plateau" biasing scheme is said to offer the sonic benefits of pure class-A without the massive energy inefficiency and correspondingly huge, hot heatsinks. The energy-conscious KSA-200S idles at 120W. If it ran in pure class-A, it would have to idle at around 1kW—over eight times as much wasted energy drawn from the wall.
The KSA-200S accepts unbalanced (RCA socket) and balanced (XLR) inputs. Optimum operation in unbalanced mode is obtained with shorting links tying the unused "cold" XLR terminals to ground. This achieves the lowest hum and noise performances.
In contrast with earlier Krells, the gold-plated, custom-designed speaker terminals accept only speaker cables which have been properly terminated with spade connections. While this is an inconvenience to reviewers and quick-change demonstrators alike, it does ensure high-quality, low-resistance speaker connections. There are two sets of terminals per channel, aiding bi- and tri-wiring arrangements.
AC power is coupled by a 20A-rated, three-wire detachable IEC cable. Full protection is provided against open and short circuits and thermal overload. Front-line protection is via a user-resettable, 20A circuit breaker. It should be noted that the KSA-200S is the central member of a family of very similar amplifiers: the 100Wpc KSA-100S, which is suited to smaller rooms and higher-sensitivity loudspeakers, and the large KSA-300S (footnote 1). (The latter, with its 200-lb shipping weight, verges on a three-man lift!) While the difference in rated power is a modest +1.8dB from the '200S to the '300S, and the same reservoir capacitance is used in both designs, the '300, thanks to its massive power supply, has the sonic advantage under real-world load conditions. The transformer rating is lifted from 2800VA to 5000VA. That "motor" (see below) ensures a fast recovery for the power supply after heavy peak current surges.
The front panels of all three amplifiers are similar, the physical differences lying in their depths: 19.25" for the '100S, 21" for the '200S, and 24" for the big '300S. All three models come with a small remote control which sets power on and determines whether the bias display is on or off. (This display pulses brightly with the music; I found it sufficiently disconcerting to prefer it switched off.) These controls are also present on the unit fascias.
The display reveals some aspects of the amplifier's operation. A central blue LED shows power on, while a triangular set of six red LEDs is surmounted by a further two in blue. For each channel, these LEDs indicate five levels of output-stage biasing according to the output power demand. The top blue LED lights at level five, signifying a full-power, class-A, 8 ohm demand. The idle, or "one," level is signified only by the central "on" LED.
Built from heavy-duty aluminum, the casework and comparatively compact side-mounted heatsinks are finely finished in dark satin grey and black, in the current Krell style. This modern and monolithic design drew blood from both yours truly and the delivery agent. So precise is the computer-controlled milling of the alloy brass plate that its acute angled facets are razor-sharp, even after anodizing. You can unknowingly cut your fingers merely by moving the amplifier into position. [Krell has rounded off those sharp edges in production since January '94.—Ed.]
At first sight, the KSA-200S's toroidal power transformer looks awesome. Heavy-duty, gold-plated, beryllium-copper bus bars carry speaker and power-supply lines to minimize internal impedances. A concentrated short-path design is centered on the heatsink-mounted output devices and the input and output terminals. While the two channels share a common ground reference, in almost all other respects they are independent. Separate secondary windings on the transformer feed Sprague "Powerlytic" reservoir capacitors. There is a total of 68,000µF for each plus and minus voltage rail, a total of more than a quarter of a Farad.
Protection circuitry is comprehensive. A heavy-duty set of relays with shared contacts is the ultimate arbiter, severing the links to the precious loudspeakers when necessary. Massive over-current, overheating, line brownout, and RF or DC faults will activate the relays; under a fault condition, the amplifier remains in standby until a cure is effected. It's heartening to see this quality of protection in an amplifier capable of putting out such high powers. As a further protection against overheating, when the heatsink temperature reaches 80°C, due to prolonged use at full power or use under high ambient temperatures, the bias circuit switches out of class-A and remains in class-AB above levels 3 and 4 until normal heatsink temperatures are regained.
While IC op-amps are used for system control functions—the DC servos, for example—the signal circuitry is all-discrete, with direct-coupled, symmetric-complementary differential stages with FET inputs. These have an intrinsically wide bandwidth and low distortion, due to heavy local feedback. This power amplifier differs from earlier Krells in having no global or loop negative feedback. While some high-feedback amplifiers can sound very good, the general feeling is that a more natural tonal quality, a cleaner high-level sound, and more natural dynamics are all associated with low loop feedback. This is, of course, provided that a respectably low output impedance can be obtained without feedback. (As well as improving static linearity, negative feedback looped around an amplifier lowers its output impedance.) While Krell's new "S"-series amplifiers don't have global feedback, their output stages are self-contained, low-distortion blocks with typical output impedances of around 0.1 ohm.
Footnote 1: Thomas J. Norton reviewed the $9500 KSA-300S power amplifier in January '94 (Vol.17 No.1, p.92).—John Atkinson