Krell Full Power Balanced 350mc monoblock amplifier Page 2

Hand-in-hand, Motorola microprocessors control optimization of Krell's Sustained Plateau Bias II system. The updated circuitry "evaluates" both the musical signal and the speaker impedance and passes that information along to the microprocessors charged with dynamically adjusting the bias plateaus to keep the amp in class-A "during any musical passage or speaker condition." Each bias plateau is sustained from 30 to 90 seconds, so the output stage is always ready to rumble. "Jonathan," Dan enthused, "it's simple. Class-A is the most linear, lowest distortion mode of operation—full current for full power is always available!"

The claimed result is class-A performance with less heat and power consumption than other more conventional designs. The output devices are anything but ordinary, of course. The 350Mc makes use of 28 Motorola high-speed, high-current bipolar transistors per chassis. No overall loop negative feedback is used, but there are "nested" local feedback loops.

The microprocessors also implement sophisticated protection schemes for a variety of conditions—short circuits, excessive heat, "irregular regulator voltage," or excessive DC at the outputs. (All FPB amps are direct-coupled from input to output.) When an "aberration" is detected, the FPB amps disengage their input and output stages, and no one gets hurt.

Low-impedance pathway Current Mode Gain stages are used throughout all components in Krell's Class A line, implemented on military-grade multi-layer circuit boards. (See the CAST section for further details.)

The whole package is contemporary and sharp-looking, big but not massive, and surprisingly well-proportioned. The slightly concave front panel looks rich and attractive, the polished and anodized finish contains an optical brightener giving it a special luster. The convex vertical center section carries an elegant, silver Power button with a triangle of blue LEDs mounted above showing operating status. The Standby LED illuminates when the back panel power breaker switch is engaged, powering up the input circuitry. The Bias LED shines when the Power button is pressed and Sustained Plateau Bias II comes up, and the Regulator LED indicates the regulator is providing power to the output stage. A teeny-tiny red LED in the center of the triangle lights up when the CAST connection is active. The LEDs flash to indicate fault conditions, but can be turned off completely if they bug you.

Around back in a vertical array, just next to a carrying handle, are two pair of speaker binding posts. These are great connectors and make child's play of lashing up all manner of recalcitrant mind-of-their-own cables. Between the upper and lower set you'll find a balanced XLR input next to the CAST connector. Over to the right are a pair of Krell Link connections (Krell Link allows remote operation of the amps), with a heavy-gauge captive power cord below.

When powering up from Standby, the amps default to the CAST input if this is connected, even if you've got an XLR cable plugged in. There is no single-ended input, although adapters are available from Krell. They just don't believe in 'em.

CASTing about for better sound
CAST (Current Audio Signal Transmission) is the keystone upon which all Krell's Class A series components are built. CAST inverts the voltage-mode wisdom that signal transfer from source to load requires a low-impedance output and a high-impedance input. Instead, CAST operates in current mode, with the signal communicated from a high-impedance source to a low-impedance load.

Why is this important? According to Krell, interconnects can "significantly" affect overall signal quality, their impedance—resistance, inductance, and capacitance—reacting with the source and load impedances to change the signal voltage at the amplifier's input. According to the Krell white paper on CAST: "This alteration can amount to as much as a few percent, depending on the exact nature of the source, cable, and load impedances. This may not sound like much, but in an industry where 'goodness' is often measured in hundredths and thousandths of one percent, it's really quite a lot. As cables get longer, these effects increase proportionately. The sonic results of these interactions range from mild to severe, and they form a large part of the mysterious art of 'matching' components and cables for a desired sonic result.

"The idea is basically simple," the documentation goes on, "even if the implementation is a bit complicated: The preamp has a high output impedance measuring hundreds of millions of ohms and delivers current rather than voltage to the load. The amplifier's input senses and amplifies this current, which is not altered by the cable's impedance as these impedances and their reactive effects will be at least a million times less than the total transmission system impedance. In short, cable impedances and nonlinearities are totally eliminated from the picture.

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