Power Supplies
Housed in a separate chassis, the power supply makes use of extensive electrical and magnetic shielding to keep radiated interference out of critical amplifier circuits. Internal high current line conditioning circuitry filters RF noise on the
AC mains, as well as compensating for asymmetric power waveforms and DC
on the mains. A dedicated 165 VA toroidal transformer with six independent
secondary windings powers the amplifier's low level stages, gain stages, and
control circuitry. Power for the high current regulators and amplifier output stages
comes from two massive 2500 VA toroidal transformers. These in turn drive eight
35-amp bridge rectifiers and 120,000 microfarads of filter capacitance. High reliability
connectors with 56 machined, gold plated, 20-amp contacts transmit these
tremendous power reserves to the regulators.
Power Supply Regulation For Absolute Voltage Stability
The rails powering the amplifier's low level and gain stages are regulated twice
for total immunity from fluctuations in the AC mains and virtually noise-free output.
All the regulators use current-mode circuitry for wide bandwidth, low output
impedance, and complete immunity to varying load conditions. The high-current
regulators powering the amplifier output stages use fully complementary gain
stages and output drivers. The resulting wide bandwidth and low output impedance
of the Evolution One surpasses even the stringent demands of the amplifier
output stages.
The driver and output stages in the high current regulators use 14 pairs of the
same robust, high performance power transistors used in the amplifier output
stages. All the regulators are located in the amplifier chassis to provide ultra low
impedance supply rails for the audio circuitry.
Evolution Krell CAST
By employing radical current mirror circuitry, the Evolution components elevate the
CAST technology to another level. This advanced use of the technology increases
the linearity, transient speed, and bandwidth of the Evolution components while
reducing the distortion by an order of magnitude. Together, CAST and
Krell Current Mode technologies unify separate Krell components into a single
global circuit. Krell Current Mode technology enjoys bandwidth increases up to an
order of magnitude greater than their voltage based counterparts. This dramatic
increase in circuit bandwidth delivers near perfection in the audible band that typically
suffers from phase distortions in voltage circuits.
Circuitry Highlights
Audio signal voltages are converted to currents at the amplifier input, and the
audio signal remains in the current domain throughout the entire amplifier until
the output stages. All signal gain is realized in the current domain using proprietary
multiple-output current mirrors with extraordinary open loop linearity. The
current mirrors in the final gain stages use 36 pairs of high voltage, high speed
transistors. Originally developed for demanding, high bandwidth video circuits,
these transistors help yield gain stages with superb accuracy and very low distortion.
The signal path is fully complementary and fully balanced from input to
output. The driver and output stages use 28 pairs of 150 Volt, 14 Amp, 50
megahertz power transistors mounted on Krell-designed high efficiency heat
sinks.
All transistors operate in Class A mode. Independent complementary pre-driver
and driver stages for the positive and negative output transistors make
the output stages extremely fast and linear. The output stage develops 450
Watts into an 8 Ohm load, 900 Watts into 4 Ohms, and 1800 Watts into 2
Ohms. This unique circuit is impervious to low-impedance or reactive loads;
the Evolution One amplifier simply drives any loudspeaker with absolute confidence,
achieving the very best possible sonic results.
Impedance Advantage
The output impedance of the final gain stages is one hundredth that of the final gain
stage in most power amplifiers. This provides extremely wide bandwidth and very
high current capability. The amplifier does not employ global negative feedback—the
final gain stages use only 8 dB of local, nested negative feedback. This is one five hundredth
the amount used in most solid-state power amplifiers. The result is an
extraordinarily open, liquid, effortless sound.
Evolution Bias Control Systems
The new Evolution Bias system features 12 individual bias levels for optimum
performance and efficiency. Evolution Bias constantly compares the audio signal
at the input stage with impedance at the output stage, and adjusts the bias current
accordingly. In addition, a proprietary bias monitor continuously measures
and adjusts the output stage bias, eliminating the need for future adjustment or
calibration.
Advanced Microprocessor Control
If the DC level at the amplifier inputs exceeds a safe level, the microprocessor
control automatically switches in DC blocking capacitors to prevent damage to
the input stages or the loudspeakers. The microprocessor control measures the
load impedance in real time to optimize the performance of the amplifier and
power supply or, in the case of a short circuit, to shut the amplifier off. It monitors
the regulator output voltages, the symmetry between the two halves of the balanced
output stages, and it monitors the operating temperatures of the three
main heat sinks. If any of the heat sinks become too hot, the microprocessor
control adjusts the operating parameters of the amplifier and power supply to
help the circuitry run cooler. If the temperature of any of the heat sinks becomes
unsafe, the microprocessor control shuts the amplifier off until it cools down.
In the event of a major protection fault the microprocessor flashes a diagnostic
code on the power/stand-by LED. The RMS output voltage and current signals
used by the microprocessor are also used by the power meter, on the
amplifier front panel, to display accurate power measurements.
Hi Fi News
"Has Krell built the best ever amp system known to man with its Evolution Series?"
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The Absolute Sound
"There is a level of excellence at which conventional words about quality and cost-effectiveness become meaningless. Like the very best cars, watches, and wines, and every other assault on the consumer state of the art that you can actually buy, price ceases to be an object. You choose because it is a personal passion—if not an obsession—and because you are lucky enough to be one of the handful of people who really can afford the very best.The Krell Evolution preamp and amplifier are cases in point."
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