Krell KSA-i400 40th-Anniversary Reference Stereo Power Amplifier

Description

KSA-i400 Power Amplifier

400 Watt Stereo Class A Amplifier with iBias™, XD, &
Sym‐Max technologies

The KSA-i400 is a state-of-the-art 400 watt/channel stereo amplifier.
It incorporates Krell’s most advanced circuit topologies and Krell’s
proprietary technologies iBias, XD, and Sym-Max. This gives the KSA-i400 an
organic, spacious, and detailed presentation along with the rock-solid bottom
end and effortlessness that Krell is known for.

iBias

iBias enables the KSA-i400 to operate in Class A up to its
full rated power of 400 watts with absolute accuracy but without the heat
produced by a traditional Class A design. It does this by using a novel circuit
to unobtrusively monitor the current flowing through the output stage which is
part of a closed loop controller that makes sure this current never drops below
a preset level (Class A means that it never drops to zero). This is what makes
the amp both efficient and accurate.

The efficiency comes from the controller adjusting the bias
to maintain that low preset current level as the signal and speaker load
changes. Traditional Class A amplifiers set it for the maximum level all the time
so they’re always dissipating the maximum amount of heat. iBias is like having
a variable size Class A amplifier. If, for example, the signal and speaker are
only requiring 10 watts, it’s like having a 10 watt Class A amplifier, all the
way up to 400 watts in the case of the KSA-i400.

The accuracy also comes from maintaining that low preset
current level. It doesn’t matter what the signal is doing or how the speaker
impedance changes with frequency, by maintaining that preset current level the
amplifier is always operating in Class A. Other variable bias schemes monitor
the input or output signal to adjust the bias but that doesn’t take into
account varying speaker impedance. This means they will sometimes apply more
bias current than necessary or drop out of Class A operation.

XD

XD is a technique for lowering the amplifier’s output
impedance. This results in better control over the speaker drivers which
improves musical dynamics, depth, and dimensionality. It is very dependent on maintaining
thermal equilibrium in the output stage so that all the output transistors are
contributing equally to the signal output. The KSA-i400 uses a massive aluminum
device bar to mount the output transistors and also employs special temperature
compensation across the device bar to limit temperature variations. With its
low output impedance, the KSA-i400 achieves a damping factor of better than
800.

Sym-Max

Sym-Max is short for “Symmetry Maximization.” The circuitry
in the KSA-i400 is differential, meaning that there is a positive half and a
negative half. The transistors in this circuitry are complimentary, meaning
opposite polarity but with equal characteristics. In practice they are not so
equal which means the circuit is operating asymmetrically. Sym-Max makes small
adjustments in the circuit parameters to remove this asymmetry. The result is
that 2nd order harmonic distortion is almost completely eliminated.

This dramatically opens up the sound stage with more detail
and a better sense of pacing, making for a more realistic experience.

Power

The heart of any power amplifier is the power supply. The
KSA-i400 has a massive power supply consisting of 5400VA of transformers and
188,000uF of filter capacitance. Delivering this power to the output circuit is
a combination of gold-plated circuit boards and silver-plated sold copper buss
bars with a very short physical path for extremely low impedance. This, along
with the audio circuitry enables the KSA-i400 to double its power from 400
watts into 8 ohms to 800 watts into 4 ohms and even 1600 watts into 2 ohms. It
can drive 1 ohm speakers and will deliver a peak current of up to 62 amps.

Circuitry

The KSA-i400 uses Krell Current-Mode circuitry with advanced
current mirroring that is balanced, differential, and direct-coupled. Each
stage is independently tuned and matched to the previous stage for optimal
operation. The output stage consists of 16 pairs of high-power audio
transistors and 8 pairs of audio driver transistors. This results in a
frequency response greater than 100kHz that is flat at 20Hz for effortless
reproduction of the audio band.

Control and Protection

The KSA-i400 is microprocessor controlled and
has Ethernet connectivity and a 12VDC trigger input for easy connection to a
home controller. The Ethernet connection allows access to the KSA-i400 web page
for status information and control. The KSA-i400 is protected against high DC
on the outputs, output short circuit conditions, and power brown outs. The
protection operation is conveniently indicated on the front panel status lights
and the web page.