Krell Duo 125 2-Ch Power Amplifier


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SKU: Duo 125
Brand: Krell


Krell Class A iBias Technology

Krell’s history is rich with breakthrough Class A amplifiers that have helped
build the Krell legacy of offering the best sounding amplifiers available.
Audiophiles have always considered Class A technology to be the best sounding
operating state for amplifiers. However, despite Class A’s unrivaled sound
quality, it has fallen out of fashion because of recent demands to reduce power
consumption and heat in home electronics products. Krell engineering took this
challenge and redefined the meaning of high performance power amplifier. Our
goal – unmatched performance, elegant design, and a compelling array of
features. The breakthrough – a patent pending circuit delivering Class A
operation without the excessive heat and wasted energy of conventional designs,
housed in a striking new form factor, with network connectivity for advanced
access and monitoring. The sound is open and unconstrained, in a manner that
rivals live performance and the true sound of voices and instruments. Music and
dialogue are reproduced with a richness, detail, and startling dynamics that
fill a room.

Plainly stated, Class A designs are the most musically
accurate circuit topology available. Class A amplifiers do not suffer from the
inherent distortions that all Class AB amplifiers experience. In a traditional
Class A design, the output transistors conduct full current at all times
regardless of the actual demand from the speakers. Often, only a fraction of
this power is needed to reproduce an audio signal at normal listening levels.
The rest of the power is dissipated through the amplifier’s heat sinks,
producing large amounts of wasted heat. With Krell’s iBias™ technology, bias is
dynamically adjusted, so the output transistors receive exactly as much power –
but only as much power – as they need.

Krell’s iBias Class A technology allows our latest amplifiers
to run in full Class A mode to full power while minimizing heat generation.
Previous efforts at using a “tracking” bias, while effective, only
measured the incoming signal and set bias levels from this information. Our new
patent pending iBias technology significantly elevates the effectiveness of
previous designs by calculating bias from the output stage. This seemingly
small change in topology results in a dramatic improvement in sound quality,
especially midrange richness and purity.

The core of the technology is an innovative, patent pending
design for a dynamic intelligent bias circuit. Our iBias Class A circuit
directly measures the output current of the amplifier and adjusts the bias to
the optimum level. Because iBias Class A measures the output current, the real
time demands of the specific speaker connected to the amp are directly
incorporated into the circuit function. In addition, iBias Class A even reduces
the bias when the signal is at very low levels, making its operation undetectable
by ear and even by standard amplifier measurements.

In sliding bias schemes, the circuit merely estimates how
much bias is needed based on the input signal and an “assumed speaker
load.” Compared with iBias Class A, these sliding bias technologies are much
less effective – and much less accurate.

Power Supply Highlights

Power supply technology has always been an important contributor to the Krell
sound. The power supplies of our Krell iBias Class A amplifiers have been
optimized for use with the iBias circuit. Depending on the model, up to four
toroidal transformers feed amplifier modules that include the audio circuitry,
rectifier, and power supply filtering mounted to an individual heat sink. This
design shortens the electrical path from the power supply to the output
transistors, reducing the overall impedance and allowing the circuit to respond
faster and control the speakers even better and more accurately.

Unlike traditional Class A amplifiers, iBias Class A amps
have a compact design that allows rack-mounting, making them ideal for custom
installation as well as traditional audiophile systems. This convenient form
factor is made possible through thermostatically controlled ventilation fans.
The fans used are specifically chosen for quiet operation, and operate at the
speed required to maintain the ideal internal temperature. They run only during
periods of peak energy demand – when the music is at its loudest – so they are
inaudible in normal use.

Network Connectivity

The new amplifiers include RJ 45 Ethernet connectivity and
an internal web page that is accessible from any smartphone, tablet, or
laptop. Network connectivity brings convenience, monitoring, and reporting to
end users. Amplifier configuration options include display brightness and
timeout. For energy conservation, the amplifiers can be programmed to power
off at a preset time of inactivity. Individual channels can be muted and
firmware updates can be initiated from the web server.

Once the amplifier is connected to a network router with
Internet access, the amplifier’s advanced protection systems are now viewable
on an Internet-connected device. Excessive current, output DC, fan speeds,
short circuit, and overheating are all monitored in real time. If an issue
occurs, the fault is displayed on the front panel and reported on the web
server interface. Emails will automatically be sent to as many as three email
addresses to notify the end user and/or the dealer of the condition.

Circuitry Highlights
The new iBias circuitry is built on a foundation of core Krell circuit
technologies. All signal gain is realized in the current domain using
proprietary multiple-output current mirrors with extraordinary open loop
linearity. Each amplifier channel uses all discrete components. There are no
generic integrated circuits or op amps used anywhere. Gain is distributed
among several stages, allowing each to have a large linear operating area.

Audio signal voltages are converted to current at the
amplifier input, and the audio signal remains in the current domain
throughout the entire amplifier.

Current mirrors in the final gain stages use a new output
power device that operates at a 73% higher voltage, delivers almost 10% more
current, and offers 120 watts of additional power handling capability as
compared to other devices.


With this combination, the iBias amplifiers may now deliver
substantially more power while using a smaller footprint. Normally used in
demanding, high-bandwidth video circuits, these transistors allow the design of
gain stages with superb accuracy and very low distortion. The signal path is
fully complementary and fully balanced from input to output. Independent
complementary pre driver and driver stages for the positive and negative output
transistors make the output stages extremely fast and linear. This unique
circuit is impervious to low-impedance or reactive loads; it simply drives any
loudspeaker with absolute confidence, achieving the very best possible sonic

Most amplifiers use coupling capacitors in the signal path
to block DC and prevent damaging offset voltages from reaching your speakers.
Krell amplifiers are fully direct-coupled, with no capacitors in the audio
signal path. This design gives the Krell amplifiers lower internal impedance,
which allows firmer, more precise control of your speakers. It also provides
flatter, more extended low-frequency response, because coupling capacitors not
only block DC but also affect the lowest bass frequencies. Krell employs
expensive, non intrusive DC servos that remove DC without impacting the musical
signal. Thus, the iBias Class A amplifiers deliver the full breadth of the
music with detail, impact, and space intact.

Everything Audiophiles and Home Theater Fans Could Want
in an Amplifier

Krell iBias Class A amplifiers are the first to deliver the rich musicality of
Class A amplifiers, the uncompromised dynamics of classic Krell amplifiers, and
the efficiency and low power consumption of Class G and H amplifiers.

Because the iBias circuit eliminates crossover distortion,
the amplifier is able to resolve more of the detail and microdynamics in even
the best analog recordings. Simply put, the music breathes. Whether an iBias
amplifier is called on to reproduce the extreme dynamic range of high-resolution
digital files, the minute intricacies of a 45-rpm, 180-gram vinyl record, or
the complexity of today’s latest blockbuster action film, it does so without
altering or abating the music in any way.

The iBias amplifiers’ unprecedented ability to retrieve the
subtlest details gives their sound an incredible dimensionality, with an
ambient, broad and extraordinarily deep soundstage. As spacious as the sound
is, though, the amplifiers are still able to produce pinpoint stereo imaging if
the recording calls for it.

It’s all the power and control for which Krell has always
been famous, with a level of resolution and musicality in the midrange and
treble that has simply never been heard before.

In short, the new patent pending iBias Class A Krell amplifers
give today’s audiophiles and home theater fans everything they could possibly
want in an amplifier.

Additional information

Weight 0.00000000 lbs
Center Piece Finish

Black, Silver


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