Krell Duo-175 175 Watt Amplifier Featuring Krell iBias Technology
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.
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.
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
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
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 results.
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.
|Frequency Response ||20 Hz to 20 kHz +0, -0.08 dB|
0.5 Hz to 100 kHz +0, -3 dB
|Slew rate||40 V/µs |
|Signal-to-noise ratio||>95 dB, wideband, unweighted, referred to full power output|
>115 dB, "A"-weighted
<0.038 , 20 Hz to 20 kHz |
|Damping factor||>210, 20 Hz to 20 kHz, referred to 8 O|
|Total harmonic distortion||<0.03% at 1 kHz, at 175 W, 8 O
<0.13% at 20 kHz, at 175 W, 8 O
|Power consumption||Standby: 1 W|
Idle: 73 W
Maximum: 1500 W
|Input Impedance||Single-ended: 100 kO|
Balanced: 200 kO
|Heat output||Standby: 3.42 BTU/hr.|
Idle: 250 BTU/hr.
Maximum: 5137 BTU/hr.
|Input sensitivity||Single-ended or balanced: 1.84 V RMS|
2 single-ended via RCA connector|
2 balanced via XLR connector
|Output power||175 W RMS at 8 O|
350 W RMS at 4 O
2 pairs binding posts
|Output voltage||106 V peak-to-peak|
37 V RMS
|Dimensions||17.1 in. W x 7.65 in. H x 21.10in. D|
mm W x 194 mm H x 536mm D
|Output current||13 A peak|
|Weight||70 lbs., 31.8 kg (shipping)|
60 lbs., 27.3 kg