Duo 125 XD
2 Channel Power Amplifier
125 Watt Amplifier Featuring Krell iBias Technology
20 Hz to 20 kHz +0, -0.08 dB
0.5 Hz to 100 kHz +0, -3 dB
>95 dB, wideband, unweighted, referred to full power output
>112 dB, "A"-weighted
Total harmonic distortion
<0.03% at 1 kHz, at 125 W, 8 Ω
<0.17% at 20 kHz, at 125 W, 8 Ω
Single-ended: 100 kΩ
Balanced: 200 kΩ
Single-ended or balanced: 1.63 V RMS
125 W RMS at 8 Ω
250 W RMS at 4 Ω
89.4 V peak-to-peak
31.6 V RMS
18 A peak
<0.015Ω, 20 Hz to 20 kHz
>530, 20 Hz to 20 kHz, referred to 8 Ω
Standby: 1 W
Idle: 58 W
Maximum: 1350 W
Standby: 3.42 BTU/hr.
Idle: 198 BTU/hr.
Maximum: 4617 BTU/hr.
2 single-ended via RCA connector
2 balanced via XLR connector
2 pairs binding posts
17.1 in. W x 7.65 in. H x 21.10in. D
434 mm W x 194 mm H x 536mm D
60 lbs., 27.3 kg (shipping)
50 lbs., 22.7 kg (unit only)
Krell's New XD Technology
Krell’s “XD” (Xtended Dynamics,
Xtended Dimensionality, Xtended Detail) upgrade for the iBias amplifiers takes
an already great sounding amplifier and raises its performance to the next
level. “XD” is a classic example of Krell’s continuous R&D efforts reaping
benefits across multiple product lines. During the development of the K-300i
Integrated Amplifier we discovered substantial sonic improvements by lowering
the output impedance below traditional norms. So we applied this technique to
our existing line of amplifiers and the improvement was so substantial that it
required a unique designation, hence, “XD.”
It was no simple matter to just
lower the output impedance. This affects the amplifier’s stability and
transient response so each stage prior to the output stage had to be re-tuned
to work optimally with the lower output impedance. This lower output impedance
exerts more control over the speaker drivers and damps out unwanted vibrational
modes, allowing a more accurate reproduction of the original signal.
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
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
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
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 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
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.