KRELL Duo 175 XD Stereo Power Amplifier

Meet the Mighty 175 XD Stereo Power Amplifier

Audio Engineering at its Finest with Krell’s New XD™ Technology

Krell’s XD (Xtended Dynamics, Dimensionality & 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. When applying this technique to our existing line of amplifiers, the improvement was so substantial, it deserved its own, unique designation—XD.

It was no simple matter merely lowering 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. Our revolutionary, patent pending circuit delivers true Class A operation without the excessive heat and wasted energy of conventional designs.

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.

Advanced Circuitry

The new iBias circuitry is built on a foundation of core Krell circuit technologies. All signal gain is realised 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 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 & Home Theatre Buffs could ever ask for and some

Krell iBias Class A amplifiers are the first to deliver the rich musicality of Class A amplifiers combined with 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 amplifiers give today’s audiophiles and home theatre fans everything they could possibly want in an amplifier. 

Technical Specifications

Frequency response
20 Hz to 20 kHz +0, -0.08 dB
0.25 Hz to 100 kHz +0, -3 dB

Slew rate
40 V/μs

Output impedance
<0.015Ω, 20 Hz to 20 kHz

Damping factor
>530, 20 Hz to 20 kHz, referred to 8 Ω

Power consumption
Standby: 1 W
Idle:73 W
Maximum: 1500 W

Heat output
Standby: 3.42 BTU/hr.
Idle: 250 BTU/hr.
Maximum: 5137 BTU/hr.

Inputs
2 single-ended via RCA connector
2 balanced via XLR connector

Outputs
2 pairs binding posts

Dimensions (W x H x D)
434 x 194 x 536 mm 

Weight
31.8 kg (shipping)
27.3 kg (unit only)

Warranty
5 Years for Parts and Labour