Krell – Duo 175 XD – 175W Stereo Class A Power Amplifier with iBias

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.

“The New XD amplifiers have… warmth… tonal saturation… a level of bass slam that made Krell famous.”

~ Tone Audio

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.

Krell’s 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.

Krell’s Major Technological 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.

“I was absolutely blown away.”

~ Dennis Burger, Home Theater Review

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.

Krell’s New XD™ Technology

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 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 of 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.