KRELL DUO-300XD 300w Stereo Power amp class-A w/ iBias technology

KL 12 AS D300
NZ$ 20,995.00 ea (incl. GST)
Krell

The Leader in Audio Engineering

New

KRELL DUO 300 - Winner of Hi-Fi News Editor’s choice award

"The Krell showed blissful attack with authentic decay, and just the right amount of dryness with the percussion that opens ‘Up On Cripple Creek’. It picked up the snap of the percussion, the kick-drum air movement, with true ‘feel’. Yeah, this is a Krell, alright. Salvation came from Lou Rawls’ At Last [Blue Note], a bit of recording perfection. It was suitably silky, Rawls’ vocals were languorous perfection, while Dianne Reeves sang as clear as a wine glass from Zalto".

A comment on the new Krell sound from our XD upgraded amplifiers…Initially, there’s no comparison between the two models (pre and post XD upgrade) – the jump in resolution and musicality is massive. It’s like they are channeling the sound that made Krell famous, while incorporating the changes in technology that have come to be over the last 35 years

When developing the DUO300-XD, David Goodman, our Director of Product Development, whom has been with Krell since 1987 and was largely responsible for design work on all Krell products, discovered that with modifications to the output stage, we could vastly improve sound quality, across the board, to all of our amplifiers.

THE RESULT
A deeper, darker, blacker background that provided significantly better macro and micro dynamics, more silence between the notes. Vocals and midrange took on an organic, yet more vibrant tone, enabling us to hear much more body, and even though our amps were great before, there was a very significant improvement. 
The speakers disappeared; yet, instruments were more focused with appropriate size and specific soundstage locations. All in all, a much better sounding, much improved experience that was easily, immediately heard by all that listened.

A few comments on the new Krell sound from our XD upgraded amplifiers…
“Initially, there’s no comparison between the two models (pre and post XD upgrade) – the jump in resolution and musicality is massive. It’s like they are channeling the sound that made Krell famous, while incorporating the changes in technology that have come to be over the last 35 years. 

The raw emotion that generated so much excitement over this brand in the 80s is back in spades, yet without losing that magic, there is more low level detail, and more refinement. This is a tough combination to master, yet Krell has. It’s like taking a car with a great paint job, wet sanding it with 2500 grit sandpaper and then buffing to a higher luster than you thought possible – yet there you are.”...... -Jeff Dorgay – Tone Audio - Full comparison review, here.

“I hate the Doors. Hate 'em! Yet I loved the sound of "Riders on the Storm" from the big system in VPI's demonstration room. The source was VPI's brand new 40th Anniversary Direct Drive turntable, Krell Illusion II preamp and two new Duo 300XD amplifiers, and Wilson Alexx loudspeakers. The system had the sort of extraordinary, large-scale spatial performance that paid musical as well as sonic dividends—each instrument was completely separate from the others—and was relentlessly, engagingly propulsive.”......-Art Dudley – Stereophile

“Powered by Krell Audio, tied together with Transparent cable and Genesis Maestro speakers at the end of the chain, the system had a large sound/sound-stage. The sound was natural, organic, and still took apart the notes in a clean and detailed way.”.......-Mat Weisfeld – VPI Industries

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 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 and Home Theatre 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 amplifiers give today's audiophiles and home theatre fans everything they could possibly want in an amplifier.

Features

Specifications

Reviews

Testimonials

Features

Circuitry Highlights 
Krell iBias Class A technology allows our latest amplifiers to run in full Class A mode to full power while minimizing heat generation. All signal gain is realized in the current domain using discrete, proprietary, multiple-output current mirrors with extraordinary open loop linearity. There are no generic integrated circuits or op amps used anywhere. 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 previous devices. The signal path is fully complementary and balanced, Independent complementary pre driver and driver stages for the positive and negative output transistors make the output stages extremely fast and linear. 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. 

Power Supply Highlights 
Two 750VA 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 better and more accurately. 

Feature Highlights 
Network connectivity brings convenience, monitoring, and reporting to end users from any smartphone, tablet, or laptop. 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. Excessive current, output DC, fan speeds, short circuit, and overheating are all monitored in real time. Messages are displayed on the front panel and reported on the web server interface. Additionally. end users can be sent an email notifying them of any operational anomalies.

Krell Class A iBias Technology
Krell's new line of iBias amps promise class A performance without the usual pitfalls—high power consumption and lots of heat. The new amps use a fan-cooled chassis to pull off that trick and the result is a series of highly powerful yet compact amplifiers. The new iBias design allows the amp to actively measure the current flow to the speakers and adjusts the amps bias to meet demand—if full power is called for, then full power is what you get. However, if full power is not needed, the amp adjusts the bias so that energy isn't wasted the way it is in traditional class a designs.

The new amps fit in a standard rack—something that was not possible with old-school Krell amps. This was achieved by using ultra-quiet fans—active ventilation. That makes these new amps a great fit for high-end audio and home theatre installations. The new amps are as transparent and dynamic as we have come to expect from Krell.

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 unrivalled 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.

Specifications

2 Channel Power Amplifier
300 Watt into 8ohms featuring Krell class-A iBias technology 
2x Single-ended RCA and 2x Balanced XLR Inputs
1 RJ 45 Ethernet Port
1x 12V Trigger Input
WBT Speaker Binding Posts
Optional Rack Mount Kit
Size: 433W x 177H x 531D mm
Weight: 32kg

Reviews

KRELL DUO 300 - Winner of Hi-Fi News Editor’s choice award
Ken Kessler

The Krell showed blissful attack with authentic decay, and just the right amount of dryness with the percussion that opens ‘Up On Cripple Creek’. It picked up the snap of the percussion, the kick-drum air movement, with true ‘feel’. Yeah, this is a Krell, alright.

Salvation came from Lou Rawls’ At Last [Blue Note], a bit of recording perfection. It was suitably silky, Rawls’ vocals were languorous perfection, while Dianne Reeves sang as clear as a wine glass from Zalto.

Krell’s new iBias range is claimed to be more efficient, or less power-hungry, than pure Class A. Paul Miller suggests that iBias is a modern take on the popular sliding bias circuits of the 1980s. So what is the motivation for it?

Statements from the company suggest that Krell is doing its part to modernise the high-end, to increase its appeal to audiophiles who are not comfortable with bulky intrusions into their living spaces in a manner acceptable in the past. And yet nothing differentiates the Duo 300, physically, from hundreds of other ball-buster amps.

It’s a big, metal-cased block, with the usual back panel fittings. Yes, the styling is tasteful – but there’s only so much you can do with an amplifier’s looks. This is a Krell by any name and any measure. Which is as it should be.

What does differentiate the Duo 300 and its siblings from the mainstream – though other companies are fitting web links, too – is the Ethernet connection, so each amplifier can be accessed on its own web page through any device that can run a browser, eg, an iPad.

The user can then view heatsink temperature, fan speed and other information. This will also provide alerts for conditions like overheating, fan failure and shorting of the output terminals.

Blissful attack

Finding something suitable to play through the Krell for the crucial, initial impression, we chose vinyl in the form of The Band’s eponymous second LP on MoFi. In part, it’s because of the astonishing bass and that incredible drum sound, but primarily because we love the album, period!

The Krell showed blissful attack with authentic decay, and just the right amount of dryness with the percussion that opens ‘Up On Cripple Creek’. It picked up the snap of the percussion, the kick-drum air movement, with true ‘feel’. Yeah, this is a Krell, alright.

Lou Rawls’ At Last [Blue Note], a bit of recording perfection. It was suitably silky, Rawls’ vocals were languorous perfection, while Dianne Reeves sang as clear as a wine glass from Zalto.

If we seem to lack just a little in enthusiasm here, it’s only because the last Krell product we reviewed blew us away: the astonishing S550i integrated (so it should at 500w! - model since discontinued) 

The Duo 300 is certainly a good amp, but our exposure to assorted Constellation masterpieces and six months with a D’Agostino Momentum Stereo – all much more expensive than a Duo 300 (at 3 to 5times the price!) – have altered expectations of modern solid-state amplification, regardless of the Class of operation, price, or any other respects.

Consider, though, that the Duo 300 is an easy product to live with in many ways, not least the cool running and easy set-up. Moreover, there is a bonus for those who harbour insecurities about massive high-end power amps, thanks to its on-line nanny.

Verdict

Assuredly solid-state in its demeanour, Krell’s Duo 300 doesn’t, for a moment, suggest the (sonic) warmth of a true Class A amplifier likes its ancestors … which may be music to the ears of those who can’t abide ‘valveness’.  this one is for rockers.

.....Ken kessler

Everything about this amp is absolutely superb
Bob Furstenberg

I just loved listening to this system.
Jason Victor Serinus |

SUMMARY: Dean Martin’s voice, on LP, sounded drop dead gorgeous. The sound was open and inviting,  I could hear every overdone portamento, every little extra bubble of sentiment-laden vibrato. 

Krell Illusion II preamp and Duo 300 XD amplifier, 

 

Krell’s immensely energetic Walter Schofield was in the middle of a system description when I entered the room, so I didn't know exactly what I was hearing, but whatever it was, the fact that I’d just submitted my review of the Krell K-300i integrated before boarding a plane for Denver left me very eager to hear it.

Dean Martin’s voice, on LP, sounded drop dead gorgeous. The sound was open and inviting,  I could hear every overdone portamento, every little extra bubble of sentiment-laden vibrato. But as gorgeous as the sound was, I also knew that the plush midrange of Martin’s voice, like the warm core of Ella Fitzgerald’s instrument, rarely fails to show off a system to best advantage. I needed another sample.

On another track, James Taylor’s “Her Town Too,” there was no fall-off in transparency or pleasure. Once again, the sound was open and revealing, with every bit of post-production reverb exposed. I just loved listening to this system.

As I learned after listening, components included the Krell Illusion II preamp and Duo 300 XD amplifier, VPI's 40th anniversary HW-40 turntable with Fatboy Gimbal 3-D printed tonearm and Audio Technica Art 1000 cartridge the brand new Alta Audio Alec loudspeakers.

Truly Impress The Most Demanding Audiophile!
McEvoy

With KRELL’S NEW XD upgraded to their I-Bias amplifiers comes a new level of demand & respect from audiophiles everywhere while we believe Krell sounded great before, the XD upgrades continue to astound listeners with the major sonic improvements. The “XD” upgrade for the Chorus/Duo/Solo amplifiers reduces the output impedance to less than half of its original value. This lower output impedance better damps unwanted speaker vibrational modes. Changing the output impedance also necessitated re-compensating each amplifier stage to achieve optimal critically damped transient response. This ensures that the amplifier accurately follows the dynamics of the input signal.

Here’s a Couple of Press Reactions to The Krell Poers System:

“The electronics were supplied by Krell Industries and Rondi D’Agostino, Managing Director, and Walter Schofield (COO) were on hand to explain their Krell products. The Posh speakers were driven by a Krell Duo 300 XD amp and Krell Illusion preamp with an Oracle Delphi Signature turntable and Oracle PH-200 phono stage as the source. The Grand Studio speakers were fed by an Oracle Origin turntable, an Oracle Origin tonearm, an Oracle PH-100 phono stage, and a Krell K-300i amplifier. All cables were by Kimber Cable.

If there were one word I could use to describe the sound in this room from both speaker models, it would be ‘fun’. The expression on the faces of the audience said it all. It was very obvious that everyone in the room was having a great time. We waited for 10 minutes to get a good seat but it was in vain. The people in the room were having too much of a good time to give up their seats. We were therefore forced to listen by standing at the back of the room. Despite this, we could easily tell that the sound quality was quite outstanding. The PRaT (pace, rhythm and timing) was spot on, the highs and the mids were very seductive and the bass was taut and well controlled.”– Part Time Audiophile

“Oracle, Krell and Gershman Acoustics have joined forces to offer a flawless presentation around two systems at very different price levels. The most ambitious of the two systems consisted of an Oracle Delphi Signature turntable, an Oracle PH-200 phono stage, a Krell Illusion Preamplifier, a Krell Duo 300 XD amplifier and the famous Gershman Acoustics Posh loudspeakers. It is a very ambitious audio system with immense qualities in terms of sound.

However, what really caught my attention is the second audio system whose proposition is geared towards a much more affordable budget. It consists of an Oracle Origin turntable, an Oracle Origin swivel arm, an Ortofon MC-1 Turbo cell, an Oracle PH-100 phono stage, an integrated Krell K-300i amplifier and Gershman Acoustics Studio 2 loudspeakers. All cables were from Kimber Kables. What a beautiful HiFi system with a very lively swing! First, the Platinum Origin injects a serious dose of life into a system. The surprising Krell K-300i amplifier is rock solid and drives the Gershman Studio 2 with an iron fist. The sound filled the room with a jazz tune with an excellent sense of rhythm and I actually caught my foot accompanying the tempo. Nice demo, we will surely have the opportunity to listen this set-up at the Montreal Audio Show 2019 in March. Don’t miss this !” – TED (Trends, electronics and designs)

“And that segues conveniently into the Gershman Acoustics demonstration room where the lovely Mrs. Gershman showed their fabulous $129,000 Posh flagship as well as the Studio II in a pair of integrated metal stands. Sharing the suite were Krell reps Rondi D’Agostino and Walter Schofield displaying their advanced new Krell K-300i integrated  The priority in this room was music and when the discussion turned not to equipment details but incredible sounding demo material, you knew the hardware had properly done its job. – Six Moons

Testimonials

Everything about this amp is absolutely superb