KRELL SOLO-375XD 375w Monoblock power amps class-A w/ iBias technology / cast

KL 15 AM S375
NZ$ 32,995.00 pr (incl. GST)

The Leader in Audio Engineering


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

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 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.

When developing the SOLO375-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.

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.

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 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 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.
Krell CAST™
Reserved for the Solo 375 and Solo 575 is our CAST inputs and circuitry. Current Audio Signal Transmission, termed CAST, is a revolutionary method of connecting analog audio components for unparalleled sonic performance. In traditional audio systems, each component is a discrete entity with unique characteristics that act upon the musical signal independently. Each component is unaware of the other components in the system. The cables that connect the components also have their own electrical characteristics, which affect the sonic presentation of the entire system. Krell CAST™ unifies individual components and interconnects them into an electrically linked whole. The original signal remains unaltered from source to speaker. Still equipped with standard connections for use with other components, the full expression of Krell technology is realized when the system is connected using Krell CAST MMF interconnects. Krell CAST connected systems offer significant improvements in every performance area: speed, precision, dynamic range, depth and width of the sound stage, transient impact, and tonal balance.
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.





Output power
300 W RMS at 8 Ω
540 W RMS at 4 Ω

Frequency response
20 Hz to 20 kHz +0, -0.13 dB
0.5 Hz to 100 kHz +0, -3 dB

Signal-to-noise ratio
>98 dB, wideband, unweighted, referred to full power output
>116 dB, "A"-weighted

Gain: 26.2 dB

Total harmonic distortion
<0.045% at 1 kHz, at 400 W, 8 Ω 
<0.17% at 20 kHz, at 400 W, 8 Ω

Input Impedance
Single-ended: 100 kΩ 
Balanced: 200 kΩ

Input sensitivity
Single-ended or balanced: 2.4 V RMS

Output voltage
138 V peak-to-peak
49 V RMS

Output current: 16 A peak

Slew rate; 40 V/μs

Output impedance: <0.017 , 20 Hz to 20 kHz

Damping factor: >470, 20 Hz to 20 kHz, referred to 8 Ω

Power consumption
Standby: 4 W
Idle: 180 W
Maximum: 1600 W

Heat output
Standby: 13.7 BTU/hr.
Idle: 615 BTU/hr.
Maximum: 5677 BTU/hr.

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

Outputs; 2 pairs binding posts

Dimensions; 434 mm W x 194 mm H x 536mm D

36.4 kg (shipping)
31.8 kg (unit only)


I've never encountered another amp that combines the Solo's 375 warm, wonderful, involving sound with such a practical and versatile design.
Brent Butterworth

REVIEW SUMMARY: Spending this sort of money on a pair of mono-block amps is a lot, but the Solo 375 delivers a lot. It combines a very smooth, un-solid-state, un-hifi sound with loads of power and dynamics, plus a design that works great whether you're plopping the amps on the floor by the speakers or shoving them out of sight into a closet or equipment cabinet. In fact, I'd go so far as to say I've never encountered another amp that combines the Solo's 375 warm, wonderful, involving sound with such a practical and versatile design.

EXTENDED REVIEW: The Krell Solo 375 mono block demonstrates how the amplifier business has exploded with classes, so many classes that even people in the audio industry often get them confused. Twenty years ago, almost everything was Class AB or Class A. Now it's also common to see Classes D, G, and H. We also see "made up" classes--marketing terms rather than official designations--such as Class I, Class T, and Class AAA. We can find most of the above classes executed with compact, efficient switching power supplies or with traditional analog supplies using transformers and big storage capacitors.

What's the best? That depends on how you define "best," but audiophiles generally believe Class A delivers the best sound quality. With Class A, the amp's output transistors or tubes never switch fully off, so there's no crossover distortion--that ugly, high-frequency artifact caused when an amp's positive-polarity transistors or tubes hand off the signal to the negative-polarity transistors or tubes.

Why isn't everything Class A, then? Because Class A wastes a lot of power. It dissipates the entire output of the amp's power supply either as sound through the speakers or as heat through the amp's heat sink...but mostly as heat, which makes it impractical to use Class A amps in places where heat can build up, such as in equipment cabinets or closets.

Krell's Solo 375 and the other amps in the company's new iBias Series adapt Class A to a world in which electronics power consumption is an increasing concern and the desire to hide the electronics is a top priority for many customers. The iBias technology uses a Class A output stage in which the bias--the ever-present voltage that keeps the transistors turned on all the time--is continuously adjusted so there's only as much as needed for the signal the amp is playing at that moment. Thus, there's not that huge amount of excess power that must be dissipated as heat. Power consumption is lower, less heat sinking is needed, and the amp can be made smaller. Assuming the circuit that controls the bias works as intended, the iBias amps should give you all the sound quality of Class A with none of the drawbacks.

If this technology sounds vaguely familiar, it should. It's similar in ways to Classes G and H, which use a "tracking" power supply that reduces voltage at lower signal levels but typically employ a Class AB output stage. A few years ago, Sony introduced a high-end Class A amp with a tracking power supply.

However, Krell's iBias approach is different. Rather than using the input signal to adjust the bias or the power-supply voltage, iBias tracks the output current. The advantage of this approach is that iBias can optimize the amp's performance for your specific speakers, rather than for an assumed speaker load. Even though iBias should result in more accurate optimization of the amp's operation--cutting the bias "closer to the edge," if you will--my assumption is that Krell chose to supply a comfortable margin of bias voltage to the transistors. Why do I guess that? Because despite the Solo 375's large chassis, it has cooling fans: two thermostatically controlled, low-RPM fans that are managed so that their sound should be inaudible. Clearly there's some wasted heat being generated.

Krell-375-mono.jpgNot only is the Solo 375's amplification technology innovative, but its control system is, too. If the amp is wired to an Ethernet network through the RJ-45 jack on the back, you can access a web page for each amp. The web page shows current operating temperature, fan speed, overload conditions, etc.

The Solo 375 is rated at 375 watts into eight ohms and 600 watts into four ohms. The iBias line also includes the 575-watt Solo 575 mono block, as well as two-, three-, five-, and seven-channel models. All use a similar chassis design, and all can be rack-mounted.

All of the amps in the line use fully balanced, fully complementary circuits through the entire audio path. In essence, each circuit comprises two "mirrored" halves, one of which operates on the positive half of the audio signal and the other on the negative half. This is the way most of the bigger, more expensive high-end solid-state amps are made; it reduces noise and improves the slew rate (the speed at which the amp can go from zero volts to full output).

The Hookup

The moment I unpacked the first of the pair of Solo 375s I received for review, it immediately became my favorite Krell ever. Or at least, my back's favorite Krell ever. Despite its bulk, it weighs just 60 pounds.

For some audiophiles, this will be a problem. Krell has built its history on amps with back-breaking weight, and some Krell enthusiasts cherish the fact that their amps require two strong people to lift. When a visiting headphone manufacturer saw the two Solo 375s on my floor, awaiting setup, he picked one of them up, and an immediate look of shock crossed his face. "That's a KRELL?" he blurted. I explained the whole iBias technology and pointed out the fans, but he just rolled his eyes. I've seen at least one other audio reviewer express similar sentiment.

I put the Solo 375s on thick MDF platforms to elevate them above my carpet. I connected them to two different pairs of speakers: my usual Revel Performa3 F206 towers and my cherished Krell Resolution 1 towers. I don't often use the Resolution 1s because they weigh 200 pounds each and are thus impractical to move in and out of my system often, but I thought the occasion merited the effort.

The Solo 375s got their signals primarily from a Krell Illusion II digital preamp, using either a laptop computer or a Music Hall Ikura turntable (with an NAD PP-3 phono preamp) as the source--mostly the former, using my own ripped WAV files or tunes streamed from Tidal. I used balanced professional Canare Star Quad XLR cables to connect the preamp to the amp and AudioQuest CinemaQuest 14/2 speaker cables.

The whole time I used the Solo 375, including some crank-it-up rock listening sessions and a couple of action movies, I only ever heard the fans when my ears got within a couple feet of the amp.


I've never been a Diana Krall fan, but it's hard not to be captivated by Wallflower, her new album of covers of classic rock tunes. In just the first 20 or 30 bars of her take on Elton John's "Sorry Seems to Be the Hardest Word," I learned a lot about the Solo 375. I was struck by how intimate and warm Krall's voice sounded. She sounded like she was right in the room with me, about eight feet away, with very little ambience. In fact, based on her voice, I'd almost have thought someone deadened up my listening room with about 30 square feet of Sonex foam. But the instruments sounded huge and spacious, much as in Elton John's original recording. The spaciousness didn't sound like the result of exaggerated treble or phasiness, and it rarely produced a "wow" reaction from me; it merely sounded natural. In terms of sheer involvement, this was a higher level than what I'm used to hearing from my Revels.

You're probably sick of hearing the Mark Ronson/Bruno Mars tune "Uptown Funk," but it happened to come up on the home page of the Tidal app, so I played it just out of curiosity. It'd be easy to dismiss this as insubstantial pop fluff; but, through the Solo 375 and the Resolution 1 speakers, I could hear that it's actually a musical and sophisticated production. The Solo 375's sound suited Bruno Mars' voice, which is smooth but not deep and thus might sound grating through some amps. Through the Solo 375, it sounded positively liquid, yet there was nothing soft about the bottom end; the Solo 375 kept each Resolution 1's dual woofers in perfect control, producing tight, deep, powerful bass tones. Again, the unexaggerated, natural-sounding spaciousness pulled me in. 

Based on these and some cuts I'd heard before, I was starting to wonder if the Solo 375/Resolution 1 combo could conjure a really huge sense of space. I found out fast when, on its own, Tidal went straight into Mars' "Locked Out of Heaven." The tune's background vocals almost literally jumped out of the speakers, actually seeming to come from behind me. This is a pretty easy trick for big panel speakers like MartinLogans and Magnepans, but not many systems using conventional dynamic drivers can so convincingly wrap sound around you.

Having heard enough pop singers for a while, I shifted over to one our greatest anti-pop singers: James "Blood" Ulmer. Ulmer's Odyssey album is an idiosyncratic masterpiece, consisting only of drums, violin (often played through a wah-wah pedal), hollowbody electric guitar (with all the strings tuned to A), and Ulmer's inimitable vocal stylings. The Solo 375 got all the spacing right, the natural reverb of the space in which the drums were recorded contrasting perfectly with the much more intimate sound of the close-miked vocals and the reverb-soaked violin lines. Ulmer's vocals also sounded just right: smooth and soulful, but with that little trace of edge that makes Blood Blood. (BTW, I've seen Ulmer live more than any other artist, in widely varying venues and numerous musical settings, so I'm pretty familiar with his sound by now.)

Not surprisingly, the Solo 375 sounded great with rock, too. R.E.M.'s "Pilgrimage," from Murmur, the group's first full-length album, isn't something anyone would mistake for a Led Zeppelin or Deep Purple tune, but all the elements are there: a dynamic, insistent drum sound with a huge-sounding snare and a powerful vocal performance backed by highly reverberant background vocals. (OK, so it has chimes in unison with the guitar. That doesn't mean it's not rock.) The spaciousness that worked so well for the other recordings came through on "Pilgrimage," too, and I especially loved the power of Bill Berry's kick drum and the way his firm snaps of the snare drum came through with loads of dynamics but not a track of edge.

Basically, the Solo 375 sounded like the world's most powerful tube amp. The tonal and spatial character, combined with the warmth of the mids, reminded me of some of the big push-pull tube amps with quartets or octets of KT88 tubes. By and large, that's a good thing.

The Downside

One of the things that made the Solo 375 remind me of a tube amp is that the top end is smooth and not in any way "hifi sounding." Personally, I like that. But I know some audiophiles don't--they want to hear every last little detail in a recording, even if they need a somewhat elevated or edgy treble to get it. If that's you, that's okay. In audio, you gotta go with what makes you happy. Just know that, if what makes you happy is a lot of treble detail (apparent or actual), the Solo 375 probably isn't your amp.

Comparison and Competition

I had a chance to compare the Solo 375 with a couple of other big solid-state amps: Classé Audio's CA-2300 and Pass Labs' X350.5. The latter, incidentally, runs in Class A for the first 40 watts; so, for all intents and purposes, it's almost always running in Class A and thus makes an interesting comparison for the Solo 375. Using a one-kilohertz test tone, I matched the amps' output levels within ±0.1 dB and connected them all to the Resolution 1 speakers.

A particularly illuminating track for comparing almost any kind of audio gear is Trilok Gurtu's "Once I Wished a Tree Upside Down," a light saxophone melody backed by shakers, tabla, and synthesisers. In the intro, the shakers swirl around your listening room; the degree to which they wrap around my listening chair is one way I judge a system's soundstaging capability. With the CA-2300, the treble sounded wonderfully detailed and delicate, but the action all seemed to be taking place in front of me rather than around me. With the X350.5, I got a greater sense of spaciousness and wraparound, but the treble didn't sound as smooth as with the Classé or the Krell. The Krell got the spaciousness just right, but because its treble was smoother/softer, it didn't have quite that level of excitement that the others did. 

I listened to some more jazz and pop cuts through all three amps, but the comments were the same thing over and over. All three had ample dynamics and bass; it's mostly the character of the treble and the spaciousness of the sound that varied. Which one will you like better? That depends on your personal taste. But if smoothness and spaciousness rank high on your list of priorities, the Krell seems like the best bet to me.


Spending this sort of money on a pair of mono-block amps is a lot, but the Solo 375 delivers a lot. It combines a very smooth, un-solid-state, un-hifi sound with loads of power and dynamics, plus a design that works great whether you're plopping the amps on the floor by the speakers or shoving them out of sight into a closet or equipment cabinet. In fact, I'd go so far as to say I've never encountered another amp that combines the Solo's 375 warm, wonderful, involving sound with such a practical and versatile design.


KRELL SOLO 375 with YG Acoustics Hailey