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 Classés 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 Classés 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.

Not 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 $8,750 Solo 375 is rated at 375 watts into eight ohms and 600 watts into four ohms. The iBias line also includes the $11,250, 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.

Performance
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 $7,000 CA-2300 and Pass Labs' $11,500 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 synthesizers. 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 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.

Conclusion
Spending $17,500 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.

Additional Resources
• Visit our Stereo, Mono and Audiophile Amplifiers category page for similar reviews.
• Krell Foundation AV Preamp Reviewed at HomeTheaterReview.com.
• Krell Announces the Digital Vanguard Integrated Amplifier at HomeTheaterReview.com.