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.
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 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.
Click over to Page Two for Performance, The Downside, Comparison & Competition, and Conclusion...