Krell KSA-i400 Stereo Power Amplifier

It’s a monster, indeed, packing 400Wpc into 8 ohms and a whopping 800Wpc into 4 ohms. The amp offers 164V peak-to-peak output voltage, 58V RMS, 62 amps of peak output current, and a damping factor of greater than 800. Add to that 5400VA of transformers and 188,000uF of capacitance, and you can power those monsters from the id for decades. Of course, they’re not the Krell’s 9200 thermonuclear reactors working in tandem—that would likely be overkill for a stereo amplifier. Nonetheless, those fancy statistics mean that the amp can drive pretty much any speaker and maintain absolute control over its drivers. But that’s not the amazing part. It achieves these otherworldly specifications while operating completely in Class A all the time and not reaching temperatures that would melt Krell steel. In fact, the KSA-i400 never got hotter than warm to the touch. See? This must be some type of alien technology, magic of the darkest type, or more likely Krell’s patent-pending iBias technology, XD low-output impedance control, and symmetry maximization.

To avoid getting too deep into Krell’s new tech (of which there is a boatload),   iBias essentially permits the amp to maintain Class A output all the time without meltdown by using new circuit topology that monitors current and adjusts bias to maintain a preset low current in a feedback design that monitors output-stage current. The result is perpetual Class A operation with improved accuracy and efficiency. XD controls output transistor temperature to maintain the output-stage thermal equilibrium required to lower the amplifier’s output impedance well below traditional norms, which increases damping. Symmetry maximization appears to improve the inherent balanced nature of the circuit by making small adjustments to perfectly symmetrize opposing polarities and reduce subsequent second-order harmonic distortion. Krell offers white papers that made my eyes hurt and my head ache, but if you want more, reach out to the mad scientists at Krell, and they will be happy to assist. Just don’t blame your headache on me! There is one more cool feature, the unit is microprocessor controlled and connected to the Internet through a web page, which offers status, temperature, and control features, as well as software updates. The system works well and is nice to have, although I never really depended on it.

The front of this very large (17.25″ x 9.4″ x 24″) amplifier has two vertical lights, which indicate its status (on, standby, update, shutdown, etc.) and a central button that helps change those states. The back has large handles (thank you for that!), a pair of speaker binding posts that can accommodate banana and spade lugs, RCA and XLR inputs, a 20-amp IEC power cord receptacle, and an RJ45 Ethernet port, as well as a 3.5mm, two-conductor, 12-volt DC trigger plug. The amp is beautifully crafted in all black and has very large side cutouts for heat dissipation and machismo.

I set the amp in front of my rack, on the floor, on an oversized Symposium Ultra amp stand. It was happy there, and I was happy with it there. Everyone was happy.

So, let’s jump to the opinion part. I can’t wait! This was the best (insert inappropriate explicative) Krell amplifier I have ever heard. And not by a little bit. Anything I have ever criticized about Krell was gone. Anything I have wanted but was lacking was there. Was it the best amplifier I have ever heard? No. But to get truly better will take moving up two-to-three times in price, and that’s serious dinero, like Porsche money! I always love reading the marketing material and technical jargon to see the manufacturers explain how this advance made this sound better or that technology makes that even better. It’s like the marketers stick their hands into a barrel of audio superlatives and use whatever they pull out. Well, I can say that as I read about Class A’s organic sound and how iBias improves accuracy, it correctly described what I heard. When Krell opined that XD improves musical dynamics, depth, and dimensionality, I heard that, too; it also lowers the noise floor and improves bass impact and dynamic texture. In reading how symmetry maximization reduces second-order harmonic distortion dramatically, opening up the soundstage with improved detail and a better sense of pacing, I nodded my head in agreement. When Krell commented on the massive available capacitance and use of direct-coupled, balanced differential, current-mode circuitry providing unrestricted, limitless power with effortless reproduction of the entire audio band, there was another big ol’ yup from me. Score one for marketing sincerity and delivering on what is promised. Congrats Krell—you have a hit on your hands.

I really could stop here, but I won’t. Not because I didn’t make my point, but because I simply like rambling on about this thing. I listened using what I had in my room at the time. That included my resident Pilium Alexander preamp ($40k’ish) and a Triangle Art L-200 mk2 Dual Mono Tube preamplifier ($25k’ish). As a source, I alternated between my Pilium Elektra DAC fed by my Taiko Extreme server/streamer and an old Marantz CD player, because everyone likes spinning some tiny silver discs every once in a while. It was a luxury having such a quality tube preamp on hand, and I will admit I ended up doing over 50% of my listening with the tube preamp in the system. There was a synergy between the Krell amp and the Triangle Art preamp that I was not expecting but couldn’t resist. My Pilium preamp is crazy good, like the last-preamp-I-will-ever-own good. But synergy is synergy, and as awesome as the Alexander sounded with the i400, the Triangle always beat it when it came to a raw sense of reality and natural tonal character. It gave up some of the magical accuracy those Krell marketers raved about, but that was OK because what I gained was still accurate enough and oh so engaging. I lost hours on this review, and I literally have no idea where they went. Again, I don’t want this to be interpreted as the i400 needing tubes because it’s too this or to that with solid-state preamplification. That’s just not the case. Take it for what it is.

Regarding sonic signatures, I find it fascinating that components of different design and manufacture seem to be trending toward a single point of what I now call idealality. Gone are the days when Wilson sounded bright and Soulution too analytical. I find brands, as a whole, migrating toward a more uniformly accepted ideal. I am presuming this has to do with the quality of components now available, as well as the fact that many have established ideal design factors when it comes to circuit-board topology, power-supply implementation, the material and isolation of the enclosure, cable and accessory selection, etc. To look at another industry as an example, my new Subaru Outback Wilderness does 0-60 in 5.9 seconds—stock from the dealer. Not, of course, after I loaded it with roof rack, lights, and accessories. But you get my point.

My notepad had a paucity of notes. Scribbles of “it just sounds right” and “wow” made it to the paper when I forced myself to stop and write something. What was obvious was a very black background upon which three-dimensional images presented themselves as if by an audio 3D printer. The channel separation was practically monoblock level and presented a constantly natural and palpable soundfield with presence of performer and near-visible texture. The massive power reserves allowed my Wilson Audio Alexx V speakers to punch low with an iron grip on its drivers. I had the mental image of walking a leashed Velociraptor. And then I had the mental image of getting eaten by the Velociraptor. The first mental image was unquestionably due to the Krell’s high damping factor. The second image was due to the fact that the first image was ridiculous. Frequency extension was limitless as you would expect. Vocal presentation was transparent and organically natural and real. The i400 was Galium Nitride fast, but I don’t think it uses GaN. Timbral reconstruction was wonderful, and the presentation of complex sonic palettes was like Bob Ross painting happy little trees out of sound waves in my room. I’m not 100% sure what that means, but it made perfect sense to me while I was listening. Just go with it.

Classical music was great. Pop was great. Show tunes were great. Classic rock was great. I know this is a ridiculous thing to say in a stereo magazine, but it’s simply true. Anything I threw at the system sounded world-class. The KSA-i400 performed incredibly well with tube and solid-state preamps. It sounded great with $75k worth of digital front end and with $800-worth of digital front end. It was remarkably flexible, played well with everything else in the playground, and was not needy or finicky. What it did was constantly satisfy, unerringly deliver, check off every audiophile box, and blissfully abscond with hours of my life.

If you have $36,500 and are in the market for a stereo amplifier, buy it. If I didn’t already have my Pilium amp (which is a few grand more and equally spectacular), I would have kept the review unit without hesitation. In fact, I tried to think of how to justify keeping it or convincing Krell to let it hang at my house for another year or three before calling it back. Alas, nothing came to me, and they came and picked it up. It’s gone. I am sad. Fortunately for me as a reviewer, there is always something fun and new heading my way. But the Krell i400 now has a special place in my heart. I first watched Forbidden Planet when I was about seven years old at night with my three-year older brother, Jeff, in our family room. It was a Core Memory. The Krell and Robby the Robot became a permanent part of my brain waves. To me, the concept of the Krell represented something inconceivably sophisticated and advanced. As such, I have always had what is likely an unreasonable expectation of the brand named after it. You can blame my id. But the i400 scratched my Krell itch. It clearly wasn’t made by the Krell themselves, but I can see Dr. Morbius coming up with the design after having his IQ “amplified” with the Krell intellect enhancer. There is a rumor Krell is coming out with a matching preamp. Me and my id can’t wait to hear it!

Specs & Pricing

Output power: 400Wpc into 8 ohms; 800Wpc into 4 ohms
Inputs: Balanced on XLR jacks, unbalanced on RCA jacks.
Control: Ethernet via RJ45; 12 VDC trigger input
Input impedance: 34k ohms (balanced); 17k ohms (unbalanced)
Frequency response: 20Hz–20kHz, +0.1, -0.07dB
Signal-to-noise ratio: >102dBr, wideband, unweighted, referred to full power output; >117 dBr, “A”-weighted
Gain: 26.4dB
Input sensitivity: 2.8V RMS balanced; 2.8 V RMS single-ended
Total harmonic distortion: <0.015%, 1kHz, 400W, 8-ohm load; <0.12%, 20kHz, 400W, 8-ohm load
Output voltage: 164V peak to peak; 58V RMS
Output current: 62A peak
Slew rate: 70V/µs
Output impedance: <0.01 ohm, 20Hz–20kHz
Damping factor: >800, 20Hz to 20kHz, referenced to 8 ohms
Dimensions: 17.25″ x 9.4″ x 24″
Weight: 160 lbs. (net); 180 lbs. (shipping)

KRELL INDUSTRIES, LLC
45 Connair Road
Orange, CT 06477-3650 USA
(203) 799-9954
sales@krellhifi.com