Krell FPB 400cx Stereo Power Amplifier 
Home Theater Power Amplifiers Stereo Amplifiers
Written by Augie Bettencourt   
Tuesday, 01 April 2003

Movie buffs may remember the name Krell from the movie "Forbidden Planet," where the Krell were the most powerful race of an alien civilization. Krell Industries, manufacturer of some the world’s most exotic AV equipment, named its company after this superior species. Like Morbius, another “Forbidden Planet” character who sat in front of the Krell’s computer terminal trying to understand their science, I sit in front of Krell’s amplifier, learning about its science.

The amplifier I’m talking about is the Krell FPB (Full Power Balanced) 400cx, Krell’s latest “mid-priced” offering in the FPB stereo amplifier line-up, priced at $10,500. The 400cx is rated at 400 watts per channel into eight ohms, 800 watts in four ohms and 1600 watts into two ohms. Upon unboxing the Krell FPB 400cx, I was immediately struck by its build quality. Weighing in at 110 pounds and measuring 19 inches wide, 10.3 inches tall and 19.7 inches deep, this doesn’t exactly make it an easy amplifier to haul around - I hoped that what my back didn’t like, my ears would love. The 400cx’s large black anodized face and three small blue LEDs give the amplifier a very appealing, distinguished appearance. The front panel also has an infrared sensor for remote operation and interaction with other Krell products. The rear panel has two pairs of speaker binding posts per channel for bi-wire applications. These can be easily thumb-tightened, but they will not accept bare wire, banana plugs or pins - only spade lugs will work. It also offers both single-ended and balanced connections, as well as the proprietary Krell CAST inputs, which I’ll cover later in this review. The rear panel also includes the remote control Krell Link to turn the amplifier on and off, a very substantial non-detachable power cord, as well as a power breaker switch and a pair of handles to carry the beast around.

I placed the Krell FPB 400cx on the bottom of my rack and connected it to my Krell HTS 7.1 via the balanced inputs with my reference Cardas Golden Cross XLR interconnects. I made the speaker connections using the Cardas Golden Cross bi-wire speaker cables to my Martin Logan Prodigies. I’ve found the Cardas Golden Cross cables are a great value and work exceptionally well in my system. My Arcam DV-27 was used for both the audio and video source of my evaluation and the analog outputs were used for CD playback, while the DACs in the Krell HTS 7.1 were used for all movie watching.

The FPB 400cx is a Full Powered Balanced X Series Stereo Amplifier, which is what Krell calls their latest iteration in circuit design. The latest X Series Amplifiers use the Krell Sustained Plateau Bias III microprocessor control system that maintains Class A operation regardless of music or movie demands. Class A bias is the most accurate method used to amplify musical signals, but has it’s engineering challenges. Class A operation means that all transistors in the amplifier draw current all the time. However, the large current consumption gives us one of the benefits of Class A operation, namely a distortion that is low and kind to our hearing. It’s a design that’s neither cheap nor easy to engineer, but has obvious advantages. As I mentioned earlier in the review, the FPB 400cx has its own proprietary output design, called CAST Technology (Current Audio Signal Transmission), or in this case, CAST II Technology, Krell’s latest update. Krell claims the CAST II system improves every performance area, including speed, precision, dynamic range, depth and width of the soundstage, transient impact, tonal balance, harmonic distortion, and more. Unfortunately, I’m not able to confirm this, because my reference preamp, the Krell HTS 7.1, doesn’t include CAST outputs. With all the benefits that Krell claims CAST II technology delivers, it seems to me it would be a feature they’d want to include in their best and latest A/V processor, or at least create a Class A Series A/V processor that includes CAST technology. I’m always skeptical when a manufacturer makes claims of superior technological or sonic advantages of any type, whether it is Class A, Class AB, Zero-Feedback, Single-Ended Triode, or anything else for that matter. My thought is that it is implementation that rules the sonic world, rather than claims about technology.

The Music and Movies
Before beginning my listening session with the Krell FPB 400cx, I left the amplifier playing for three days. The first thing I noticed about this amplifier was how much hotter it made my listening room than other amplifiers I’ve listened to, with the exception of the Audio Research VT-200 MK II tube amplifier that has an internal fan blowing hot air off the tubes and into your room. I definitely didn’t have to turn the heater on before I started listening to my first choice, Keb Mo’s self-titled CD and the song “Every Morning” (Epic Records). Keb Mo’s vocals had a very open, natural sound with excellent soundstage width and depth. His guitar had proper acoustic tonal quality with an exceptional airy feel. With every pluck of his guitar, I heard very distinct accuracy. Keb Mo’s voice had perhaps the most clarity and best pitch definition that I’ve ever heard in my system. The second song I listened to from the same CD was “Am I Wrong.” I was reminded of Krell’s reputation for excellent dynamics and great bass slam, with the deepest and most powerful bass my Prodigies have ever produced. I was treated to a newfound clarity that other amps that I have auditioned, such as the Classe CAM-350s or the Audio Research VT-200 MK II, hadn’t been able to achieve.

Next up, I picked up the pace with Santana’s Shaman CD (Arista Records). I’ve been a Carlos Santana fan for years (pre-Fedora) and still have Inner Secrets, the first Santana LP I purchased as a kid. Santana’s latest CD has a huge range of musical styles from rock, rap and blues to classical. In the first song, “Feels Like Fire,” Dido’s haunting voice had a holographic quality when played through the Krell, perhaps the most three-dimensional I have heard my system sound. High frequencies were smooth and grainless and Dido’s voice sounded upfront and detailed, but never harsh or etched. Dynamic peaks seemed to come from nowhere and sometimes had me scrambling for the remote’s volume control, which seemed to happen much more frequently than with other amplifiers I’ve heard. This amplifier has the ability to scale dynamic peaks with great ease, a definite testament to the power reserves of this amplifier. I don’t think I’ve ever had another amplifier in my system that can handle dynamic transients like this amplifier can. The next song from the same CD, “Sideways,” has become my favorite song from this disc. Its recording quality is excellent, with great soundstage depth and width but, unfortunately, it’s the only blues song on the CD. When listening to this song, Citizen Cope’s voice was firmly planted deep in the middle of the soundstage, with Carlos Santana’s guitar playing off to the left side. The Krell FPB 400cx has a great ability to create an ambient feel for the recording studio, with a natural and airy sense of space. It placed all the performers and instruments very well and had glorious midrange quality. I don’t remember ever hearing an amplifier in my system with a soundstage as clear and specifically rendered as the Krell FPB 400cx. Citizen Cope’s delicate guitar strumming had a very natural sound, while the electric guitar licks from Santana’s Paul Reed Smith guitar had all the bite you’d expect.

I then moved to movie watching and started with M. Night Shyamalan's latest, “Signs” (Buena Vista Home Entertainment). “Signs” is well-written and I really enjoyed this sci-fi thriller that never tried to beat you over the head with its morality, for all its religious implications. The Krell FPB 400cx’s ability to reproduce detail shined, since “Signs” makes some of the best use of ambient sound design I've heard, from all the scary cornfield sounds to those eerie alien radio transmissions. The Krell FPB 400cx never faltered in its ability to extract every possible nuance from the soundtrack, and I heard detail that I’d never heard from my system before.

The next movie I watched was “The Bourne Identity” (Universal Home Video). What I initially thought would be another Bond rip-off actually turned out to be a tightly written and well-acted spy film. “Bourne” had a very active and enveloping sound mix right from the beginning, and the dynamic range was well rendered, with excellent spatial detail. The front soundstage is very nicely spread out and the soundtrack and surrounds are almost constantly engaged, down to the subtlest ambient detail. Bass response was spectacular and the soundtrack was faithfully duplicated. Movie watching is all about dynamics and this amplifier does dynamics better than any other I’ve heard. In listening comparisons with other comparably-priced amplifiers like the Classe CAM-350s and Audio Research VT-200 MK II tube amplifier, the Classe amplifiers seemed hazier through the midrange frequencies than either the Krell FPB 400cx or Audio Research VT-200 MK II. This can actually be an advantage with some recordings, but the Classe CAM-350s lack the ability to create the sonic sense of space and three dimensional soundstage of the Krell and it’s bass was weak and anemic by comparison. The Classe CAM-350s have the ability to drive the Martin Logan Prodigies to reasonable levels, but they can’t come close to competing with the Krell FPB 400cx’s bass authority and ability to scale dynamic peaks. The Audio Research VT-200 MKII has great lifelike midrange quality with softer upper frequency response, something that some may prefer over the Krell FPB 400cx, but the bass response of the Audio Research VT-200 MK II is rounder and less well-defined than the much more powerful-sounding Krell FPB cx400, as well as having a much weaker transient attack. I often felt like the Audio Research VT-200 MK II was running out of steam, something I never felt with the Krell FPB 400cx. Even with the slight midrange advantage of the Audio Research VT200 MK II, once you’ve heard the transparent and effortless sound of the Krell FPB 400cx, it’s hard to give it up.

The Downside

It’s difficult to fault the Krell FPB 400cx, unless you’re seeking a “soft,” forgiving or very dark-sounding amplifier – the single-ended triode guys need not apply here. The Krell has a very natural, precise and transparent sound that will be unforgiving to poor quality recordings and soundtracks or inferior electronics upstream. One man’s detail could be another’s ear strain, so I would audition the heck out of this amplifier with your own music before you sign on the dotted line. Another thing to consider would be any size or weight constraints.

This amplifier is very large and must be placed in a well-ventilated space. Because of its Class A operation, this amplifier burns AC like a ‘71 Dodge Barracuda burns fuel. If you leave the amp on full-time, which is really unnecessary these days, expect to see a little spike in your electricity bill.

After spending hours in front of the Krell FPB 400cx and the two other similarly-priced amplifiers, I believe the powerful Krell lives up to its name and have concluded that yes, Morbius would be very impressed with its science. It’s hard to deny that this is one very nicely engineered, well-built and great-sounding amplifier, with the latest in Krell features and technology. You’d be hard-pressed to find another amplifier with better bass slam, dynamic attack, transparency or more open, natural sound than this amplifier. It sounds great whether you’re listening to Santana or watching the latest sci-fi thriller. My hat goes off to Krell for improving weaknesses of the past, and building upon its strengths. They have created an amplifier that can be officially classed as one of the very best-sounding amplifiers available at any cost. If you’re looking for an amplifier that’s uncolored and true to the original source material, I highly recommend the Krell FPB 400cx.
Manufacturer Krell
Model FPB 400cx Stereo Power Amplifier
Reviewer Auggie Bettencourt

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