Krell F.B.I. (Fully Balanced Integrated) Integrated Amplifier 
Home Theater Power Amplifiers Integrated Amplifiers
Written by Brian Kahn   
Friday, 01 December 2006

The Fully Balanced Integrated Amplifier or F.B.I. is Krell’s no-holds-barred solution to an integrated two-channel audio system. Integrated amplifiers are often considered to be more or less lifestyle pieces. Not so with the 104-pound FBI. This massive $16,500 integrated amplifier started life as a separate amplifier/preamplifier combination. During the design process, it became apparent that the combining the two pieces would create a synergy that was not likely to be achieved when they operated separately.

Krell has long been known for their large, high-power amplifiers, capable of providing some of the industry’s most powerful and detailed bass. The amplifier portion of the FBI is comprised of the popular FPB 300cx stereo amplifier. The FPB 300cx is part of the Full Powered Balance (FPB) series of amplifiers, which consists of completely discrete, fully balanced, dual differential single-channel and stereo amplifiers. These amplifiers, as well as the FBI, feature a Class A output, which is arguably the most linear and accurate amplifier circuit topology. Krell is able to eliminate notch distortion that exists in the more typical Class AB or B biased amplifiers by sticking to a pure Class A design.

The amplifier section of the Krell FBI is a dual differential design all the way from the inputs to the binding posts, instead of the binding posts being hot and ground as with most amplifiers. The binding posts on the FBI are hot and cold. This design provides significantly more control over the speakers and requires twice the power, as both the original signal and its inverse are being amplified. Why would Krell do this, you might ask. The answer is simple: more control, which means more detail and accuracy. The dual differential design used by Krell has one half of the amplifier pushing the signal, while the other half pulls. This design maintains much more control over the signal than an amplifier that simply pushes. Think about a wood saw. A typical wood saw is made out of fairly flimsy sheet metal, which will flex tremendously when worked through a firm wood. Now take that same saw and add another handle on the other end of the saw. When one person is pushing the other is pulling and vice versa. This keeps the blade relatively ridged and free from distortion.

Unfortunately, having double the output devices and having them run in a Class A configuration is extremely inefficient, as the full amount of potential current for the increased number of output devices is always being delivered. If not utilized by the signal, the wasted energy is converted into heat. Accordingly, Class A amplifiers typically have large heat sinks and run very warm.

The Krell FBI reduces the amount of wasted energy while operating in Class A by the use of a proprietary technology called Sustain Plateau Bias. As described above, Class A amplifiers have the output transistors conducting the maximum amount of current. The Sustain Plateau Bias circuit monitors the incoming music signal and adjusts the amount of current that the transistors can conduct. The transistors can be set at different current handling levels and are automatically set to a level, which allows the amplifier to remain in Class A operation while minimizing wasted current.

Another unique Krell technology implemented in the FBI is the Krell Current Mode technology. In traditional amplifier designs voltage, rather than current, is manipulated to conduct the signal. In the FBI, the use of current mode maximizes the integrity of the signal. In current mode technology, the signal is transmitted from a high-impedance source to a low-impedance receiver, minimizing the effects of cables. Krell’s CAST system is also accommodated by the FBI, allowing the signal to be transmitted in the current domain between components. Krell states that when you have an entire CAST system the components act as one unified circuit, thus maximizing their potential. Unfortunately, we did not have any CAST-equipped sources available to confirm this.

All of these technological marvels are packed in a stunning enclosure measuring a little over 17 inches wide by 10 inches high and 20-and-a-half inches deep. The front plate of this 104-pound beast is a thick piece of brushed silver aluminum that takes its styling cues from the Krell’s new Evo line of products. (A black finish is also available.) The center of the front panel features a highly polished diamond cut convex accent. The top half of the accent carries an attractive yet discrete model identification above an LED window that displays the volume and an IR sensor. Directly beneath the volume window is the most luxurious-feeling volume knob I have ever had the pleasure of spinning. The bottom half of the accent panel below the volume knob contains a mute and a power button. On either side of the center accent rest three vertically arrayed buttons on each side for source selection. The sides are finned for heat dispersion and the back panel features another pair of vertical handles towards the outside. Inside each handle are four binding posts for each channel, allowing for easy bi-wiring. The center portion of the rear panel contains three pairs of single-ended inputs, a single-ended tape loop, one pair of balanced inputs, a pair of CAST inputs, and a preamplifier output. In addition to the signal connections, there is an IEC power plug, a power switch, a 12v trigger input and output and a RC-5 input. The entire chassis sits on four large vibration-isolating feet.

The remote supplied with the FBI is a heavy aluminum-bodied unit. The finish is reminiscent of a prior generation of Krell products. While it is extremely well-made and functions flawlessly, it does not have the same aesthetics and tactile feedback as the main unit itself.

Inside the FBI is a huge three-kilowatt custom-made toroidal power supply that comprises a large portion of the FBI’s mass. The FBI is capable of 300 watts per channel into an eight-ohm load, 600 watts into four ohms and 1,200 watts into two ohms. Total harmonic distortion is less than .04 percent at 1kHz and less than .3 percent at 20kHz. The signal to noise ratio is 108 dB, “A” weighted. The frequency curve from 20 Hz to 20 kHz is +0.0 dB and -0.05 dB. The big FBI uses 70 watts at standby, 185 watts in idle and up to 1,800 watts in operation.

The FBI was very simple to set up once I got it into position. Due to its size and weight, this is one integrated amplifier that I would suggest getting a second pair of hands to help with moving it. Make sure that the location that you move the FBI into is extremely well-ventilated. The FBI runs very warm due to its Class A topology. My primary source unit was Classe’s CDP-202 CD/DVD player, which was connected to the FBI via Cardas’ Golden Presence Balanced Interconnects. I tried several different speakers with the FBI, including Martin Logan Summits, B&W DM604S3s and Krell’s own LAT2000 (review forthcoming). Speaker cables included Monster Cable Sigmas and Cardas’ Golden Presence. The majority of my critical listening was done with the Summits.

The FBI can also be set to control other devices with its 12-volt trigger connections. For those who wish to integrate the FBI into a theater system, the FBI can be configured in a theater throughput mode, which allows the volume to be controlled from the systems multi channel pre-pro. Lastly, as mentioned before, during this review all the connections were traditional and we did not have the opportunity to test the CAST system.

Once everything was hooked up, I let the Krell play at background levels for a few hours a day for approximately two weeks before performing any critical listening. All of my listening notes were taken while the Krell was driving the Martin Logan Summits, but I also listened to the Krell with several other sets of speakers from Dynaudio, B&W and even Krell. The FBI’s characteristics noted below in the listening section were fairly consistent from speaker to speaker, with no strange interactions.

I started with a simple yet revealing recording, Elvis Presley’s Elvis is Back (DCC). This album contains Elvis’ rendition of “Fever” which, while being one of my favorite renditions of this song, is also very well-recorded. I could easily hear the minute details from Elvis’ cufflinks to the various background noises. At the same time, the FBI did not get in the way of the music coming through. Elvis’ voice and each of the backing instruments were solidly placed within the appropriately-sized soundstage.

Robbie Robertson’s self-titled album (Geffen/Mobile Fidelity) followed Elvis and provided the FBI with a workout with a larger and more complex soundstage. “Fallen Angel” opens up with a subdued bass line that was portrayed with the right balance of detail, control, power and rhythm to create a sound that was tight and powerful, yet not overly controlled and analytical. Both Robertson’s and Peter Gabriel’s vocals were well-textured and defined within their own space. The track “Sweet Fire of Love” features U2 playing along with Robertson on a fairly large soundstage that extended a few feet beyond the outside edge of my speakers both front and back, as though the sound was emanating from beyond the front wall of my listening room. There is a lot going on with this track and the FBI was incredibly quick and detailed enough to catch everything without sacrificing detail. The FBI’s massive power reserves were fast enough to reproduce the leading edge of the notes with immediacy and no smearing, yet at the same time could reach deep enough to put plenty of convincing muscle behind the appropriate notes. This track is both busy and dynamic and I never felt as though the FBI was straining even a bit to keep up.

While I was working on the FBI review, I came across one of my favorite jazz discs that had been somehow misplaced, Bill Berry's "For Duke" (Realtime Records). I planned on only listening to a few minutes of a couple of tracks, taking a few notes for the review, but ended up playing the whole album through without writing a thing down. This is one of those few albums that features truly great jazz, as well as being masterfully well-recorded. The album was recorded in 1978, using the direct to disc method. The first half of the album is Bill Berry and His Ellington All Stars. The sense of rhythm and timing was right on which, coupled with the detail, speed, accuracy and control, let me get lost in the music. The piano had a good sense of presence and body to it and the notes had a fast and strong leading edge, giving the listener a sense of immediacy. The horns were portrayed likewise, with a good amount of textural detail that captured some personality rather than just the notes themselves. While the entire album was remarkable through the FBI, the opening of “I Got It Bad” was chilling in how realistic the Marshall Royal’s alto saxophone piece sounded. Throughout my listening, the FBI’s sound was consistent with what I have heard from Krell in the past, powerful and accurate. The FBI was revealing and detailed without any extra midrange flesh on the bones to hide what you are feeding it. This is not to say that it stripped anything away – if you fed it a full-bodied signal, you’d get full-bod sound; it just didn’t add it itself. Having used the smaller Krell integrated amplifiers as reference pieces for several years, I noted that the FBI was without a doubt more powerful, seemingly having no bounds to its dynamic range. The FBI was also faster with a more open and forward upper end, which created a better sense of space, smoothness and extension at the high end without any added harshness.

The second half of the Bill Berry disc is actually a second album, Earl Hines' Fatha. “Birdland,” the first track of Fatha, features Red Callendar on the tuba. This piece has a solid tuba line, which the FBI had no problems with. I figured that the FBI is built like a tank and has tons of power, so I wanted to turn it up a bit in pace and volume, so I found something with the depth and power of the tuba line I had just heard that was just a bit more modern.

Crystal Method’s Vegas (Outpost Records) fit the bill. This frenetically-paced album is replete with dramatic transients and structures rattling bass lines. While nearly the entire album is synthesized or so heavily processed it makes the album impractical to use to check accurate reproduction of an instrument, the same traits make it a great album for checking dynamic range and control. The opening track, “Trip Like I Do,” has dubbed vocals, and a bass line that covers the range form subtle to pounding. This is all done with over overlaid tracks playing at the same time. The FBI had no problem separately reproducing each note. Nothing was smeared; the transients were razor sharp and powerful. Next up was my favorite track, “Busy Child.” I especially enjoy the deep and powerful synthesizer at “concert” volumes, otherwise known as “Check and make sure that your neighbors are out before playing at this level.” Even at these insane levels with this busy piece, the Krell never hinted at losing its composure. The bass lines were razor sharp, deep and powerful. At the same time, all of the synthesizer pieces were being reproduced in a clean, coherent manner. Nothing stressed the FBI out. Whatever I threw at it, from complex jazz riffs, rock and roll or fast paced electronica, the FBI handled with apparent ease.

The Downside
I can’t find anything bad to say about the way the FBI sounds, no matter how hard I try. There is no question that Krell has their sound, and to me, their sound sounds good. Alas, there is no such thing as the perfect component, and the FBI, as good as it is, is not perfect. In a perfect world, the FBI would not run as hot as it does. During my listening sessions, I could feel the heat coming off the FBI when I walked by. Those of you with small children and/or animals should take the necessary precautions to avoid any potential heat-related injuries. Placing the FBI in a well-ventilated and “off limits” area would solve a lot if not all of these potential pitfalls.

I would also have changed the remote control. Aesthetically, the remote is more in line with the last generation of Krell products rather than the new Evolution line. Lastly, I would have added additional inputs, as I have more two-channel sources than the Krell has inputs.

Krell’s FBI is, by far, the best integrated amplifier I have ever had the pleasure of hearing. If you can scrape together the funds necessary to purchase it, you owe it to yourself to give it a serious audition. I felt that the FBI could handle anything I threw at it with great detail, accuracy and ease. It performed as well as expected in many areas, and even better in others, particularly the higher frequencies. Its soundstage was portrayed with excellent scale and space. The individual instruments were reproduced in a palpable manner that provided a greater sense of realism previously unheard of with other integrated amps. When listening to the FBI, it was easy to close my eyes and picture the musicians in front of me and it did nothing to get in the way. I felt like I consistently had a seat close to the front of the stage, near enough to hear each individual instrument, yet far enough away for them to came together. The FBI is truly one of those products that, if you’re lucky enough to be able to afford it, makes a statement in more ways than one.
Manufacturer Krell
Model F.B.I. (Fully Balanced Integrated) Integrated Amplifier
Reviewer Brian Kahn

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