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

Set-up
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.


 

 
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