Power Amplifier Reviews

PRICE $9,500

AT A GLANCE
Plus
A new take on amplifier classes with iBias
Superb dynamics and soundstage
Ethernet capability for system monitoring
Minus
LED illumination too bright
Extremely heavy

THE VERDICT
Krell’s iBias technology has allowed them to deliver the benefits of a Class A multichannel amplifier in a way that will have audiophiles grinning from ear to ear.

Do you remember what it was like sitting for your high school or college lessons? Well, get ready for a trip down memory lane, because to give the Krell Chorus 7200 the praise it’s due and explain just how much this “little”-amplifier-that-could is going to change the audio industry, we’ll need to start with a short class in “classes.”

There are many different established amplifier topologies out there, designated by class, as in Class A, B, A/B, D, G, and H. Each has its own set of plusses and minuses, but in the audiophile world, Class A has always been king for a variety of reasons. First and foremost is sound quality, which is virtually unmatched to those with golden ears—those things attached to the sides of your head, not the speaker company that Darryl Wilkinson always raves about. Audio signals are basically alternating current—the sine waves you learned about in grade school—with both a negative and positive voltage. Remember, the goal is to make a loudspeaker diaphragm move out (positive voltage) as well as in (negative voltage). The Class A amplifier has the ability to conduct the full audio signal, both the positive and negative portions of the cycle, on each output device, reducing distortion in the process.

Unfortunately, there’s no such thing as a free lunch. There’s one negative—and it’s huge: Class A’s efficiency is about as green conscious as a Lear jet carrying one passenger across the country. A pure Class A design has the output transistors operating at full power all the time; they’re never idle. This means any energy not required to drive the speaker is released through the amplifier’s heatsinks, turning the amp into a power-wasting space heater. Furthermore, the ability to place multiple channels in the same chassis is all but impossible due to the heat buildup, which has essentially shut out the technology for the majority of home theater installations.

What Exactly Is iBias?
Krell’s audio legacy is built upon Class A amplification, and it’s no surprise that their engineers have been able to develop a patent-pending circuit delivering traditional Class A–like operation without the excessive heat and wasted energy of conventional Class A. Furthermore, the design can be housed in a form factor fit for home theater applications. It’s called iBias, but a better name may be iReallyLikeIt!

Krell’s innovative iBias technology allows the amplifiers to run in full Class A mode as needed, while at the same time minimizing heat generation. Krell isn’t the first to attempt using a “tracking” or “sliding” bias that reacts based on the nature of the audio signal, but their approach is quite different. In the past, the tracking monitored the incoming signal and set the bias based upon this information. The iBias technology takes a different approach by calculating the bias from the output stage; it directly measures the output current of the amplifier and adjusts the bias to the optimum level. Because iBias measures the output current, the real-time demands of the specific speaker connected to the amp are directly incorporated into the circuit function. The amplifier monitors the load, accounting for the variables present at any given moment, rather than blindly reacting to the incoming audio.

The president of Krell Industries, Bill McKiegan, likes to compare this technology to a 12-cylinder automotive engine, which shuts down some of the cylinders when you don’t need a lot of power. But when you slam the accelerator to the floor, the engine can deliver 600 horsepower—or more—almost instantly. iBias works virtually the same way. It can be cruising along in efficiency mode yet in a matter of microseconds give you hundreds of watts of full Class A amplification for musical peaks or when the action kicks up in the latest Hollywood blockbuster.

With this new topology comes other benefits. Krell has been able to fit seven channels of amplification into a relatively small—though extremely heavy—rack-mountable chassis, making iBias practical for use in environments where a traditional Class A amplifier would be too large. That’s not to say the Chorus doesn’t generate heat; it certainly does. Krell cools the amplifier using thermostatically controlled fans, which are generally eschewed by audiophiles and home theater aficionados. Still, in all of my testing, I was never able to detect any audible noise from the four fans on the rear of the amp, and the output temperature measured with an IR thermometer never exceeded 115 degrees F, even under the most strenuous tests.

Oh, My Aching Back
I was out of town when UPS delivered the amp, and the arduous task of bringing the 100-pound beast (110 including packing materials) fell to my 16-year-old son and one of his friends. It took two strapping teenagers to get this baby into the house, and while it’s not the heaviest amp I’ve reviewed, it certainly is one of the most dense, and getting it into the rack was a serious chore.

Aesthetically, the Chorus 7200 is quite beautiful, as far as black boxes go. The front façade is matte black highlighted by a silver band running vertically through the center of the facing, where a backlit Krell logo protrudes slightly from the box. The left side features a small circular power button, while the right has a rectangular LCD that gives you the amp’s IP address when it powers up.

Yes, I said IP address. You see, the rear of the amp has all the connections you typically see on an amplifier: both balanced and unbalanced inputs for all seven channels, the aforementioned fans, a 12-volt trigger input, a detachable power cord, a master power button, and, unusually, an Ethernet port.