It seems like, every few years, Krell goes through a major cosmetic (and sometimes engineering) revamp. Visually, the song always seems to be the same: variations on the unashamedly macho look that began with the KSA-100 amplifier way back in 1981. The new Illusion II digital preamp embodies the same industrial design ideas as the new iBias amplifiers: a more understated (or less overstated) look, with a form factor conducive to rack-mounting. It's still macho, though.
Engineering-wise, there's nothing particularly "revamped" about the Illusion II ($7,000). It's a straightforward high-end digital preamp--i.e., an analog preamp with a built-in digital-to-analog converter, a concept that's rapidly becoming the norm in two-channel audio. The Illusion II has five digital inputs feeding its 24-bit/192-kilohertz ESS Sabre DAC: one AES/EBU, two coaxial RCA, and two Toslink optical. If it seems like something's missing, it is: there's no USB input. USB audio inputs have become ubiquitous only over the last couple of years; and, according to company president Bill McKiegan, the Illusion II was too far along in its development cycle to add USB. This isn't a big deal; you just need to add a USB-to-coax converter, which I'll discuss in the Hookup section below.
The Illusion II also has four stereo analog inputs: three RCA and one XLR. There's no built-in phono preamp; so, if you want to use a turntable, you'll have to provide the phono pre. XLR and RCA stereo outputs are provided, as is a quarter-inch front headphone jack.
For the Illusion II, Krell uses much the same control system as in its other recent preamps and integrated amps (including the S-300i integrated I usually use for speaker reviews). You can do all the normal stuff, like volume and input selection, from the front panel or remote. A front-panel alphanumeric display with a menu system lets you access more exotic features, such as balance, home theater bypass, input trim and naming, and the function of the 12-volt DC trigger jack on the back (which can send out a trigger voltage to turn on your amps when you turn on the preamp).
Krell's website touts four main features of the analog circuitry: current mode design; a fully balanced circuit topology, in which separate circuits amplify the positive and negative halves of the audio signal; dual-mono layout, with separate circuit boards for left and right channels; and a power supply with 40,000 microfarads of storage capacitance, which is a lot for a preamp. Of course, all of this reflects well on the construction quality, but none of it really tells you how the Illusion II is going to sound.
Let's get past that big issue I cited above: the Illusion II's lack of a USB input. It seems to me that almost everyone who buys this stereo preamp would want to use a computer as a source device, so finding a way to connect a computer to the Illusion II is priority number one in any installation. What you need here is a USB-to-coax converter: a "dumb box" that simply converts the USB from your computer to an SPDIF coaxial RCA digital output that the Illusion II can accept. I ended up using a Peachtree Audio T1 converter--which costs $79, worked instantly with my Toshiba laptop running Windows 7, and required no driver installation. The T1 only passes resolutions up to 24/96, though; if you want 24/192, spend $149 for the Peachtree X1. Of course, other manufacturers offer higher-end options you can explore. (Unfortunately, I started with the NuForce U192S converter, which I picked up for $49, but I couldn't get it to work with my PC or a friend's Mac, even after installing the drivers downloaded from the company's site. NuForce's only product support was a phone number with an answering machine that promised a company rep would call back, but no one ever did. So I recommend that you get one of the Peachtree models instead.)
I used the Illusion II with a couple different amps: the Krell Solo 375 mono blocks and a Classé Audio CA-2300 stereo amp. Speakers were either the Revel Performa3 F206 midsized towers or the Krell Resolution 1 large towers. Interconnects were all Canare Star Quad, and speaker cables were AudioQuest CinemaQuest 14/2. Most of my listening was using my Toshiba computer as the source, but I also used my Music Hall Ikura turntable and NAD PP-3 phono preamp to get an idea of how the Illusion II's analog section sounds. I also used it in home theater bypass mode along with my Denon AVR-2809CI receiver (used as a surround processor only), with the Krell amps handling left and right channels and an AudioControl Savoy seven-channel amp handling the center and surrounds.
The only thing I didn't like about using the Illusion II is that its digital inputs are accessed differently from the analog inputs. Each analog input has its own dedicated button on the remote and the front panel, but accessing the digital inputs is much less intuitive. From the front panel, you press the Digital button to switch over to the digital inputs, then hold the button down for a second to scroll to the next input. From the remote, you press the Digital button, then scroll through the inputs using the Select button. I think an extra row of buttons that allows direct access to the digital inputs would be welcome.
All comparisons cited in the review were conducted with levels matched to within 0.1 decibel.
Click over to Page Two for Performance, The Downside, Comparison & Competition, and Conclusion...