Krell KRS-2 preamplifier

One of the most striking aspects of high-end audio is that you can never take any component for granted. Most of the radical change in audio at present takes place in new front-end and speaker technologies, but other components are changing as well—and with at least as much impact in making recorded music seem believable.

The Krell KRS-2 preamplifier is a case in point. I have now spent several months comparing it with every other unit I've been able to find. The end result is that this Krell has emerged as the equal of the Audio Research SP-11 in defining the state of the preamplifier art. Equally importantly, it is the only active preamp I have yet heard which meets the challenge posed by the new generation of passive preamplifiers.

When the output of the Krell KRS-2's high-gain stages are compared with the output of a purely passive preamp like the Mod Squad's Line Drive (reviewed in Vol.10 No.3), it produces a virtually identical degree of transparency. No other active preamp that I have yet been able to find has met this test to the same degree. While this may not be the result of a radical change in technology, it does seem to be an important advance.

Technical Details
Krell's flagship preamplifier is the dual mono, four-chassis KRS-1A, available in conventional unbalanced form for $8250, or completely balanced for $10,000. At $4500, the two-chassis KRS-2 represents designer Dan D'Agostino's assault on the real-world preamplifier market, but with many features in common with the cost-no-object KRS-1A. For example, the KRS-2, like the KRS-1A, is direct-coupled throughout. It does not have to rely on the designer finding the world's best-sounding capacitors—it simply eliminates them. The sound goes directly from the moving-coil cartridge input jacks to the output jacks without passing through a single capacitor in the signal path.

You also get one hell of a power supply, and this may explain a great deal of the KRS-2's transparency. The four 350VA transformers are housed in a separate unit and feed the psu box proper, which contains eight tracking regulators and 200,000µF of decoupling capacitance. The end result is that the Krell preamp's complete power supply outweighs most receivers and at least some high-end power amplifiers. Even so, the preamplifier chassis has another eight discrete tracking regulators and 20,000µF more capacitance.

The styling and construction of the KRS-2 is classic Krell—a Rolls-Royce standard of finish and quality of manufacture that ensures the unit is likely to outlive several generations of owners. As for features, there are inputs and outputs for even the most complex system, the usual controls for balance and volume, and separate controls for the main and tape outputs. The volume control is a top-quality, United Technologies dual instrument control.

The phono front end is an all-FET moving-coil stage, driving a FET/bipolar moving-magnet/RIAA stage. The RIAA compensation is adjusted for maximum error no greater than 0.1dB. There are internal DIP switches to select either moving-coil or moving-magnet operation, and a separate set of DIP switches for selecting the moving-coil load. Only the Audio Research SP-11 offers greater convenience in changing loads, but it offers only about one-third as many settings. If you leave the KRS-2 top panel unscrewed, you can make virtually instantaneous adjustments to loading, ensuring the proper match with a given cartridge to a degree I have seen rivaled only by the Klyne preamps. The high-level circuitry consists of all-FET gain stages, running in class-A, with FET outputs; the output impedance is a mere 6 ohms. As with all Krell designs, all the stages feature complementary symmetry.

In common with the other Krell preamplifiers, the KRS-2 features phase compensation for CD players. The CD input circuitry includes internal dip-switch settings to compensate for the phase error introduced by a CD player's reconstruction filters. Krell recommends different settings for different players, based on a diagnostic disc and computer modeling of the correct filter response. These settings do make an audible difference. They do not restore the fine musical detail missing in CD reproduction, but do make the sound richer, cleaning up some of the harmonics that are there and adding a trace of depth. Given the volatile state of the CD/DAT market, it may be better to have this feature in your preamp than to spring for something similar in an expensive CD player.

Passive vs Active
My experience with the newer CD players and an experimental DAT unit has led me to regard a "passive versus active" preamp test as increasingly important. It has now become clearer that the high-level stages of even the most expensive tube and transistor preamps introduce significant changes in sound character. There is no question that, even when these changes are euphonic, they come at an audible cost.

I should emphasize that I am not arguing that a passive preamp costing several hundred dollars is a proper substitute for an active preamp costing several thousand. Quite aside from the issue of giving up a phono stage and the need to buffer tape recorders, I have never yet been able to hook up a passive preamp in a way that did not slightly affect dynamics or upper octave response. My experience with CD players and open-reel tape units with variable volume controls has shown that the passive components and wiring/circuit board in a passive preamp are audible.

Further, it has shown that audiophiles who care about flat extension from about 1kHz up, or who want the full dynamics of music from the lowest to the loudest passages, are not going to get a "free lunch" by giving up the high-gain stage in an active preamp. Good as the latest passive units can be, the gain, buffering, and impedance matching of active units have important payoffs in allowing music to have the effortless energy present in any live performance.

However, if you use a top-quality passive unit like the Mod Squad Line Drive with very short, low-capacitance cables, you can come very close to the sound of a cheap straight wire without gain. Certainly you can get a neutral device with colorations tending more to be subtractive than additive.

Listening Tests
I have been conducting such comparisons with top-quality preamps ever since I received samples of the PS Audio 4.5 and Mod Squad Line Drive preamps; the results make an important preface to my review of the Krell KRS-2.

Source : stereophile[dot]com