Krell KAV-300i integrated amplifier Page 2
Inside, the compact, low-profile chassis packs a modest-looking, toroidal, steel-band-shielded transformer. This is rated at 400VA, but to a high-regulation Krell specification. (If this were not the case, that 300W into 4 ohm specification would be well out of reach.) Considerable thought has been given to protection, which in this design is by fuses (the bigger Krells use electronic protection operating via power relays). Gross short-circuit and speaker-fault protection is provided by 12A fuses at the output; these are located beneath the output terminals and user-accessible from the outside. A 12A fuse should be good for continuous full power into 2 ohms so I don't reckon it would ever blow in normal use. There are four additional 12A fuses inside protecting the plus and minus voltage rails to each channel in case of device or circuit failure. Finally there is the AC mains fuse, which is integral with the IEC AC input receptacle.
Remarkably compact in layout, the circuitry is largely symmetrical for both channels, with a short path from input to output. It is also dual-mono from the separated transformer secondary windings onwards. The smaller, upper board is the input and control section, noteworthy for its desirably high input impedance of 210k ohms. Earlier Krell products often had rather low input load impedances, but the '300i will load source components lightly, even suiting D/A processors with tubed output stages. The inputs are AC-coupled using polypropylene dielectric capacitors. The potentially noisy microprocessor, complete with its own power supply, is located well away on the front panel board together with its associated buttons and LEDs.
Balanced/single-ended input conditioning is achieved with Krell's discrete FET-input, class-A amplifier. This has a low output impedance appropriate for driving the microprocessor-programmed volume-control section. This features a precision MDAC chip (a PMI DAC8043, one per channel) used as a precision multiplying attenuator. This stage outputs a signal in current form; current/voltage conversion is accomplished by a well-respected op-amp chip, a SM2131, which feeds the resultant voltage signal to the DC-coupled stereo power amplifier stages on the lower board.
This larger board also incorporates the power supplies, which are integrated with the stereo channels. A dual winding on the transformer feeds separate rectifiers and moderate-capacity Nichicon supply reservoir capacitors, two 8200µF electrolytics per channel. Krell practice is followed for the driver-section power supplies, which use multiple paralleled reservoirs for a controlled low impedance over the audio bandwidth.
All-discrete, the power amplifier circuitry follows Krell design principles, with arrays of complementary differential stages possessing a wide bandwidth. The fully complementary output stages use custom, high-current TO3-can Motorola bipolar transistors, six per channel. These have an intrinsic peak rating of over 40A per set and are operated in class-A/B at moderate bias. The '300i amplifier stages employ low values of global negative feedback, reflecting current thinking that the quest by designers in earlier years for high specific damping factors and very low distortion was misguided, especially if their realization required high feedback. Although not true in all cases, it does seem that amplifiers that achieve a good overall technical performance with low global feedback sound more natural than those that rely on high feedback for linearization.
Installation was a breeze. The KAV-300i ran fairly cool and reached a stable sound quality in a matter of 10 to 15 minutes from standby. In operation, the remote control was slick and smooth-acting, with no funny noises, offering positive input selection as well as an even, wide-ranging volume control action of good resolution (just how the KPS-20i/l's volume control should have been—see Stereophile, April '95, Vol.18 No.4).
Serious listening commenced after a week's conditioning and informal use. The KAV-300i turned out to be a seriously good amplifier. Fresh out of the box the '300i sounded promising. It could play loud and neither change in tonal character nor appear short of breath. In its raw state, however, it lacked some of the clarity and refinement which became nicely apparent after the initial 20-40 hours of use.
Writers and engineers have recently been discussing the advantages of well-designed one-box CD players with their jitter-free direct link between transport and decoder. There is a similar advantage with a single-box integrated amplifier. Not only is the signal path shortened, you also eliminate typically two soldered connections for each channel as well as a length of external cable. More importantly, you get rid of the ground currents that occur between audio separates and the audio connectors at each cable end. Knowing in advance the matching criteria for the pre- to power-amp connection, the designer doesn't have to over-engineer the interface in expectation of a range of load problems. This may help simplify the preamp section as well as further boosting the performance of the integrated amplifier.