Krell Illusion II/KSA-i400 Pre/Power Amplifiers Class A All The Way?

Class A All The Way?

Over 40 years since Krell's iconic Class A power amps [HFN Nov '21 & May '11] hit the high-end, and over the last two decades of the brand's steady evolution, the company is now offering very refined, sophisticated amplifiers, packed with proprietary thinking but still underpinned by prodigious power. The fully balanced, direct-coupled 'current mode' amplification has been the backbone of its amplifiers for some time but the 'XD technology' [not to be confused with Cambridge Audio's XD 'crossover displacement' output stage technology – HFN Jun '14] is more recent. Here the amplifier's output impedance is reduced as far as possible, without impacting on stability, to improve its resilience to low and variable impedance speaker loads. The KSA-i400's input and driver stages have all been optimised for the wide open-loop bandwidth and enhanced transient performance of this revised output stage.

Krell's iBias (intelligent Bias) output stage technology has been a staple for some ten years [HFN Oct '14], claiming to offer the 'sonic benefits of Class A operation without the heat and power consumption of traditional designs'. In practical terms, rather than maintain a constant, high standing current (or bias) in the output stage – the effective, but inefficient, Class A approach to eliminating NPN/PNP transistor crossover distortion – iBias dynamically adjusts the output stage idle current to suit the music signal. 'Sliding bias' technologies have been with us since Technics' 'New Class A' and JVC's 'Super-A' regimes of the 1980s but iBias claims a better correlation with the music signal. Instead of 'guestimating' the optimum bias from the input signal voltage and presumed 8ohm load, iBias tracks the real output stage current and moderates the bias within the closed-loop of the feedback network. Thus the multiple output devices in the KSA-i400 are always maintained in their 'on' state regardless of the music signal level or content, or variation in the loudspeaker load. PM

Krell Industries LLC
Orange, CT, USA
Supplied by: Absolute Sounds Ltd
0208 971 3909