Krell Illusion
High-end Krell preamplifier with dual-mono circuitry and separate power supply, designed to pair with CAST-linked Krell amplifiers.
Description
The Illusion preamplifier follows in the tradition of Krell's reference preamplifiers, which physically separate the power supply from the audio circuitry. It was designed to form a cohesive system with Krell power amplifiers from the Solo range, notably via the proprietary CAST link, and is aimed at systems where control of the signal takes precedence over any other consideration.
The design adopts a dual-mono topology: the left and right channels each have their own power regulation and individually sized boards, which limits interaction between channels. Gain is achieved through a current-mirror topology with multiple outputs, offering an open-loop bandwidth of 1.5 MHz and open-loop distortion typically below 0.003%. The preamplifier operates without global feedback, and volume control is handled by a balanced resistor ladder, which preserves bandwidth and transient response regardless of the volume level set.
The power supply is housed in a separate, dedicated chassis, with fully discrete circuitry rather than the integrated-circuit topology common elsewhere. It features a 290 VA transformer and 41,600 μF of filtering capacitance, a generous specification for a preamplifier that aims to respond quickly to current demands while remaining undemanding at idle, with a stated standby consumption of 2 W.
A crossover function is available as an option via a dedicated card. Once installed, it opens up independent crossover frequency settings for the high and low channels, with a choice of 40 Hz, 55 Hz, 75 Hz and 100 Hz, along with filter slopes of 12 dB, 24 dB, 48 dB, of Butterworth or Linkwitz-Riley 24 dB type. The high-pass section of this crossover uses the preamplifier's full circuit for its output stage, allowing a subwoofer to be paired with satellite speakers without routing the signal through a lower-quality crossover circuit, as is often the case in conventional subwoofers.
This preamplifier is aimed at high-end systems, particularly those built around a complete Krell chain where source, preamplifier and amplifier communicate via CAST links, thereby reducing the number of voltage gain stages. It is also suited to configurations pairing a subwoofer with satellite speakers in space-constrained environments, thanks to the built-in crossover option.
What to check before buying used
When buying used, it is advisable to check the condition of the CAST connectors and the links between the preamplifier chassis and the separate power supply chassis, as well as the correct operation of the optional crossover card if the unit is fitted with one. The volume potentiometer, implemented as a resistor ladder, and the power supply capacitors, sized for a high energy reserve, also warrant careful inspection given the complexity of the circuit and the number of electronic stages involved.
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