Dual Chassis Mono Preamplifier
The Evolution Two preamplifier is the ultimate expression of Krell preamplifier technology. A monaural preamplifier with a separate power supply, the Evolution Two is capable of rendering a near lifelike soundstage with stunning detail and presence. The balanced, zero feedback, proprietary Krell Current Mode and CAST circuitries results in a two megahertz open loop bandwidth that guarantees near perfection in the critical listening spectrum. Although the Evolution Two embraces the simplistic pleasures of a high-end two-channel preamp, it also features sophisticated control capabilities for better integration into a state-of-the-art home environment.
All signal gain in the Evolution Two is realized in the current domain using proprietary multiple-output current mirrors with nearly 500 times the open loop linearity of other designs. Negative feedback is not used anywhere in the Evolution Two preamplifier, nor is it necessary. Open-loop distortion is typically less than 50 parts per million, and the open loop bandwidth extends to two megahertz. The result is an extraordinarily wide and deep image, with an open and effortless sound.
The Evolution Two volume control is realized with a digitally controlled analog circuit with 216 step resolution, which uses low resistance, high-linearity solid-state switches and discrete precision resistors. Control signals for the switches are optically coupled for low noise and maximum signal integrity. The bandwidth and transient response of the Evolution Two circuitry are virtually unaffected by the volume setting. Typical volume controls, working in the voltage domain, introduce frequency response and phase anomalies that change in accordance with the degree of attenuation. The current domain attenuator in the Evolution Two avoids these anomalies, revealing high frequencies that sound especially smooth and extended. Music seems to emerge from ÂÂjet black silence. The analog and digital signal paths are not interconnected in the Evolution Two. Where necessary, digital control signals are optically coupled to analog circuits. The Evolution Two has two buffered tape outputs: one linked to the main signal selection bus, and another with its own independent signal-selection bus. This allows the option of listening to one source while recording another, or the option of connecting two tape decks having different input signals. Both tape outputs have single ended and voltage outputs. Either the fourth single-ended input or the third balanced input can be user-configured as a tape monitor input, allowing the opportunity to use home or professional recording equipment. Housed in a separate chassis, the Evolution Two power supply makes use of extensive electrical and magnetic shielding to keep radiated interference out of critical preamplifier circuits. Internal line conditioning circuitry filters RF noise on the AC power, and compensates for asymmetric power waveforms and DC on the mains. Power for the analog stages comes from a large 170 VA toroidal transformer.
This transformer drives four 8-amp bridge rectifiers and 39,600 microfarads of filter capacitance. Each Evolution Two preamplifier channel has as much power supply capability as many stereo power amplifiers. Most stereo preamplifiers use 10 VA transformers. The Evolution Two power supply utilizes 30 times the current capability of typical designs, thus guaranteeing its ability to deliver enormous transients without hesitation. The Evolution Two main regulators for the analog stages are scaled-down versions of those used to power the output stages of the Evolution One amplifier. Operating in current mode, with fully complementary gain stages and output drivers, wide bandwidth, low output impedance, and high current capability, the main regulators easily exceed the requirements of the analog stages. The driver and output stages in the main analog regulators use five pairs of 150 Volt, 8 Amp, 40 megahertz power transistors.A dedicated 90 VA toroidal transformer with three independent secondary windings powers the Evolution Two digital control circuitry. The regulators for the control circuitry team ultra-stable voltage reference chips with the same discrete high-current driver and output transistors used in the main analog regulators.
