6
SECTION
ONE
Evolution 202 Features and Technology
This section describes the innovative features and technology of the Evolution 202
preamplifier, and defines CAST and other key terms used in this reference.
The Evolution 202 Stereo Preamplifier, evolving from the flagship Evolution Two,
works to carefully transmit audio signals without damaging the ephemeral staging
and dimensional components of the music. The Evolution 202 is a two chassis
design with a separate power supply that isolates the critical audio stages for the
lowest possible noise floor. A combination of advanced technologies and inspired
design elevate the Evolution 202 preamplifier’s performance to the reference level.
Features
Preamplifier
The Evolution 202 has a 1 MHz open loop bandwidth in a zero feedback, bal-
anced Krell Current Mode design, terminating in Evolution CAST ™. CAST inputs
and outputs drive a newly designed current mode balanced input stage. Class A,
balanced circuitry is used throughout. Cascoded, current mirror input and output
stages utilize an LED voltage reference to achieve an exceptionally linear state of
operation.
There is no feedback employed anywhere in the design. A digitally controlled, ana-
log volume control is accomplished entirely in the current domain. The bandwidth
and transient response through this circuit is virtually unaffected by the volume
setting. A new, Krell-designed digital control circuit monitors and optimizes the
preamplifier’s operating parameters.
Power Supply
The separate power supply makes extensive use of 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 preamplifier’s analog stages comes from a large 170 VA toroidal
transformer, with four 8-amp bridge rectifiers and 39,600 microfarads of filter
capacitance. A dedicated 90 VA toroidal transformer powers the preamplifier’s
digital control circuitry. The preamplifier has as much power supply capability as
many stereo power amplifiers.
