KRELLEvolution TwoMonaural Preamplifier

  • KRELLEvolution TwoMonaural Preamplifier


KRELLEvolution TwoMonaural Preamplifier


Power Supplies
Housed in a separate chassis, the 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 monaural preamplifier has as much power supply capability as many stereo power amplifiers.

The 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 preamplifier's 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. These ample power reserves are brought to the Evolution Two via high reliability 12-pin connectors with machined, gold plated, three-amp contacts. Secondary voltage regulators for each analog stage in the Evolution Two provide ultra-quiet, rock-solid supply rails, as well as complete isolation between all the analog stages.

When the Evolution Two is placed in stand-by, all the analog circuitry continues to operate at the same bias current; however, the main analog supply voltages decrease by 30 percent. The result is a corresponding 30 percent reduction in power consumption and heat without the need to wait for the electronics to "warm up" before listening.

Zero Feedback Balanced Current Mode Design
The signal path is fully complementary and fully balanced from input to output. All transistors operate in Class A mode. CAST inputs drive a newly designed current mode balanced input stage, which uses a unique topology able to reject common- mode input noise over an unusually wide bandwidth. Single-ended and balanced inputs drive a dynamically-cascoded JFET buffer, which has tremendous open-loop linearity and very high input impedance, to eliminate loading effects. The voltage signals are then converted into balanced current signals, the form in which they remain through the remainder of the signal path. The audio signal remains in the current domain throughout the entire preamplifier. Only at the single-ended and voltage outputs is it converted back to the voltage domain.

All signal gain 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 0.005% (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, effortless sound. The signal switching relays are hermetically sealed and use gold-plated bifurcated contacts for maximum signal integrity. Where needed multiple relays are connected in a "T" configuration to virtually eliminate crosstalk from unselected inputs. The volume setting can be adjusted and displayed either numerically, from 0 to 151, or in dB, from - inf to +12.0 dB.

Fully Balanced Discrete Volume Control
The volume control is realized with a 16-bit balanced resistor ladder, 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 preamplifier circuitry are virtually unaffected by the volume setting. As a result high frequencies sound especially smooth and extended, and music seems to emerge from "jet black" silence. The analog and digital signal paths are not interconnected. 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.

Advanced Microprocessor Control and Custom Software
A user menu allows extensive customization of the preamplifier operation, including (but not limited to) input names, input-specific gain trims, Theater Throughput ™, display brightness, and 12-volt triggers. Integration is supported via RS- 232,12 Volt trigger, and RC-5 inputs, as well as an IR transmitter for programming a learning remote control.

A built-in digital network allows multiple preamplifiers to be linked together, blending the convenience of a single-chassis stereo preamplifier with the sonic performance obtainable only from individual monaural preamplifiers. Six or more preamplifier channels can be "daisy-chained" together and controlled simultaneously for multi-zone or home theater applications. The digital link allows the user settings in one preamplifier channel to be conveniently "cloned" to other linked preamplifier channels. Similarly, upgraded software in one preamplifier channel can be installed in other preamplifier channels using the digital link.

In the event that the DC level at any of the preamplifier channel outputs exceeds a nominal level, sophisticated protection circuitry safely mutes the outputs, traces the source of the excess DC, and displays a diagnostic message on the main display. On power-up, the microprocessor verifies that all the circuit boards in the preamplifier channel are properly connected and are compatible with the version of software installed.