Krell Evolution 202 preamplifier & 600 monoblock power amplifier
And I whined. "I don't like to review complete systems," I moaned to Krell's Randy Bingham. "That's too many changes, and it takes forever to do comparisons."
"We don't want to impose, but with the CAST current gain system, the only way you can evaluate how good any individual component is is to hear it in concert with the other components running CAST. That way, from source to output, the system consists of a single gain stage, which is about as simple as it gets. Besides, we don't want you to review the universal player—we promised it to Fremer."
"Actually, we insist on it—and while you're at it, disconnect the CAST cables and hear the Evolutions in balanced and single-ended modes. We're convinced that even an audio reviewer—um, I mean, especially an audio reviewer—will hear the difference."
Oooh, a challenge. I love a challenge—especially a cheeky one.
Evolution of a technology
The Evolution 202 and 600 are jam-packed with technological buzzwords (not to mention the technology that drives them), so I'll just surf the highlights and point you to Krell's website for the complete list.
The 202 is a two-chassis design; the power supply, with its quad-rectified 170VA toroidal transformer and 39,600µF of filter capacitance, is housed in its own shielded chassis. The volume is controlled through a balanced 16-bit resistor ladder. The signal path is a zero-feedback, high-bandwidth, 1.5MHz open loop in a balanced Krell Current Mode design, terminating in Krell CAST (Current Audio Signal Transmission).
CAST uses Krell's Current Mode Technology (CMT) to transmit the signal as current rather than voltage. Normally, you'd want a system's preamp output to be low and the power amplifier's impedance to be high, but that creates a situation in which the interconnect's impedance could affect—even distort—the signal voltage operating the amplifier. CAST, says Krell, transfers current from a high-impedance source to a low-impedance load, essentially eliminating the cable's effect on signal transmission. And, if you're using a CAST CD player, the signal can be taken straight off the DACs without going through a current–voltage conversion stage.
That, says Krell's CEO and chief designer, Dan D'Agostino, is crucial. When D'Agostino was working on Krell's CMT, "I noticed that every time we did an I-to-V conversion, the converter added noise and grain and messed with detail, so I just said, 'Wouldn't it be wonderful if we didn't convert this at all and ran it as a pure current signal from input to output?' Once we thought of it—well, it would be wrong to say it was simple, but getting out of the voltage-gain mindset was the 'ah-ha!' moment, and the rest was just engineering. And engineering is what we do."
Of course, to run CAST, you need to use CAST cables ($500/m), which are thin and flexible and sport four-conductor LEMO connectors. The Evolution 202 has two CAST inputs, three single-ended RCA inputs (and a tape loop), and two balanced XLR inputs. There are four outputs: one single-ended (SE), one balanced, and two CAST.
The Evolution 600 monoblock, obviously, also employs CAST, but also has SE and balanced inputs. It puts out 600W into 8 ohms (1200W into 4 ohms) and employs Krell's Active Cascode Topology. ACT is not precisely the same as conventional cascode technology, which generally means combining a transconductance amplifier stage with a current buffer stage.
Krell's technical support manager, Jim Ludoviconi, likens Krell's ACT to the saying "'many hands make light work.' Cascoding is a simple method of doubling the amount of devices per rail in a push/pull configuration for linear operation. With our FPB amps, we split the audio waveform and assign an output stage to each phase of the waveform. Where typical push-pull designed divide up the current, ACT splits the voltage as well—but here's where it gets unusual for Krell: We're not in class-A anymore.
"Class-A is a needy technology in terms of space, heat management, and efficiency, which nobody knows better than Krell. The problem with not using class-A is notch distortion, which is where our driver stage comes in. The driver stage, designed like a mini-output stage, takes over and shoulders the load, just bulling the output stage through the area where notch distortion would occur if it weren't being controlled by the driver. The pre-driver stage is designed to deliver massive throughput—Dan D'Agostino calls it 'Hoover Dam'—and the input stage is a triple-cascoded current mirror, which is low distortion, which means we aren't introducing errors at that point that get amplified down the line."
The guts of the 600 are its 5000VA power supply, which is electrically and magnetically shielded to keep radiated interference out of the signal paths. The 600 also sports internal high-current line-conditioning filters, which Krell says not only remove AC noise but compensate for asymmetric power waveforms and DC on the mains. Additionally, the rails that power the amp's low-level and gain stages are dual-regulated.