Krell Full Power Balanced 600 power amplifier:
Page 2 Krell's regulation feature is still more impressive when you take into account Krell's "Sustained Plateau Biasing," a patented Krell technique that provides an effective equivalent to class-A biasing—without a long-term power dissipation penalty—by anticipating the size of any and every musical event (footnote 1). It uses high-speed current-feedback circuitry to do this, then holds these required levels in a static condition for tens of seconds after the event is over. This minimizes any possible dynamic interaction of bias level with sound quality. The FPB 600's seven stages of bias represents the highest evolution yet of this technique.

Driving this powerhouse are two massive Avel Lindberg toroidal transformers, wired in parallel to give maximum power to the regulated supplies. The gain in dynamic performance from such paralleling outweighs considerations of dual-mono working with a potential for better channel separation. In any case, the regulators ensure high channel separation.

The 600's output stage is run without overall loop negative-feedback, something that can confer excellent high-frequency stability and low coloration. Such output stages tend to have more consistent lower-order harmonic distortion, and, if well executed, have the potential for more natural dynamic expression, the latter often impaired to some degree in conventional high-loop-feedback designs.

More problematical is the design and specification of output devices to provide a sufficiently low output impedance and moderate distortion, lacking the benefits in those areas that are generally offered by conventional loop feedback. This is solved by paralleling a large number of output devices within a short local loop. According to Krell, the 600 is wholly bipolar transistor, with not a FET in sight. The monster arrays of metal-can TO3 output devices are made to Krell's own specification by Motorola, acknowledged leaders in the field, and are specifically graded 25A devices. The '600 uses a total of 120 of these, both for output and regulation duties.

The final factor—one that, a priori, requires an amplifier with a sound heart to deliver its full benefit—is sheer power. If the intrinsic performance of a power amplifier can be kept uncompromised when the design is scaled up, the bigger amplifier should sound better than the smaller one, even at the same operating level. This is because the larger amplifier will be proportionately less stressed.

Compare the FPB 600 with the '300, which shares a very high percentage of engineering technology with the big design while possessing half the output current capability. Thus at 100W peak, a comfortable level for both models, proportionately less of the '600's current potential will be used, so providing greater linearity and clarity. This can be confirmed on the test bench.

Circuitry
The FPB 600 is DC-coupled and complementary push-pull, with a jumper to allow inclusion of an input rolloff at low frequencies, for use where a signal source (eg, an incompatible tube preamp), might provide DC or excessive LF. This would otherwise trip the protection, of which the final element is the circuit breaker on the rear panel.

The circuit is configured as a true balanced design from input to output. When driven unbalanced, the negative input is shorted to signal ground (with a supplied link), while the internal cross-coupling ensures that balanced integrity is obtained in successive stages. The all-discrete input and driver circuitry is run in class-A throughout, with high current ensuring a very wide bandwidth and near zero phase-shift in the audio bandwidth.

Seriously thick, plated-copper bus bars link the amplifier boards to the paralleled pairs of output terminals, and there is no relay or output inductance in the output path—there's nothing in the way. Four "small" sub-reservoir sections ("small" is relative: each is a block of four capacitors, each of these rated at 6800µF, 100V) serve each regulator rail, independent plus and minus for left and right channels, these bypassed by still smaller and faster capacitors. With all of these capacitors integrated with the amplifier's printed circuit boards, supply link inductance is held to a low value. The detachable power cord is a heavy-duty 30A type.

There is a microprocessor support in each power channel for protection, bias compensation, thermal control, startup, and to decode infrared commands (from a remote control).

Preliminaries
As I wasn't willing to sacrifice my back for a review product—not even a Krell—two strong men were brought in to elevate the '600 to my main listening room. The amplifier was installed between and well behind the main speaker positions in order to avoid spurious acoustic reflections that could have marred the stereo image.

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Footnote 1: Class-A bias means that the output current swings work within a larger defined standing current. In principle, this imparts greater linearity, total freedom from crossover or zero-point switching distortion, and more stable operating conditions for the driver and output transistors. A more focused, purer sound quality is often achieved. The downside is that the amplifier is inherently inefficient and its heatsinks have to dissipate large amounts of heat.—Martin Colloms