Krell FPB600 can it be bridge to mono

I found Krell's patent on what is being discussed. It appears that rather than adjusting the bias among the different possible levels based on the input signal, the adjustment is performed based on sensing of the load current. Which explains the reference by Martin Colloms to "high-speed current-feedback circuitry." The patent goes on to indicate that bias increases necessitated by abrupt transients will be "jumped to," but subsequent decreases, if called for, will occur one level at a time, and slowly.

So George is correct that in the event of a large and abrupt transient there will be some very brief instant of time during which the amp will not be operating in class A. I have no opinion regarding the degree to which that may be audibly significant.

Regards,
-- Al

Since the FPB-600 has balanced outputs, it cannot be bridged. From the manual:

The differential circuitry employed within the Full Power Balanced amplifiers requires special attention when connecting speakers. Be careful not to connect the negative speaker terminals together. Do not connect the negative speaker terminals to ground, or attempt to bridge the left and right channels together....

Regards,
-- Al

Jeff, Martin Colloms' review of the FPB-600 in Stereophile states as follows;

... 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.... 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.

The manual states that:

Sustained Plateau Bias is a KRELL patented process that enables your amplifier to play all music (up to full rated power) in Class A, yet greatly reduce the heat dissipation and energy consumption associated with conventional Class A designs. Sustained Plateau Bias is a true Class A circuit, as opposed to sliding or adaptive biasing schemes.

I suppose the last sentence quoted from the manual is debatable, and comes down to a matter of definitions and semantics, but in any event Mr. Colloms' comment seems to explain the relevant aspects of the design.

Best regards,
-- Al

Also, regarding "how would it know a transient is coming to switch it to a higher mode?," while I don't quite understand the reference by Mr. Colloms to "high-speed current-feedback circuitry," presumably the delay from amp input to adjustment of output stage bias is less than the propagation delay through the signal path from amp input to the input of the output stage.

There have been a few designs that have appeared over the years from other manufacturers which also changed output stage operating conditions in anticipation of signal requirements, except that what was being changed were the DC voltage rails supplied to the output stage. In those cases that I am aware of the purpose was to reduce power consumption and heat, and consequently cabinet size, weight, and cost, rather than to maintain what might be considered to be class A operation. Some of Bob Carver's older "magnetic field power amplifiers" were one such example.

Best regards,
-- Al