Krell FPB600 can it be bridge to mono

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

"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."

That sounds like how my old Hitachi sr804 Class G receiver worked if I recall correctly.

It was a very nice receiver for its day, but the sound quality paled compared to many more modern designs. I tended to attribute that to the Class G operation as described, but there could easily be more to it than that.

Class G never seemed to take off though. Newer Class D amp technology seems to have taken over that niche. Sound quality is quite good with Class D I have heard and own already but I tend to think there is still useful untapped potential still to be realized down the road with that particular technology.

Krells class A plateau biasing network, is not as good as normal high biased or pure class A, to spell it out simply.

1: The network senses the transient and raises the class A automatically to keep the transient in class A.

2:It then keeps that class A level for a few seconds just in case there's another transient following or not, if not it dies back down to Class B

3: The trouble with this system of sensing and raising is that the initial transient that comes along has long gone through to the speakers before the network has had a chance to raised the Class A. So it or the first part of it still in class B

4: So your not hearing the initial transient in class A it's B, and that's why this system will never be as good as normal class A biasing but then you get the continuous heat problem with that, and much larger heat sinks,

I use to water cool my heatsinks jackets, it was the only way for me to get 150w pure class A without the size and weight.

Cheers George

Perhaps it is possible to respond fast enough with the bias to peaks at the input, since signal is subjected to small delay passing thru the amp?