Krell anticipator circuits of the 1990s

I owned a FPB-300 for about 20 years. Great sounding amp. This replaced a pair of Krell MDA-300 monoblocks. I thought the FPB-300 sounded as good and ran a lot cooler. 
 

I don’t think it really matters if the bias can adjust instantly. In fact, I suspect it odd preferable for the bias to increase gradually. There are many class AB amps that have no problem providing excellent dynamics, so running class AB for a free milliseconds Should not be a problem. 
 

At any rate, these were very nice sounding amps and did receive excellent reviews when the first came out. The only real problem is the capacitors used don’t last very long and are time consuming to replace. 

Here is a link to Krell's patent describing their plateau biasing method. It adjusts the bias based on the amp's load current requirements, not the input signal. It seems pretty clever to me. 

Krell patent

@clearthinker - it sounds like you have already decided that Krell's plateau bias technology is just marketing BS. I read through the patent in detail and, as an EE, I do think it's a clever approach and not just hype. Whether it really improves the sound quality compared to operating the same circuitry with a fixed class A/B bias, I can't say. But I was pretty impressed with the sound quality I got from this amp. When driving various Magneplanar and Revel speakers, I thought the sound quality was on par with the much less efficient Krell MDA-300 monoblocks that my FPB-300 replaced. 

When it failed after 20 years, I decided to trade it in instead of getting it fixed or I would probably still own it.

I like my Pass XA60.8 monoblocks a bit better (as well as my 300B SET amps), but my current speakers are a lot more efficient, my musical tastes have changed, and I generally don't listen at as high a volume as I used to. 

@yyzsantabarbara  - yes, I think that @clearthinker is misunderstanding the purpose of this type of circuit. It is not designed to anticipate the bias requirements to always be in class A. It is designed to adjust it's bias based on the user's load requirements so that it operates in class A "almost" all the time. 

I think most of us don't always listen to extremely dynamic music or at very high volumes all the time. For me, most of the time I am only needing a few watts of power from my amps. The plateau bias mechanism allows the amp to operate much more efficiently when high power is not needed, but has plenty of power reserves when it is. Once the bias has been ratcheted up, it will stay in high bias as long as the requirements persist (unless the heatsinks get too hot). 

@yyzsantabarbara - I was thinking about the bias mechanism used in the older FPB series which is what I used to own. There newer amps could use something completely different. 

Based on the way this circuit works, it should easily be able to keep up with a low frequency dynamic spike. By the time the waveform returns to the zero crossing, the bias will have been adjusted so the other half of the output transistors are biased into class A. I would expect this would even be the case for a high frequency spike unless the designer intentionally prevented this. 
 

@charles1dad - yes, I use both of these amps with the same speakers. The speakers are fairly efficient at 92-93db/w and are augmented by powered subs which handle up to about 70hz. So the 15w PSET amps play plenty loud. 
 

the Pass amps have a bit better control over the mid bass and are perhaps just a bit more detailed, but the tube monoblocks sounds more engaging with a deeper and broader sound stage. After going back and forth a few times, I have only been listening to the 300B amps and the Pass amps are tucked away in their boxes. I’m trying to decide whether I should sell them.