Do you use your parallel 300b amplifier and Pass Labs XA 60.8 driving the same pair of speakers? Is so, this seems an interesting comparison of two very different topologies. Nice!
Charles
Krell anticipator circuits of the 1990s
"Krell FPB-600 Stereo Power Amplifier
This big power amp features the evolution of the plateau biasing circuit introduced in the KSA series of amps. This circuit anticipates the power demands of the output by monitoring the incoming signal as the demand for power increases, the more power the amplifier supplies. After a grace period of fifteen seconds and no additional high current signal demands, the Krell FPB-600 amplifier returns to its appropriate power setting. This feature allows for Class A bias output without all the wasted electricity and heat."
Do you believe the anticipator can up the bias quickly enough? A guy hits a huge bass drum, the anticipator circuit senses this and ups the bias in time for the hit to be amplified in Class A?
We are talking a micro second. Once he hit it the start of the moment was over. This was a con. Created by Krell because they were under pressure from the emerging green lobby to cut power consumption. Qualified Krell service engineers have not been able to explain to me how it can work.
Me? I still have my KRS200s. Pure Class A. So there's my answer.
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Maybe Martin Collums in his Stereophile review, gets it. “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…”
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Martin Collums doesn't add anything to the debate. He eulogises the anticipator circuit. But he doesn't say how long it takes for the bias to be changed. That still hides behind the emperor's new clothes. At this point it is necessary for me to repeat the question in my OP: how long does it take for the amp to react to the increased current generated by the signal (the big bass drum hit) before the bias is restored to Class A. No-one has told me. But it will be longer than it takes for the initial pulse of that drum hit to pass. |
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. |
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