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.
If anyone is interested. I re-read the thread I posted earlier about They also have an iBias system similar to the KRELL XD. The designer of those amps explained how the iBias predicts the needed power. His amps go into Class AB for a second if they go beyond the existing limit of Class A. Then the Class A ramps up.
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@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). |
@jaytor The type of iBias described by the WestministerLab designer seems to me different than what KRELL seems to be saying with the new iBias XD amps. I am under the impression that the new KRELL stays in Class A all the time. I was going to email Walter Schofield at KRELL and talk about the iBias but then I saw that he is no longer with KRELL. All I know is that I put back my KRELL 175XD into my office system this morning and it sounds great. So smooth and yet powerful.
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@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. |
@yyzsantabarbara I am not up with the new Krell to which you refer. However, surely if it stays in Class A all the time then it is a (pure) Class A amp. If this is the case it has no need of iBias monitoring or adjustment as it is in Class A all the time. So what am I missing? |
