2. amps-preamps
  3. 178665-147550

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.

 

128x128clearthinker
jaytor's avatar
jaytor
765 posts
 

@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
4,903 posts
 

@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.

 

jaytor's avatar
jaytor
765 posts
 

@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. 

clearthinker's avatar
clearthinker OP
1,582 posts
 

@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?

jew16384's avatar
jew16384
59 posts
 

So the Bias slides up, big deal and down. Krell would have done time domain tests to determine if there spurs appear off the fundamental.  Something like this would not be done by a field but an engineer. We would do this in the digitally domain for the floating dynamic amps and  converters we built. It not simple but not rocket science either but timing has to perfect as did this to 60 channels in parallel. This was at Texas Instruments and several others. I doubt I changed any minds but keep an open mind that analog computing is fast.

I’m in on Pass Labs, Cherrs

 

More to discover