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  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.

 

clearthinker
yyzsantabarbara
5,101 posts
 

That baloney works pretty good on the new XD lineup.

I currently have the KRELL 175XD stereo and it is supposed to be 175 watts in Class A. Though that max power in Class A is only available when demanded. Thus, the unit is not heavy and nor very hot since it is not wasting Class power when the music is not demanding it.

I went and demoed the CODA #16 amp since I was considering upgrading from the CODA #8. I felt the KRELL 175XD was better than the #8. When I heard the CODA #16, which has the first 100 watts in Class A, I could hear a very similar sonic characteristic between the 175XD and the #16. I thought the #8 sounded not as smooth and relaxed (a little grainier), while the other 2 sound smooth and relaxed. Maybe the #16 was a little clearer, like a Benchmark AHB2.

There is another amp from Japan (I think) that does something like the iBias. This is my question to the designer or distributer. The thread is a good read.

Re-imagining "Class A" Amplification | What’s Best Audio and Video Forum. The Best High End Audio Forum on the planet! (whatsbestforum.com)

 

 

clearthinker's avatar
clearthinker OP
1,582 posts
 

Thanks for your posts.  I think so far 3.5 to 0.5 in support of my position.

@jew16384   You offer suggestions as to elements of the design that might be engineered with the objective of moving instantaneously from Class B to Class A bias.  (Put like that it sounds an impossibility).

You say ' Of the speed of the gain decision must be quick.'  Indeed it must.  What I asked is can it be quick enough to amplify a signal just heard in pure Class A.  i.e. to change bias instantaneously.  That is an impossibility.  The only way of doing it would be to buffer the signal and amplify it later. when the Class A bias had been achieved.  But that would entail all sorts of dither and clock distortions, turning an analogue signal effectively into digital when I wanted to listen to LPs.

 

mceljo
1,357 posts
 

A friend of mine has one of these Krell amplifiers, I think.  It has about 5 different ranges that are identified by lights on the front.  The example you give is very specific and it's almost always possible to find a specific example to expose a design flaw because every design has compromises.  The way I would look at it is that there could be something lost when the amplifier is first triggered to the next level and then anything within that level would be unaffected until it dropped down a level or was triggered to go to the next.  I would think that by watching the lights it would be possible to estimate the number of times that something was potentially lost during the initial instantaneous need to go up to the next level.  I think it's obvious that the sound would be expected to be something less compared to another Krell running wide open all the time, but there's reasons that make that design not ideal (though not released to the SQ most likely).

jew16384's avatar
jew16384
59 posts
 

If it changed the bias in few milliseconds you will not know the difference. However can the power demand keep up, will it compress the dynamics in the outputs. The main reason for doing this is to save money in the power supply and lowering the weight and size of the chassis.
 

Example,  If a fiber is Cut on a long haul fiber network happen the system will switch to a backup fiber with in 50msec . ( telecom voice and data ) it has been proven that at most you will hear tick in that time period of 50msec to restore the path. The data is buffered and request a retransmission and it works.

In the long run you’ve added unneeded complexity to a simple amplifier to reduce cost those element I mentioned. Keep it simple stupid is the cry. But it can work, I never compared one type amp to another 🤷‍♂️

 

 

 

 

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