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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Yes it is unneeded complexity.

Note: I can't get a 'retransmission' of what comes out of the groove on my LP.

I don't think cost was a main motivator; look at the price of Krells until D'Agostino sold and the price of D'Agostinos now.  More like 'price no object'.  Neither was lowering the weight and size an issue.  Just look at today's big amps, even if they are all AB hybrids.

No, the green lobby was the big motivator as I suggest in my post.  That is why there are few pure Class A amps today and to my knowledge no really big ones.

I want the dynamics I paid for in my media (mainly LPs).  That's why I keep the KRS200s (in fact uprated to 400w/side for a UK Krell dealer, from whom I bought them way back.

I own a Krell FPB-600c (no longer in service) and it did a fantastic job of controlling the notoriously low impedance woofer sections of my Thiel CS5i’s, often at very high volume. I would submit that few have heard the full glory of Tin Pan Alley the way this combo reproduced it. John Atkinson measured the output of the prior model (FPB-600) at over 6kW (yes kilo watts) into 1 ohm!

But it ran hot, very hot. Much too hot to touch the heatsinks for even a few seconds. Despite running supplemental feeds from a second AC unit to my listening room, I was reduced to listening in my underwear (alone) on many occasions in the summer months. The black anodizing on the massive heatsinks turned the tell-tale purple due to the immense heat the circuitry generated. Alas, the many capacitors inside the unit succumbed to the heat and failed.

@mceljo There are not five ranges of plateau biasing and there are only three lights arranged in a triangle on the front panel. In no way do they indicate the switching of the circuitry in operation. They simple light up progressively as the protection circuitry moves through its cycle upon power up and stay fully lit once the unit switches into operational mode.

Liquid cooled hybrid tube / SB mono-blocks …ROCK solid bias… love them…. and a SS sub :-) 1.2 KW per side… fun

Enjoy the music

KRELL is going to release a uber XD amp around Christmas time, KSA 400i. It will be an iBias amp with some new enhancements. It is supposed to be better than the base XD amps. Which makes it a must listen for me. The 2 base XD amps I have owned have been very good to excellent (K-300i and 175XD).

The power rating for the new KSA 400i:

400 -> 800 -> 1600 -> 3200 Watts

8 -> 4 -> 2 -> 1 Ohm | - Talks

The iBias designer is on this video talking about the new amp. He actually explains how iBias works in this video.