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.



I always thought the same thing, and those amps never received the enthusiastic praise of their predecessors.

It depends on multiple elements in the design.

The Bias point the amp runs at at idle. The speed of the output stage ( slew rate ) How long long do you hold the amp stage at a class A bias before returning to idle. There are multiple way to increase the bias but end the end it must be summed to the output to the bias circuit.  We use to design analog amps for or Seismic equipment for exploration. One method is to monitor or sample the signal, it will take have multiple stages of opamps is series then compare it to a the decision point then sum the decision to the bias to increase it current and voltage p. Of the speed of the gain decision must be quick. The purpose of was the  digital control of gain was to keep the ADC at 3/4 full scale. That is the point that an ADC is most linear. This works great as we would ranging the gain of 64 channels at a time.  I have always found to keep it simple it the best approach. I have left off details because to in theory become complex. But it should work fine if was implemented to meet the change in bias in the correct time domain.  Straight class A is is more simple of course.  I always thought my Krell KSA250 was a little harsh they must have improved but it did not have dynamic biasing.







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! (



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.


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).

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 🤷‍♂️





Post removed 


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.

I owned a FPB-300 for about 20 years. Great sounding amp. This replaced a pair of Krell MDA-300 monoblocks. I thought the FPB-300 sounded as good and ran a lot cooler. 

I don’t think it really matters if the bias can adjust instantly. In fact, I suspect it odd preferable for the bias to increase gradually. There are many class AB amps that have no problem providing excellent dynamics, so running class AB for a free milliseconds Should not be a problem. 

At any rate, these were very nice sounding amps and did receive excellent reviews when the first came out. The only real problem is the capacitors used don’t last very long and are time consuming to replace. 


Thanks for letting me know.  But I will stay with my KSA200s.  Don't want computerised bias shuffling.  Keep it simple, as someone here rightly said.

The ksa amps with the anticipator circuit also have a couple switches on a circuit board that say relaxed/normal, if switched to relaxed mode it becomes a class A/B amp.

@clearthinker Did you watch the video I posted? I think that answers the initial question you posed on this thread.

I might be in over my head on this, and I really don’t know how Krell actually manages this, but I can imagine that if there was an opto-coupler at the amps input it could possibly alter the bias faster than the speed of sound and closer to the speed of light to adapt the output accordingly.

If Krell uses digital manipulation to accomplish this, there might be an issue of close proximity of digital noise contaminating the analog signal,

 I am curilous to know how Krell actually does accomplish this.

@yyzsantabarbara   Yes I did watch the video.  This adds nothing to the story.  The Krell personnel say the Class A watts output is varied according to monitoring of the watts of the signal output.  It does not say how it is done - 'we've found a way...', 'a very high degree...'., 'this is completely new...'.  All this is marketing puff.  It does not say how long the amp takes to adjust the bias in response to the measured output.

Since Krell must know the length of the delay, we are left to assume it must be relatively long.  If it were so short that there is an argument that its effect is inaudible, Krell would surely have told us the details.  This is all very unsatisfactory and in the result I continue to use my KRS200s.

If that guy does not spill the beans, then I doubt anyone else will since I am under the belief that he was the inventor of the iBias circuit.

Indeed.  If they don't tell us the details then we can be pretty sure it's a pig in a poke.

It may even be the iBias circuit is a spoof.  If there is anything good to tell then they will tell it.  They are protected by patents, after all.

If it sounds impossible then it probably is impossible.

It's interesting that the subject of my initial post:1990s anticipator circuits, continues into new Krell products being launched right now, nearly 30 years later.  Brought my subject right up to date.  Maybe they're still selling the same old tat.

Not at all like Krell's first product, the KSA50 which was and remains wonderful.  I still use the one I bought nearly 40 years ago, in my second system.  Not even a service required, although use has been sparse these 30 year.

I still don't believe it.


Qualified Krell service engineers have not been able to explain to me how it can work.

In my opinion they could not explain how it works because they knew it was marketing baloney.

Example of marking baloney in audio see below:

This feature plateau biasing circuit allows for the Krell FPB 600 amplifier to have Class A bias output without all the wasted electricity and heat. Really?

This is all very unsatisfactory and in the result I continue to use my KRS200s.

I agree, keep the Krell KRS200s if want new ''real'' class A amps see Pass XS300 mono block amps, they will give you all the heat you need to keep your English castle warm. 😎




@dlcockrum - My friends amplifier is a Krell KSA-100S.  It has sustained plateau biasing with four lights on the front to indicate the various levels.  Not quite the same implementation, but similar concept.

There is opto-couplers in my ksa 300s amp. It will give you 300 watts of class A power for only a little while as the heat sinks aren't big enough to sustain it. If it gets to a certain temperature it will be class A/b on the top two bias levels until the temperature comes back down.

I heard  a set of bryston  7b monoblocks  compared  to the krell fbp600. I am not s bryston  fan but I found them to sound much better in that system  that day. A friend used to have a set of krell kma monoblocks  I really liked them. I never liked the 600. 

ksa 80 from 1991 never been serviced.

Brings it everyday.

So much power it scares me.

Here is a link to Krell's patent describing their plateau biasing method. It adjusts the bias based on the amp's load current requirements, not the input signal. It seems pretty clever to me. 

Krell patent

@jaytor   I think you've probably been taken in by the hype, because if it could change the bias instantaniously it would indeed be 'pretty clever'.

@retiredfarmer   As in many cases, a manufacturer's first products are the purest, possibly because they are conceived without thought of saleability and profit.

As I recall it, the progression from the start in I think 1983 was: KSA50  KSA100, KMA100 - two bridged KSA100s essentially, KSA80 - replaced KSA50, [possibly a couple more], KRS100, KRS200.  As I recall that ended the pure Class A series by the late 1980s.  I still have KSA50 as well as my KRS200s, and a very good sounding amp it remains after 40 years.  All progress is not good progress.

@clearthinker I agree with you new is ot a given that it is better. The idea that technology has advanced so much I find interesting what is really new technology in amplifiers? Preamplifier s? Phono stages? There have been both forward and backwards steps in digital. Some components have gotten better. In my personal opinion the levinson 33h s i have, got better with the new caps better than the original ones. I think powercords have got much more balanced in the last thirty years. The hifi receptacle s are much better than the hospital grade which were much better than standard receptacle s. Things have progressed but not everything and the old top of the line new product s are still contenders and the new ones have to be really good to be as good. The only amps that I have heard that I think are better than what I have or possibly are is a pair of de agustino mono blocks I heard in a store demo a couple years back they were there top dog at the time and were really pretty wonderful. Very few times do I listen to a demo and think I could live with that system. This was one of the few times I could say that.

@clearthinker - it sounds like you have already decided that Krell's plateau bias technology is just marketing BS. I read through the patent in detail and, as an EE, I do think it's a clever approach and not just hype. Whether it really improves the sound quality compared to operating the same circuitry with a fixed class A/B bias, I can't say. But I was pretty impressed with the sound quality I got from this amp. When driving various Magneplanar and Revel speakers, I thought the sound quality was on par with the much less efficient Krell MDA-300 monoblocks that my FPB-300 replaced. 

When it failed after 20 years, I decided to trade it in instead of getting it fixed or I would probably still own it.

I like my Pass XA60.8 monoblocks a bit better (as well as my 300B SET amps), but my current speakers are a lot more efficient, my musical tastes have changed, and I generally don't listen at as high a volume as I used to. 

My favorite sounding Krell monoblock amplifiers to present day are the Krell MDA 300. I think the quiescent current per monoblock is 400 watts.🤒 😎


 it sounds like you have already decided that Krell's plateau bias technology is just marketing BS. I read through the patent in detail and, as an EE, I do think it's a clever approach and not just hype.

Totally agreed. 👍

The same can be said for class G and class H topology.


@jaytor ​​@imhififan      Well, if you read my first post you will have seen I do not believe the claims made.  I decided that in the 90s when the FPB amps were introduced.  I examined the proposition and discussed it with a Krell service engineer.  I do not believe the current of the signal can be determined and the bias changed before that signal is amplified.  I am not an EE but I have read the patent and it says nothing of the quantum of this time delay.  If this is not known then the system's suitability for audio use cannot be approved.

Amplifiers have very many functionalities apart from audio and Classes B thru H have their valid uses where their differing topologies suit the required functionality.  But for audio amplification none can deliver the sound quality of Class A.

"it sounds like you have already decided that Krell's plateau bias technology is just marketing BS. I read through the patent in detail and, as an EE, I do think it's a clever approach and not just hype."

Then as an EE you also recognize each effort to modulate the power supply models as a non-linear s-curve - a hysteresis loop with rose and fall times that must be less than any changes in the input signal. 




@clearthinker there is something  to  be said about owning  your amps for many years. First off you know them very well. Secondly  you  are off the rat race to continually  buy and sell. In my opinion  it takes a bunch of time  to get a system  at its peak. That is not in a month or two. That takes a few years. You can get them fairly close in a few months  but top performance  takes time and effort and listening.  As you eluded  to before many times the biggest effort  is put into early products.  When the person who formed the company  was in control and had the passion  for what he was making long before the bean counters  were there.  As far as time goes most everyone who plays with stereo  equipment  at this level works very hard in what ever they do. Most blue collar people think that the wealthy  are lazy that is far from the truth more like some of the hardest working  people out there. So that also brings up a couple of points spare time is limited  so what does a person  get there happiness  from in audio,  listening  to music  or discovering  each new piece that comes out?  None of those things are bad but a person  wants to get enjoyment  out of the hobby and that in a nut shell  is what the goal is. I always maintain  that what a person is doing is trying  to paint a picture with your hifi system.  There is no right artist just different  styles just because  a person's taste is a monet  doesn't  mean a DaVinci  is bad. I love hearing  other people s choices  as it is a view into that person's soul it shows in ways that people rarely show you. What makes me enjoy this thread is the passion  that many people are showing that is one thing that is wonderful  about this quest is it brings out passion.  

@retiredfarmer     Thanks for your understanding and kind words.  I agree with all you say.

I do take Stereophile (not AS as I have been a John Atkinson fan for the whole of his journalistic career) and am keen to keep up with all the new products.  But like with my cars I don't change equipment very often and very rarely sell anything.  Now approaching my mid-70s I am increasingly finding new products, especially cars, are not to my liking and that I get more pleasure and satisfaction with the older stuff I already have.

Neither am I much of a tweaker.  I use stock mains cable and have had my heavy Kimber speaker cables 20+ years and all Kimber silver interconnects for longer than that.  I am sure someone will tell me they oxidised years ago but the music still sounds great to me.

Perhaps one should not generalise about blue collar people, but one thing most of them do not do is love their work; maybe it's not that kind of work.  I loved my work and got as much out of it is from my down time.  Spending most of the time working it is crucial that it is found enjoyable.

Yes I love Da Vinci and Monet but also Dali, although I can't do the really modern stuff.




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.


@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.


@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?

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



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!


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…”




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.

The bias is always in class A unless you exceed a certain temperature, then the top two bias levels stay in class A/B until the temperature drops. 

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. 


Thanks for your reply and I understand your predicament. Once heard, engagement is very hard to do without.


I’m sure the circuit can keep up with any musical dynamics, you may have an argument on if it sounds better than straight up class A because of the added circuitry, but there is no way to compare it unless someone comes up with an amp with a switch from this to straight up class A.