Krell Class A/B power amps, do 'anticipator circuits' work?


My thread 2

In my hi-low speaker sensitivity thread, 8th-note mentions his Krell 300S power amps.
He shares my passion in this department.  I have run Krell Reference KRS200s, upgraded to 400wpc since 1990.
Like all Krells from the first decade of production these are 'pure class  A' all the way up.  The 300S runs class A/B

At this point, the Stereophile review of the 300S by Thomas J. Norton is very instructive and as a subscriber for many decades I acknowledge this source:
          https://www.stereophile.com/audaciousaudio/krell_ksa-300s_power_amplifier/index.html

[Isn't the Rikki Lee Jones hard to find 'Girl at Her Volcano' 10 inch just wonderful?  And the huge drum shots just the stuff we speak of below.]

By the early 1990s Krell felt under pressure from the climate change lobby to reduce the huge power consumption of full Class A operation.  The KRS200s draw more than 1kW per side.  So they abandoned it and moved to Class A/B which applies a sliding bias according to the exigencies of the music signal.

But Krell still highly valued the benefit of instantaneous power availability to cope with peaks in the musical output.  So they created 'anticipator circuits' that they said continually analyse the input signal and instantaneously increase the bias to one of four successively higher levels to accommodate peaks.

I never believed this is possible as the reaction cannot be fast enough to increase the bias before the moment is past.
How can it be implemented the moment the skin of that big bass drum is deflected by the first fraction of a millimeter.  Mr Norton covers the same issue in his review.  It seemed to me the only way to do it is to buffer the signal for at least as long as the amplifier takes to react to the input (this would of course have the massive downside of subjecting LPs to the clock and dither problems of digital).  Or perhaps beforehand to create a log of the programme that would be fed to the amplifier and applied to adjust the biasing in advance.

My KRS200s were in for full refurb in the early 2000s.  Since I was considering changing to newer Krells, I took the opportunity to make these points to the engineer doing the work.  He was not able to explain to me how it is possible but said there is no buffering.

So I have always considered the 'anticipator circuits' to be a pig in a poke or, to put it more politely, advertising flannel.

I note that in nearly 30 years no other amplifier manufacturer has sought to make such a claim.

So I retain my KRS200s as keepers; relics of a past age now gone forever in a dull world of digital amps and Class D.

I find them to be superbly dynamic and generally of extremely high SQ, if perhaps rather warm during the summer as a/c would interfere with the music.

My questions are:
Without buffering how can it be done?  Was/is Krell speaking truth?
Would buffering create the clock and dither problems I anticipate?
Has any designer tried buffering and what was the outcome?

I do rather like the concept of applying advance programme logging.  It would be a bit burdensome but, subject to the step changes of bias not being discernable, allows almost the full benefits of continuous Class A operation while keeping the Greens at bay.






128x128clearthinker

Showing 4 responses by clearthinker

Thanks for the response.
Yes a2d, at the outset Krell said that once raised the higher bias will be held 15 to 20 seconds before it is dropped back if there is no further hi-level signal.
But that doesn't answer how the first rise is triggered in time and what happens if there is a sudden new peak after 25 seconds.
One might postulate a single heavy stroke on a large bass drum every 25 seconds.
And Krell informed me there is no buffer=delay circuit.
Thanks George.

Quite.  That's rather what I thought.  Your second para describes exactly the issue I am highlighting.
Yeah.  When you've got all that heat the caps don't last too long.
I had my KRS200s totally rebuilt 10 or 15 years ago.  Perhaps they will need another cap replacement before I'm done with them.  I'm keeping them, they are the daddy of them all.
Thank you Jaytor, that is extremely helpful.  I had not read it previously.  It may be seen to settle the question I put.

I have read the patent.  It relates to a methodology of adjusting the bias current in response to measured changes in current flowing in the load.
At no point does the patent claim the bias adjustment can be triggered so fast that when a sudden high current event occurs, that event can be presented in either pure Class A or employing a higher bias current than that being employed immediately prior to the occurrence of the event.

The system can adjust the bias in quite complex ways but it cannot provide a raised bias current in time to catch sudden dynamic increases in the programme.

Such effect is advantageous to SQ and can only be achieved by running full-time in Class A.

I am satisfied the natural order is restored.  The application should not have been described by some as 'anticipator circuits'.  That would entail time travel.