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.
