Why not stick with Aestetix all around? My personal experience is fairly limited, but my local shop carries the full Focal Utopia lineup and the last time I stopped by they has Asthetix gear to go with them. Having said that, I was never that impressed with the combo but was always blown alway with Krell amps with the Focals.
15 responses Add your response
Since the FPB-600 has balanced outputs, it cannot be bridged. From the manual: The differential circuitry employed within the Full Power Balanced amplifiers requires special attention when connecting speakers. Be careful not to connect the negative speaker terminals together. Do not connect the negative speaker terminals to ground, or attempt to bridge the left and right channels together....Regards, -- Al |
Jeff, Martin Colloms' review of the FPB-600 in Stereophile states as follows; ... Krell's "Sustained Plateau Biasing," a patented Krell technique that provides an effective equivalent to class-A biasingwithout a long-term power dissipation penaltyby anticipating the size of any and every musical event.... 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.The manual states that: Sustained Plateau Bias is a KRELL patented process that enables your amplifier to play all music (up to full rated power) in Class A, yet greatly reduce the heat dissipation and energy consumption associated with conventional Class A designs. Sustained Plateau Bias is a true Class A circuit, as opposed to sliding or adaptive biasing schemes.I suppose the last sentence quoted from the manual is debatable, and comes down to a matter of definitions and semantics, but in any event Mr. Colloms' comment seems to explain the relevant aspects of the design. Best regards, -- Al |
Also, regarding "how would it know a transient is coming to switch it to a higher mode?," while I don't quite understand the reference by Mr. Colloms to "high-speed current-feedback circuitry," presumably the delay from amp input to adjustment of output stage bias is less than the propagation delay through the signal path from amp input to the input of the output stage. There have been a few designs that have appeared over the years from other manufacturers which also changed output stage operating conditions in anticipation of signal requirements, except that what was being changed were the DC voltage rails supplied to the output stage. In those cases that I am aware of the purpose was to reduce power consumption and heat, and consequently cabinet size, weight, and cost, rather than to maintain what might be considered to be class A operation. Some of Bob Carver's older "magnetic field power amplifiers" were one such example. Best regards, -- Al |
"There have been a few designs that have appeared over the years from other manufacturers which also changed output stage operating conditions in anticipation of signal requirements, except that what was being changed were the DC voltage rails supplied to the output stage." That sounds like how my old Hitachi sr804 Class G receiver worked if I recall correctly. It was a very nice receiver for its day, but the sound quality paled compared to many more modern designs. I tended to attribute that to the Class G operation as described, but there could easily be more to it than that. Class G never seemed to take off though. Newer Class D amp technology seems to have taken over that niche. Sound quality is quite good with Class D I have heard and own already but I tend to think there is still useful untapped potential still to be realized down the road with that particular technology. |
Krells class A plateau biasing network, is not as good as normal high biased or pure class A, to spell it out simply. 1: The network senses the transient and raises the class A automatically to keep the transient in class A. 2:It then keeps that class A level for a few seconds just in case there's another transient following or not, if not it dies back down to Class B 3: The trouble with this system of sensing and raising is that the initial transient that comes along has long gone through to the speakers before the network has had a chance to raised the Class A. So it or the first part of it still in class B 4: So your not hearing the initial transient in class A it's B, and that's why this system will never be as good as normal class A biasing but then you get the continuous heat problem with that, and much larger heat sinks, I use to water cool my heatsinks jackets, it was the only way for me to get 150w pure class A without the size and weight. Cheers George |
I don't think you could delay the signal so the initial plateau biasing can stay in front of it, without recording and playback either by digital or tape means, this would be even more detrimental to the sound quality. Back when I was building those water cooled class A monsters in the 1970-80's. My then boss and mentor Steven Deratz invented and patented the first electronic sliding class A bias system, it was good but never out performed the real thing. After a few years he let the patent run out. Then I believe the first commercial system I saw of the same, came from Technics and many years after Krell bought theirs out. Cheers George |
Looking at the size of the heat sinks of the new Gryphon's and the quoted Class A, I don't believe they could be pure class A, maybe sliding bias as well. There were ones going back 10 years that were huge and only 100watts total they could have been pure class A or 3/4 and the rest A/B. There's a lot of fudging going on with quoted class A figures. The only ones I know for sure, were the water cooled ones I built, and maybe the forced fan cooled early Krell KSA50 and KSA100 not the 100s And also a 20w Nelson Pass A40 project also a fan force little monster, from the 70's which was the basis of my 150w water cooled ones. [url]http://www.google.com.au/url?sa=t&rct=j&q=&esrc=s&source=web&cd=6&cad=rja&uact=8&ved=0CDAQFjAF&url=http%3A%2F%2Fwww.firstwatt.com%2Fpdf%2Fart_a40.pdf&ei=SKBnVYLoI4Xj8AXY5YPIAg&usg=AFQjCNFzV3J8CTmkP6fI6mreRC5uu3Ymaw&bvm=bv.93990622,d.dGc[/url] Here was one for sale one ebay. [url]http://www.ebay.com/itm/Nelson-Pass-A-40-Class-A-Stereo-Amplifier-A-Very-Rare-Classic-/291381081421?nma=true&si=ph3vIqnZtW%252Bz6wdpAIVab%252BCAJoI%253D&orig_cvip=true&rt=nc&_trksid=p2047675.l2557[/url] Cheers George |
I found Krell's patent on what is being discussed. It appears that rather than adjusting the bias among the different possible levels based on the input signal, the adjustment is performed based on sensing of the load current. Which explains the reference by Martin Colloms to "high-speed current-feedback circuitry." The patent goes on to indicate that bias increases necessitated by abrupt transients will be "jumped to," but subsequent decreases, if called for, will occur one level at a time, and slowly. So George is correct that in the event of a large and abrupt transient there will be some very brief instant of time during which the amp will not be operating in class A. I have no opinion regarding the degree to which that may be audibly significant. Regards, -- Al |
So George is correct that in the event of a large and abrupt transient there will be some very brief instant of time during which the amp will not be operating in class A. I have no opinion regarding the degree to which that may be audibly significant. I would say if your "good enough" and you have an aural microscope for an ear, you may hear the class B xover distortion which could manifest itself as a smearing, or hardening of the first transient after a quite section, if the plateau biasing has come back down from the last transient. Cheers George |
