Thanks for review. I was trying to remember the specs on Threshold 400 and 4000. I worked at a store where they were sold, and a good very friend of mine bought a 4000, and another guy I know had an 800. I've also always wondered what Nelson meant by "dynamic bias Class A." How does that work, and what kind of power is dissipated at idle, and how much at peak? Interesting that now Nelson, who pioneered a non-traditional bias system now goes for pure and true Class A. Even if you leave your Threshold on all the time you don't have total temperature stability because the amps are not true Class A. The power dissipation depends on power delivered to some degree.
BTW, at the store where I worked, the amplifier the staff really liked best was a cheap Nikko Alpha III, a cheap and simple Class AB Mosfet amp. Of course we didn't tell anyone because we didn't sell them. But we had one in the back room. We also liked tube amps. I'm not saying we were right, but that's what we thought back then.
I just bought a used Class A amp myself, a Krell FPB-300 which looks, works, and sounds beautiful, and I need that kind of power for my Acoustat 1+1's which suck gobs of current that shut down my old 125/200 watt amplifier. The Krell has a "plateau" bias system with 7 plateaus IIRC. I hooked it up temporarily to a kill-o-watt meter to see how the bias system works. The idle is at 300 watts which is already like a moderate power Class A amp. Give it a little kick and it moves up to 500 watts. The highest plateau I measured (didn't want to break my meter) was 1200 watts.
Krell is quite specific about not recommending you keep the amp idling all the time. They recommend you put into standby mode when not in use, which draws 60 watts and keeps the rail regulators (regulated supply for outputs!) charged. I think this is better than leaving amp fully powered as it reduces heat (which shortens component lifetime) and saves energy. BTW, I use 100% wind power through Windtricity and Native Energy.
Now certainly this doesn't fit Nelson's current thinking, which emphasizes temperature stability. Unfortunately, temperature stability costs a lot if you want 300 watts (or actually 1200 watts into 2 ohms!) pure class a with no dynamic bias or any other kind of bias trick. Such an amp would draw about 3000 watts continuously! Nelson's biggest XA design does burn indeed quite a lot of power constantly, though a bit less than that IIRC because it doesn't have quite the 2 ohm power of the Krell. I decided, given my need for power, that temperature stability was something I'd have to sacrifice. I still get most of the benefits of class a. Though I wonder how important these benefits actually are, and it seems that good old class AB designs nowadays may do equally well. The Bryston 4BSST has distortion even lower than Krell, though without the Krell's 2 ohm power. And the lowest distortion amp of all, Halcro, DM80, with parts per billion distorion, is also class AB. It may be that high bias Class AB like the Bryston is all you really need. (I am not able to do the comparison...the Bryston would have cost me just as much or more than the Krell, and I don't live near any high end dealer. I just came across an exceptionally good deal on the Krell and decided to go for it, always wanted one anyway, kinda like having a Ferrari.)
And, in fact, ignoring the "dynamic Class A" stuff, what you have with your early Threshold amps is something like high bias Class AB, and not really "pure" Class A like his newer designs.
Often you Threshold guys seem to forget there were other designers too. For example, James Bongiorno pioneered some important concepts, and designed some really hot amps back before the Threshold, and later, and is now back again with some new hot amps. (His fans also seem to forget there were other amplifier designers...) And a lot of amps like the Krell, Aragon, and Mark Levinson have fairly anonymous designers and/or teams of designers. IIRC, my Krell was actually designed by Jon Snyder. And also John Curl, who designed some of the earliest Levinson electronics (although Levinson's first preamp was designed by Levinson himself...), and now has award winning Parasound amps like the JC-1. According to James Bongiorno, one of the first good transistor amps was designed by his mentor Sid Smith, the Marantz Model 15. At the time of his death a few years ago, Sid was still using Model 15's. He was also responsible (along with Dick Sequerra) for some of the most highly regarded tube amps, the Marantz models 8 and 9, and the 10B tuner. Some wondered why Sid had not gone back to the tube designs, given their recent popularity, but he always liked his Model 15 best. And on and on. But just look at all the hundreds of highly regarded amplifiers of the last 30 years, and behind each of them is one or more incredibly talented designers, even if no one has been as successful as Nelson Pass with so many different highly regarded models over so many years under his own name. But if it was perfect, he would have only needed to design one.
BTW, at the store where I worked, the amplifier the staff really liked best was a cheap Nikko Alpha III, a cheap and simple Class AB Mosfet amp. Of course we didn't tell anyone because we didn't sell them. But we had one in the back room. We also liked tube amps. I'm not saying we were right, but that's what we thought back then.
I just bought a used Class A amp myself, a Krell FPB-300 which looks, works, and sounds beautiful, and I need that kind of power for my Acoustat 1+1's which suck gobs of current that shut down my old 125/200 watt amplifier. The Krell has a "plateau" bias system with 7 plateaus IIRC. I hooked it up temporarily to a kill-o-watt meter to see how the bias system works. The idle is at 300 watts which is already like a moderate power Class A amp. Give it a little kick and it moves up to 500 watts. The highest plateau I measured (didn't want to break my meter) was 1200 watts.
Krell is quite specific about not recommending you keep the amp idling all the time. They recommend you put into standby mode when not in use, which draws 60 watts and keeps the rail regulators (regulated supply for outputs!) charged. I think this is better than leaving amp fully powered as it reduces heat (which shortens component lifetime) and saves energy. BTW, I use 100% wind power through Windtricity and Native Energy.
Now certainly this doesn't fit Nelson's current thinking, which emphasizes temperature stability. Unfortunately, temperature stability costs a lot if you want 300 watts (or actually 1200 watts into 2 ohms!) pure class a with no dynamic bias or any other kind of bias trick. Such an amp would draw about 3000 watts continuously! Nelson's biggest XA design does burn indeed quite a lot of power constantly, though a bit less than that IIRC because it doesn't have quite the 2 ohm power of the Krell. I decided, given my need for power, that temperature stability was something I'd have to sacrifice. I still get most of the benefits of class a. Though I wonder how important these benefits actually are, and it seems that good old class AB designs nowadays may do equally well. The Bryston 4BSST has distortion even lower than Krell, though without the Krell's 2 ohm power. And the lowest distortion amp of all, Halcro, DM80, with parts per billion distorion, is also class AB. It may be that high bias Class AB like the Bryston is all you really need. (I am not able to do the comparison...the Bryston would have cost me just as much or more than the Krell, and I don't live near any high end dealer. I just came across an exceptionally good deal on the Krell and decided to go for it, always wanted one anyway, kinda like having a Ferrari.)
And, in fact, ignoring the "dynamic Class A" stuff, what you have with your early Threshold amps is something like high bias Class AB, and not really "pure" Class A like his newer designs.
Often you Threshold guys seem to forget there were other designers too. For example, James Bongiorno pioneered some important concepts, and designed some really hot amps back before the Threshold, and later, and is now back again with some new hot amps. (His fans also seem to forget there were other amplifier designers...) And a lot of amps like the Krell, Aragon, and Mark Levinson have fairly anonymous designers and/or teams of designers. IIRC, my Krell was actually designed by Jon Snyder. And also John Curl, who designed some of the earliest Levinson electronics (although Levinson's first preamp was designed by Levinson himself...), and now has award winning Parasound amps like the JC-1. According to James Bongiorno, one of the first good transistor amps was designed by his mentor Sid Smith, the Marantz Model 15. At the time of his death a few years ago, Sid was still using Model 15's. He was also responsible (along with Dick Sequerra) for some of the most highly regarded tube amps, the Marantz models 8 and 9, and the 10B tuner. Some wondered why Sid had not gone back to the tube designs, given their recent popularity, but he always liked his Model 15 best. And on and on. But just look at all the hundreds of highly regarded amplifiers of the last 30 years, and behind each of them is one or more incredibly talented designers, even if no one has been as successful as Nelson Pass with so many different highly regarded models over so many years under his own name. But if it was perfect, he would have only needed to design one.