Recommend Class a Amp

looks like Czarivey is correct in saying that the Krell KSA-250 is not full class-A. you can read a little more about that in the following link:
http://www.stereophile.com/content/krell-ksa-250-power-amplifier-when-class-class

a cut & paste from that side-bar article in Stereophile:
"Looking inside the KSA-250, the emitters of the 12 pairs per channel of output-stage transistors appear to standing on series resistors of nominal 1 ohm value. The average voltage drop across these resistors was 110.5mV, implying a standing bias for each of 110.5mA; ie, a total of 1.33A. This will give a maximum power for true class-A operation into 8 ohms of 28.5W (14.5dBW) rather than 250W."

Thanks Almarg! :-)

Nobody has mentioned the Plinius SA-250Mk4 class-A/class-AB power amp? I've heard this power amp driving B&W N803 speakers & it sounded very nice to me. There's a switch in the front that toggles between class-A & class-AB & it believe there is a DIP-switch inside that sets the time that the amp is in class-A before it transfers to class-AB if it's not being used (of course it stays in class-A if it's being used). These used to go for $5000 used.
I think it's still a very good amp.
The top-plate got scalding hot & i'm sure that the heat sinks were at 55C or higher (never measured them but taking a guess based on my experience with some class-A amps).

i re-read your post. Indeed you did. sorry about that. I must have read your post in a hurry the 1st time around....

Georgelofi
maybe this is why:-
http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1426108431&&&/Parasound-halo-JC2-what-a-joke

not directly applicable but maybe many others have had similar experiences???

I own B&W speakers that are not inefficient but which respond well to the grunt that the AB designs deliver.

isn't this a contradictory statement (it sure looks like one to me)?
Since the passing of John Bowers himself in the mid-1980s I haven't seen a B&W that is efficient. Yeah, they have a 90dB or sometimes even a 92dB SPL number but they almost always (with rare exception) respond best to high current current amps.

it's interesting to see that Pass Labs recommends their class-A/class-AB design over their pure class-A design for B&W. I'll have to dig around a little to figure out why.....

03-28-15: Abrew19
......Class A sound can be addictive however and many fans are okay with the tradeoffs....

I somewhat agree but, yeah, there are many who would sacrifice a class-A sound since getting a bigger class-A amp to satisfy their needs would be very expensive.

But the beauty of the Pass AB models like the x250.5 and x350.5 are that they deliver 90% of the sound you will be listening to at most sound levels in class A but can supply the extra grunt when you really need it.
Abrew19

the "grunt" you are talking about is, I assume, the current delivery capability of the power amp? If yes, this ability is set by (or limited by) the power transformer. For example: take the XA30.5 power amp. I'm guessing it uses a secondary 20VAC transformer. You can get a 400VA/20VAC transformer, dual secondaries. Such a transformer would be capable of 10A/ch. As a 2nd example, take the X250.5 power amp. I'm assuming this power uses a secondary 45VAC transformer. You can get a 1000VA/45VAC dual secondary transformer. Such a transformer is capable of supplying 11.1A/ch.
So, the current capability of both the XA30.5 & X250.5 amps is very similar. Yet the X250.5 is recommended by Pass Labs for speakers such as B&W. Why is that?

The answer lies in the DC rail voltage of the power amp section. For the XA30.5, the DC rail will be 29VDC. For the X250.5 the DC rail voltage will be 63V - more than 2X the rail voltage of the XA30.5!
So, when you have a speaker like B&W which has some really crazy impedance & phase responses over the 20Hz-20KHz region esp. impedances where the resistance is gets high, the XA30.5 power amp will simply run out of voltage headroom. As an example, say, the speaker resistance at a particular freq is 4 Ohms. If the max rail voltage is 29VDC, if the power amp supplies 7.25A, the voltage on the output will be 29V & the output stage of the power amp will be saturated i.e. it will start clipping. So, the XA30.5 is capable of delivery 10A max but you cannot even make use of that capability 'cos the power amp output stage saturates. OTOH, if you are using a X250.5 & the same 7.25A is supplied, the 29VDC generated in the output stage is much lower than the max 63VDC rail voltage & the X250.5 doesn't even break a sweat.
THAT is why Pass Labs recommends the X250.5 over the XA30.5 for many/most speakers. It's quite clearly stated on the Pass Labs website in the answer to the question "What is the difference between the XA.5 and the X.5 amplifiers?"

The XA30.5 would be great for a speaker with a benign impedance curve i.e. an impedance curve which remained relatively flat over the 20Hz-20KHz region. There are a few speakers that do this & for better for worse they all happen to be 1st-order x-over time-coherent speakers.
Crazy impedance curves are due to high order x-over circuits that need to compensate for sub-standard speaker drivers which change their impedance widely over their (frequency) band of operation.

Abrew, I think I understand that amp-speaker selection is not only sensitivity & nominal impedance ratings & that Qts does play a role in this selection. The Qts information is very difficult to get a hold off. However, measured resistance & phase plots are much more accessible to the public (thanks to Stereophile) & reading these impedance plots one can get a pretty good idea of what sort of power amp one might need to get superior sonics.

no problem, Abrew19.
for completion i should also add that one could have solved the issue of the amp output stage clipping/saturating by buying a bigger XA series pure class-A amp for a lot more $. I.E. if one was made of money, one could have stayed with the XA series to solve the issue of driving hard speaker loads. Since Pass Labs knows most of their clients are not made out of money they recommend the more economical route of using their class-AB designs.