Recommend Class a Amp


This past weekend I purchased some Infinity QLS-1 Speakers and the gentlemen I bought them from was powering them with a Krell KSA-250. I was impressed, how the Krell drove those speakers. Most of my amps are tube and I would like to get me a keeper used well thought of class A solid state amp. I would like to stay in the Under thew 3K price range. Look forward to your recommendations.
robmag
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)?

Hi Bombaywalla: not really the same thing. When I mentioned "not inefficient" I meant simply the Sensitivity rating of 90 spl is not considered low (or inefficient). It is fairly average (if there is such a thing). But this doesn't mean that the speakers don't work better with amps that can grip them and drive them the way the loudspeaker designers intended. It's not all about the Sensitivity and nominal impedance ratings. There is also something referred to Qts that plays a big part. I don't pretend to understand it myself; I am just pointing out that even the amplifier manufacturers recognize that the amp-loudspeaker relationship is complex and even they have some challenge explaining.
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.
Georgelofi, The JC1s have gotten good reviews and I've read posts praising them on this forum, but for whatever reason, they just don't generate a lot of buzz here.

Stereophile has them ranked in Class A with amps that cost many tens of thousands of dollars and even $100,000+ amps. They get a star for their longevity on the list and $$$ signs for their being a bargain. The description takes up 2/3 of the column and is an absolute rave. It ends stating that the amp is the "Joint Amplification Component" for 2003!!!

Maybe you should start a thread about why they aren't more popular. We might find out or maybe no one would reply.