Too good to be true?

I recently purchased an XA25, and I have been delighted with it.  It replaced a highly regarded 300B-based tube amp I had been using, a 70 watt per channel class A biased VAC Renaissance 70/70 MkIII.  I found that the following review by Terry London (Audiogon member Teajay) described its sonics perfectly:

https://hometheaterreview.com/pass-labs-xa-25-stereo-amplifier-reviewed/

I should add, though, that my speakers (Daedalus Ulysses) are very easy to drive, having a very benign almost flat impedance curve, as well as 97.5 db/1 watt/1 meter efficiency.

Also, see the comments by some other owners as well as myself in the following thread:

https://forum.audiogon.com/discussions/pass-labs-xa25-amp-and-bw-804-d3

Regards,
-- Al 

Mine's going nowhere for a long, long time.  I've had Sumo, Bryston, Classe, Audio Research and Ayre.  This is the one.  Balls, finesse, presence, extension, transparency, ease.  With my 90 dB Acoustic Zen Crescendo 2's, it's perfect for a condo.

Thank you, Gentlemen!  I appreciate your comments and opinions.  I own a pair of Martin Logan 11As.  The speakers are rated at a 91db as far as sensitivity in concerned.  Do you think this Pass amp is sufficiently powerful to drive the Martin Logans?  I don’t usually listen to extremely loud music and my tastes run to progressive rock, female vocals and pop.  Currently, I am using a Benchmark AHB2.  Again, thank you for taking time to respond and I appreciate your advice!

^^ That will depend on the room size and how lively it is- as far as will the amp drive the MLs, yes it will. In a larger room you may run out of gas; in a smaller and some medium sized rooms it will be just fine. The genre of music won't make any difference- electronics and transducers don't care about that :)

My **guess** is that you’ll be ok pairing the XA25 with the ML 11A if your room is of average size or less, and if (as appears to be the case) your listening does not include recordings having extremely wide dynamic range (such as some classical symphonic recordings that have been engineered with minimal or no dynamic compression). Especially given that the speakers incorporate powered woofers which are crossed over at 300 Hz.

I do have some uncertainty, though, stemming from three things:

1) Stereophile measured the sensitivity of the somewhat larger ML 15A speaker (which also incorporates a powered woofer crossed over at 300 Hz) as being 87 db/2.83 volts/1 meter, which is 5 db less than the 92 db/2.83 volts/1 meter spec of that speaker.

2) The 4 ohm nominal impedance of both the 15A and the 11A means that on a per watt basis their efficiencies are 3 db less than their 2.83 volt sensitivities (since 2.83 volts into 4 ohms corresponds to 2 watts).

So it appears quite possible that the efficiency of your speakers may be as low as 91 - 5 -3 = 83 db/1 watt/1 meter.

On the other hand, though, per John Atkinson’s measurements the XA25 is capable of delivering 130 watts into 4 ohms, with the 80 watt delta relative to the amp’s official rating of 50 watts for a 4 ohm load being due in part to Pass basing their power rating on much lower distortion percentages than JA bases his measurements on, and in part to the amp transitioning from class A to class AB at some power level (that I presume is not a lot higher than 50 watts).

3) JA found that the amp’s distortion performance degrades somewhat in the treble region when the load impedance drops to low values at high frequencies. And the 11A’s impedance descends to 0.6 ohms at 20 kHz.

So, again, my guess is that you would be ok pairing an XA25 with your speakers, given your listening habits and assuming your room is not particularly large. But there are reasons to be uncertain about that.

Good luck, however you decide to proceed. Regards,
-- Al

When reading the review, the first thing I thought of was the Benchmark Amp and how it would compare to the pass

the Pass dealer out West offers a generous trial/return program, call Mark if you are in USA....give it a whirl.....

Kosst, thanks for the good info!  I'll check out the Burning Amp lecture.

And +1 re Tomic's suggestion of contacting Mark at Reno HiFi.  That is the dealership I purchased my XA25 from.  In addition to providing generous trial/return privileges, he offers great advice and is a pleasure to deal with.

Regards,
-- Al

Al, there are two videos filmed at the BAF viewable on You Tube, from two different years. Roger Modjeski’s 90 minute (iirc) talk on tubes and tube circuits is also well worth your time.

 

Many thanks for your suggestions, advice and comments!

 

Here’s a new wrinkle – What about the Pass Labs XA30.8?  Since the woofers in my ML 11As are internally powered, what, if any, benefits would this amplifier offer over the XA25? 

For what it may or may not be worth, I have been powering ML Montis with Pass INT-150 for a few years now.  Never any issues, sounds marvelous.  No plans to change anything any time soon.

As an10490413 mentioned above, he has powered woofers.  I would like to ask if the quality of the amp makes any difference in the sound the powered woofers put out, or is it more like using line levels out from a preamp to a subwoofer?

@Jetter, Figure 1 of Stereophile's measurements of the Martin Logan 15A, which appears to be a fairly similar big brother of the OP's 11A, shows that even though the crossover point between the powered woofer section and the electrostatic panel is specified as being at 300 Hz, the panel is still putting out significant energy at deep bass frequencies.  For example, based on a near-field measurement the panel is only down about 8 db at 80 Hz.  So that would seem to be one reason to expect the amp to make a difference in the deep bass region.

@an10490413, I have no experience with the XA30.8, and it is a different animal than the XA25 in various respects, so I can't address your question about it.

Regards,
-- Al