Pass Labs A/B amps vs Class A amp and Actual Power

What the Pass XA.5 series does to Class A signal power when speaker impedance is reduced from 8 ohm to 4 ohm is well discussed on this board. Secondly, the design differences between Class A and Class AB amplifiers can also be searched through this board. A good number of contributors to this particular message thread is aware of these facts. Furthermore I own the XA60.5, and I talked to Pass Labs personnel at time. When impedance is halved, power doubles for Class A signal. And that's just the way it is for this design.

I forgot to mention on my last post to this thread that another big difference between the XA100.5 and the XA160.5 is the amount of current that they can each deliver - 8 amps (I think) vs. 36 amps. I can't begin to understand why this makes such a difference, but it does. There is much less strain, particularly at loud volumes, with the added current. Demanding passages on demanding loads just sound more open and less compressed. There is a sense of freedom and effortlessness with the XA160.5 that is astonishing.

Perhaps someone with more technical knowledge can talk about what the increased current actually does.

Unsound, the Stereophile review is archived on the Pass website. I think the new website was down this AM, but I found it on the old page. Perhaps your question is more technical than my knowledge goes. Nelson frequents several forums, seems to enjoy talking shop, and perhaps you should post the question or email him.

My understanding is that the original XA series was pure class A. The meter on the front did nothing, and there was no conversion to AB. When the amps ran out of juice, they were out of juice. Anyone who has compared the 'pure' class A XA series to the new XA.5 amps has remarked that the newer design outdoes the orignial XA in every way describable.

Brian, good question. There have been discussions here as to whether there is in fact such a thing as Class A/AB or for that matter even Class AB. Some here claim that there is either only Class A or Class B, but not combinations there in. It might be just that some amplifiers have part of their out-put having attributes of Class A. I don't have any where near the technical expertise to determine what is what. I will say that more often than not, those amplifiers that claim to have more Class A output, do sound better to me.

Is the "Class A" in an amp that switches to A/B at some point the same as pure Class A?

Rtn1, I don't have access to those measurements. Am I correct in assuming that those measurements are suggesting 150 Watts per channel into 8 Ohms and 200 watts per channel into 4 Ohms? Do those measurements specify that the first 60 Watts into 4 Ohms are in Class A, or are you assuming that they are?
Again, though of a different design, Nelson Pass's Class A/AB Threshold amps provided the first 20% of out-put into 8 Ohms in Class A and the balance in Class AB, into 4 Ohms the Class A output dropped to 10% and the Class AB doubled it's output. In his Class A Threshold amps the provided 100% of their rated output in Class A into 8 Ohms, into 4 Ohms they provided 50% of their output in Class A as the balance of power output doubled into Class AB.

Unsound, there are detailed measurements in the review by Stereophile and further explanation in the Soundstage review. The Stereophile measurements on the XA30.5 estimate a maximum 150 watts into 8 ohms, and 200 watts into 4 ohms after converting into AB. So, this little guy in 4 ohms provides the first 60 watts in class A, and the last 140 watts in class AB.

Peter, your description parallels what I hear. I believe many people underpower their speakers. The XA200.5 sounds incredible at high volumes. There is no strain whatsoever. It is relaxed, effortless, perfectly dilineated, and majestic. I think that recorded music has an intrinsic optimal volume, but now it sounds so good even when blatantly too loud. If I play Mahler 8 or Gotterdammerung for the neighborhood, the needles on the front will flinch slightly at 400 watts into 90db speakers at 4 ohms. The other thing that is remarkable is how good it sounds at very low volumes, even the bass. Whether at a whisper or a roar, the Pass maintains a firm grip on all the drivers.

I just bought a new speaker which is a very demanding load - 87dB, 4 ohms, sealed enclosure. I first heard it with the XA30.5. It sounded OK. My XA100.5 made it sound much better, but still a bit closed-in during complex passages. I traded in the XA100.5 for the XA160.5 and what an improvement! Open, nuanced, dynamic and effortless. I don't understand it, but the control is better, scale and staging are better and ultimately, I'm more involved with the music.

I would buy the largest XA.5 amp you can afford. If it is not large enough, try the X350.5. Contact Reno HiFi for an in-home audition.

Rtn1, I was merely asking a question, and based upon Nelson Pass's (albeit different) previous Class A designs, the question was appropriate. The Pass web site does list the XA 100.5 as "Leaves Class A at pk Watts: 200". I'm assuming pk means peak. Does that mean sustained power or short term peak headroom power and into what impedance load? It doesn't appear to be all that clear to me.

So, roughly, with 1.5 ft from each side wall and 5 ft spread, a minimum room would be 13ft wide since 20.1 is approx 2.5ft wide...

Adding another 1.5 ft on each side (total of 3ft) would make that minimum 16ft wide and one more foot spread between the speakers would be a 17ft room...

Stickman51, I would suggest having at least mininumly a foot and a half off the side walls. You could always put the Ribbon tweeters on the inside to avoid sidewall distortion. The space between the 20.1's should at least be five to six feet apart between the inside edge of the speakers. My 20's were set up this way in my old place and they still sounded great, now I have them more then six feet off the side walls and about nine feet apart and they sound even better. Hope this helps.

Remember, you want at least four to six feet off the front wall behind the 20.1's so they can breath.

Rtn1, that is exactly what Pass told me today. All of the XA.5 amps will double their Class A output into 4 ohms.

Guys, any thoughts on the absolute minimum room width for Magnepan 20.1's?

Let's be clear. The XA.5 doubles its Class A power. The XA100.5 would provide 200 beautiful class A watts before going class AB. This is indicated in the specs on the Pass website. The older XA series kept the output fixed at lower impedences.

All the more reason to make sure your buying what your expecting. Mind you, the same might be true re: doubling down power/halving Class A bias for the amount of Class A bias in the Class A/AB amps.

For me it's probably a mute point; the XA100.5 is most likely more than I can/would spend even at used prices...

Stickman, many if not most amps that are touted as doubling down Class A amps actually halve their Class A out-put as they do so. Including Nelson Pass' previous Threshold designs:
http://www.stereophile.com/solidpoweramps/1290thresh/index1.html

Teajay or other 20.1 owners - in your opinion, what is the absolute minimum room width that will work with a Magnepan 20.1?

I'm pretty sure that Pass Labs told me that all of their XA.5 and X.5 amps 'double-down', so an XA100.5 would give you 200 watts of Class A into a 4 ohm load... Pretty sure I heard them say that exactly...

Stickman, are you sure that the XA 100.5 maintains Class A bias (or something to that effect, as there has been discussion here on the technical classification as to whether Class A/AB and Class AB amps technically deliver any Class A bias(?))or is the Class A Bias reduced and the Class AB increased as it doubles down?

There is a big difference in the sound of 'Class A' between the XA.5 and the X.5. They are entirely different amps despite their similar cosmetics.

Buy the X.5 because it is a good deal and a value component. Don't buy it because you think the Class A sound is equivalent.

In the end, both are great amps.

I actually gave Pass Labs a call today and they were very patient and helpful in discussing the relative power(s) of their amps. They recommended either the XA100.5 or the X350.5. They felt that in my larger room and considering my history in using Horn speakers (In addition to the 3.6's I have a pair of Klipschorns) that the XA60.5 might not quite get the job done for me. All their amps 'double-down' into 4 ohms so I would have either 200 watts of pure Class A (that's a lot!) or 700 Watts of Class A/B (that's a lot too!)... And, even with the X350.5 I would get 80 Watts of Class A performance before it slipped into A/B mode; sounds like the X350.5 is a good deal and just the ticket!

As Elberoth sugggests, the XA.5 series is very conservatively rated, I can tell you that having owned the XA30.5 I would discuss it with Pass Labs, if they tell you it should be no problem, you can trust them.

There is a big misunderstanding when it comes to XA series power rating. Pass claims that XA-30.5 delivers 30W in pure class A. And it is true. What they do not tell, and what was shown in Stereophile measurements, is that the amp delivers extra 100W in class AB. So in real world, XA-30.5 is a 130W amp, which HAPPEN to deliver first 30W in a pure class A, not a pure class A 30W amp. Even higher powers are available into lower impedances before clipping: 195Wpc into 4 ohms, and 332W into 2 ohms.

I have not seen the exact numbers for XA-60.5, but my guess is that those numbers double for XA-30.5.

This may explain why Teejay did not have any problems with driving his Maggies, and I do not have any problems with my XA-30.5 driving W/P Sasha.

It is difficult to know whether the X350.5 or the XA60.5 would be better for you. It depends upon the size of your room, the music you like, and the loudness level. For example, classical music requires alot of power to avoid becoming congested and strained.

There is no doubt the XA.5 is better than the X.5, by a huge margin. However, you may need to consider the XA100.5 (or even the XA160.5) for the 86db load into 4 ohms.

Personally, I would be skeptical that the XA60.5 would do the job. You can arrange a demo from Reno Hifi if you first want to try. Ideally, the XA160.5 would probably be the pick.

There are not "different kinds of watts" - power, measured in watts, is the product of voltage and current, period. Amps differ in their ability to deliver power across the audio frequency spectrum into loads with complex impedance.

I can see Teajay using the 60.5's in a biamped configuration with the 60.5's on the treble side and another more powerful amp on the bass side to drive his 20's. Music demands far less power in treble than in bass. I'm skeptical that the 60.5 could drive a 20 or a 3.6 full range with satisfactory volume, though.

According to his system page, Teajay is running Threshold SA-1 monos @ 160 watts each on the bass panels and Pass 60.5's on the treble.

Thanks for the feedback. There is a used XA-60.5 on AG right now; very tempting!

It's just difficult to accept that a 60 watt amp can drive the Maggies with more or as much 'power' than a 350.5 but I we talking about different kinds of watts too.

I have had both amps/monoblocks in my system driving my biamped 20's. I loved the 350.5, until I heard the XA-100's and now have the XA-60.5's which sound beautiful in my system. So, pure power/grunt, is not the issue, in my opinion, the 60.5's will have enough juice and still make your 3.6's sing on a higher level then a 350.5. Take a look at my reviews here on the GON for details on all the above mentioned Pass Labs amps. Nelson is a true genius.