Pass ACA Class A Power Amplifier Review

[MediumRare]

As opposed, perhaps, to a more modernistically inclined, LGBTQ (add any initials I may have overlooked) wire?

A veritable rainbow of sound!

 

[m8o]

What did you do with all the other tones in your music that are each creating that harmonic spray? You know, those bass tones with much higher amplitude than the violin. Now the harmonics of those bass tones are stomping over the violin tones, including its -20 dB second harmonic.

Edit. Or p.s. to my reply above. I will also add... I imply this without saying it explicitly. Yes, most of the energy in music is in the bass region. And if "new tones" are being generated at -30dB or under to those high amplitude bass fundamentals and its true harmonics, those "generated tones" will actually be swamping mid and high frequency tones where those fundamentals and harmonics are lower in amplitude than the generated tones. Yup! Definitely agree. Related story. After I pointed an audiophile / audio collector friend of mine to this review thread, but before he read it, he told me he wanted to build an ACA a while back and would still like to do so with me. I stated to some disappointment I really was not interested as this review shows that it generates its own artificial tones that are audible. So yup, I definitely agree.

 

[dtaylo1066]

I am not sure this thread is off the rails, though civility is important in any discussion. Keep it civil, keep it respectful, and we all do not have to agree. But to me, several key points exist, and perhaps an audio conundrum. Measurements do not lie. They are factual data and cannot be dismissed. Yet in audio I have heard equipment with the most impeccable measurements that I did not think sounding great. Why is that? Does one purchase for signal accuracy and lowest distortion, or for what on thinks, in their mind, sounds best? And of course price matters. I listen primarily to a DIY Hypex amplifier with UCD180 modules and a most beefy linear power supply. Amirm in this forum has rightly extolled the impeccable design and measurements of Hypex amplifiers. I find my Hypex amp to be quite good. Dead silent background. Tremendous slam in bass. Clear and accurate reproduction. But I would have to admit that it does not quite engage me -- or get my toes tapping -- as much as my home brew, upgraded Dynaco ST70, which obviously has more distortion -- and inferior measurements overall. Do I dismiss the Dynaco for its flaws and weaknesses, or praise it for its ultimately pleasing sound reproduction? Do my ears and brain crave accuracy or a certain type of sound? Am I being lulled by H2 tube distortion? I am not an EE, but having been an audiophile for 45 years. I for sure know how to listen and I know what my brain says sounds good. I have listened to a tremendous array of audio gear, from insanely expensive to budget priced. I have heard great sound in all price levels, and bad sound in all price levels. I am happy to recognize and engage all music lovers and audiophiles, whatever their preference in gear. We share the same goal - to enjoy listening to music. But we do so with independent and different ears and experiences. And kudos to Amirm for faithfully measuring so many audio products and providing so much data.

 

[audimus]

You have lost the plot with that. The classic definition of "Class A" is that it has less distortion than class B due to lack of crossover distortion. Combine the notation of Class A with "Nelson Pass" and you have a formula where audiophiles buy this amp thinking they are getting more pure sound.
...
They want that pure "Class A" with Nelson Pass name on it.
...
With the preconditioning of above, combined with improper listening tests, they "hear" the good sound you rave about.
...
What we have done here with the measurements that are clearly compared to other amplifiers in the market, is to set the record straight. That you did not get what Class A amplifiers are famous for. That the name Nelson Pass did not bring decades of building high-fidelity amplifiers.

Self-serving framing of the issue where opinions crossover into subjective under the guise of objectivism.

Counterframing: Audiophiles don’t necessarily know what Class A does or does not vs any other class or Stasis or whatever is associated with NP. They couldn’t care less. This is conflating audiophiles with audio tech savvy, a frequent mistake in these forums. Some may have heard of Nelson Pass and his history. Some think this is worth a try and don’t mind DIY work or think it a plus to (re)indulge in a pastime. They buy and build it. Some like it, some may not.

The reason some like it may be precisely because of Nelson Pass’ philosophy of coloring amps or they may be influenced by Nelson Pass name or they may be influenced by the fact that they built it themselves or by the nice look of the case. We objectively don’t know why and to claim one of them to be true is not objectivism, it is self serving ideological dogma, the kind that is practiced here.

This amp is measured and shown to have high distortion numbers. There is evidence that some of these distortions are at audible levels. But there is no evidence that these numbers necessarily make it sound bad for all people, it colored the sound sure. But it may explain via science why the first reason above was the right one for them, not that they were necessarily fooled by labeling or biases. But the so-called practitioners of science, dogmatically attribute it to being fallible or gullible as necessarily true without proof.

Some people who liked it before they saw measurements may continue to like it for the first reason. Some people who liked it before may not like it now after the measurements but not because they are hearing different but because they have been conditioned to think those measurements should necessarily sound bad and leads to doubting their own hearing. The so-called objectivists think it is a “good thing” even though it is just another form of being fallible.

This unit and its review and its subsequent discussion is a good illustration because it is a caricature (in the literal sense of exaggerating certain characteristics to draw attention to it) of every review and test here where it shows up things that are right (measurements) but also things that are wrong (interpretations) but happen more subtly in “real” equipment.

 

[dtaylo1066]

Interesting points, Audimus. What consumer product does one ever purchase and not consider, in part, the brand, the designer, the look, the badge value, peer opinion, reviews, cost, etc. What cars do people on this forum purchase and how much of their purchase is based on the car company's overall reputation, and a particular model's measurements? How do factors such as horse power, torque, turbo vs. naturally aspirated, acceleration, suspension or steering factor into that purchase decision? What beer do you purchase -- craft, import, local craft, light beer, stout -- and why? All products have objective measurements and data, but one cannot dismiss or decouple the subjective factor in decision making and product satisfaction.

 

[amirm]

Self-serving framing of the issue where opinions crossover into subjective under the guise of objectivism.

Counterframing: Audiophiles don’t necessarily know what Class A does or does not vs any other class or Stasis or whatever is associated with NP. They couldn’t care less. This is conflating audiophiles with audio tech savvy, a frequent mistake in these forums. Some may have heard of Nelson Pass and his history. Some think this is worth a try and don’t mind DIY work or think it a plus to (re)indulge in a pastime. They buy and build it. Some like it, some may not.

Doesn't look like you have been around high-end audiophiles much. Yes, they have no idea how these technologies work. But have developed affinity for many such technical terms:

1. Class A Amp. Must be more pure than class AB. Even the lettering says so.

2. R2R DAC. Must be more pure than Sigma-delta.

3. Break-in. Surely audio gear is an engine can use break-in.

4. Tubes. They light up so sure they sound "warm."

These are stories that they are told by the audio industry and many absolutely believe it. Why else would any manufacturer emphasize that they have class A amp if they thought there is no preference for it among audiophiles? Here is a review Pass Labs has highlighted for the XS-150 amplifier:

That is a $65,000 amplifier by the way!

Really, I have not read anything this wrong on any topic in a while. Don't pontificate on people and markets you don't understand.

 

[amirm]

This amp is measured and shown to have high distortion numbers. There is evidence that some of these distortions are at audible levels. But there is no evidence that these numbers necessarily make it sound bad for all people, it colored the sound sure.

Oh? You mean other than the fact that it doesn't have enough power to power vast majority of speakers in the market? Did you read the review and my listening tests? This thing is like trying to read under candle light. It can be done, but there is no reason to because amplifier power is so easily to buy now. There is absolutely no bass whatsoever with many speakers. That, I call coloration to the extreme given how important bass is.

It also runs hot, and chews up a ton of power just sitting there.

How many wrongs does it take for you to not want to give it a pass?

 

[amirm]

This unit and its review and its subsequent discussion is a good illustration because it is a caricature (in the literal sense of exaggerating certain characteristics to draw attention to it) of every review and test here where it shows up things that are right (measurements) but also things that are wrong (interpretations) but happen more subtly in “real” equipment.

No, it is typical of spin we get in people trying to defend products with no facts of their own. It is not like you have presented a listening test with another $300 amp and declared it as good or better. You have presented no data but faulty arguments.

What we don't want in these review threads are empty arguments that just raise the noise floor but don't inform. I have other things to test than helping increase your post count.

We don't need a PR person for these products. That is useful in other forums, not here. I am an advocate of consumers and will test products and speak to their weaknesses. There are plenty of products that don't have these compromises. So I am not going to sit here and read people presenting word defenses.

 

[amirm]

Measurements do not lie. They are factual data and cannot be dismissed. Yet in audio I have heard equipment with the most impeccable measurements that I did not think sounding great. Why is that?

The key word is "think." Your brain adds things to what your ears pick up. As you can imagine, I have tons and tons of audio products across wide range of performance. I perform listing tests every other day. What I do that is different is that I remove other factors that impact the perception of sound. When I do that, there is no conflict in listening tests and perceptual analysis of measurements.

I have yet to hear a single benefit to any kind of distortion despite testing a ton of products. Maybe it exists but if it does, it is so rare that vast majority of such observations must be false.

 

[tmtomh]

Self-serving framing of the issue where opinions crossover into subjective under the guise of objectivism.

Counterframing: Audiophiles don’t necessarily know what Class A does or does not vs any other class or Stasis or whatever is associated with NP. They couldn’t care less. This is conflating audiophiles with audio tech savvy, a frequent mistake in these forums. Some may have heard of Nelson Pass and his history. Some think this is worth a try and don’t mind DIY work or think it a plus to (re)indulge in a pastime. They buy and build it. Some like it, some may not.

The reason some like it may be precisely because of Nelson Pass’ philosophy of coloring amps or they may be influenced by Nelson Pass name or they may be influenced by the fact that they built it themselves or by the nice look of the case. We objectively don’t know why and to claim one of them to be true is not objectivism, it is self serving ideological dogma, the kind that is practiced here.

This amp is measured and shown to have high distortion numbers. There is evidence that some of these distortions are at audible levels. But there is no evidence that these numbers necessarily make it sound bad for all people, it colored the sound sure. But it may explain via science why the first reason above was the right one for them, not that they were necessarily fooled by labeling or biases. But the so-called practitioners of science, dogmatically attribute it to being fallible or gullible as necessarily true without proof.

Some people who liked it before they saw measurements may continue to like it for the first reason. Some people who liked it before may not like it now after the measurements but not because they are hearing different but because they have been conditioned to think those measurements should necessarily sound bad and leads to doubting their own hearing. The so-called objectivists think it is a “good thing” even though it is just another form of being fallible.

This unit and its review and its subsequent discussion is a good illustration because it is a caricature (in the literal sense of exaggerating certain characteristics to draw attention to it) of every review and test here where it shows up things that are right (measurements) but also things that are wrong (interpretations) but happen more subtly in “real” equipment.

I think you make some good points here, and so I am not commenting with the purpose of disagreeing with you entirely.

But I also think some of your points are as imprecise as you accuse others of being, and I think that lack of precision undermines your argument.

Specifically:

1. As @amirm has noted in his response to you, there is no way in the world that a reasonable survey of the audiophile landscape cannot conclude that Class A is viewed among most audiophiles as a desirable characteristic because of superior sonics. Whether or not audiophiles are aware of switching distortion in particular is irrelevant: Class A is known to run hotter, be less efficient, and generally be more expensive, and the common-sense idea among audiophiles is that those drawbacks are worth it (or at least the price paid) for a purer/superior audio topology. In short, Class A amps are considered premium quality amps.
2. If people are buying Pass amps because of Pass' name and not because they are Class A/Super Class A, then they are falling for the Appeal to Authority logic fallacy, which is at least as bad as buying based on false technical claims. Either way I don't see how it helps your argument.
3. You say that amps with "high distortion numbers" might not "sound bad for all people," but you concede that they "color the sound." This line of argument shows, IMHO, that it's you and not others who are conflating things. The goal of sound reproduction equipment is high fidelity, which by definition means that an amplifier should amplify the signal that comes into it and do as little as possible to the signal other than that. You are 100% right that if someone prefers an amp that colors aka distorts the sound more, that's fine for them. But there is no way to objectively state or argue that such an amp sounds better than an amp with lower distortion. The only way to rationally argue that a high-distortion amp sounds better - or is at least no worse - than a low distortion amp is to accept the unstated pre-claim that all of recorded music sounds sub-optimal and that a certain amp's distortion characteristics somehow happen to perfectly counteract whatever this magically universal deficiency is in recorded music.
4. Of course everyone should buy, use, and listen to whatever they want. But no one buys an infinite number of pieces of equipment, and no one picks equipment in a truly random fashion. Everyone uses heuristics to winnow down their choices and make decisions. And to the extent that they make choices based on provably false or misleading claims - and to the extent that they participate in community forums where they might repeat and endorse those claims to others who are seeking information - it would be irresponsible not to use the knowledge we have in order to evaluate those claims and point out the errant ones as appropriate.

 

[dtaylo1066]

Well, after all my brain is tasked to "think" and interpret the world, which it does individually and uniquely. I don't believe you have a particular authority to dismiss what or how I think or hear, anymore than criticize how my taste buds interpret food. (It does not bother me in the context of this forum or debate, by the way). Do you like sour beers? Many do and consider themselves sour beer aficionados. I think they are disgusting. My taste buds tell me so, regardless of the ingredients of the beer or how the chemical analysis measures. To generalize, I find the sound of tube amps pleasing. I do not think they are warm because the tubes glow red or for some retro factor. I have listened in blind tasting sessions to various equipment and made choices on what I preferred. Sometimes that is surprising regarding cost, brand or personal beliefs or expectations. We all have our personal bias. That is called the human experience. My point is that I do not believe that the technical measurement of a piece of audio gear can be viewed as the sole or dominant factor in how it will sound to a particular individual. Yes, I understand and acknowledge you listen to the equipment you test, as you should, but that listening occurs within your own set of opinions and bias, and as a resustt of the the ways in which your brain "thinks." Absent your data, take a PS audio DAC, Hugo DAC, Topping DAC, Benchmark DAC, SMLS DAC or whatever and do some blind testing with yourself and a group. I would seriously doubt that top measuring DAC would come out on top. There may not even be a consistent winner from the group. And if that is not the case, why not?

 

[BDWoody]

There may not even be a consistent winner from the group. And if that is not the case, why not?

Likely because the ones doing their job properly would be indistinguishable from each other.

One quick suggestion...

Paragraphs.

 

[cjfrbw]

I never thought Amir would throw a grenade into the DIY chicken coop, but it was fun watching the feathers fly.

On a contrarian note, it makes me almost curious to build the ACA myself, so I guess there is no such thing as bad publicity (compared to no publicity) after all. I guess it is the masochism of subjectivity.

 

[garbulky]

I think you make some good points here, and so I am not commenting with the purpose of disagreeing with you entirely.

But I also think some of your points are as imprecise as you accuse others of being, and I think that lack of precision undermines your argument.

Specifically:

1. As @amirm has noted in his response to you, there is no way in the world that a reasonable survey of the audiophile landscape cannot conclude that Class A is viewed among most audiophiles as a desirable characteristic because of superior sonics. Whether or not audiophiles are aware of switching distortion in particular is irrelevant: Class A is known to run hotter, be less efficient, and generally be more expensive, and the common-sense idea among audiophiles is that those drawbacks are worth it (or at least the price paid) for a purer/superior audio topology. In short, Class A amps are considered premium quality amps.
2. If people are buying Pass amps because of Pass' name and not because they are Class A/Super Class A, then they are falling for the Appeal to Authority logic fallacy, which is at least as bad as buying based on false technical claims. Either way I don't see how it helps your argument.
3. You say that amps with "high distortion numbers" might not "sound bad for all people," but you concede that they "color the sound." This line of argument shows, IMHO, that it's you and not others who are conflating things. The goal of sound reproduction equipment is high fidelity, which by definition means that an amplifier should amplify the signal that comes into it and do as little as possible to the signal other than that. You are 100% right that if someone prefers an amp that colors aka distorts the sound more, that's fine for them. But there is no way to objectively state or argue that such an amp sounds better than an amp with lower distortion. The only way to rationally argue that a high-distortion amp sounds better - or is at least no worse - than a low distortion amp is to accept the unstated pre-claim that all of recorded music sounds sub-optimal and that a certain amp's distortion characteristics somehow happen to perfectly counteract whatever this magically universal deficiency is in recorded music.
4. Of course everyone should buy, use, and listen to whatever they want. But no one buys an infinite number of pieces of equipment, and no one picks equipment in a truly random fashion. Everyone uses heuristics to winnow down their choices and make decisions. And to the extent that they make choices based on provably false or misleading claims - and to the extent that they participate in community forums where they might repeat and endorse those claims to others who are seeking information - it would be irresponsible not to use the knowledge we have in order to evaluate those claims and point out the errant ones as appropriate.

I have an Emotiva XPA-1 gen 2 which has a high bias Class A circuitry. (I also have good experience with the XPA-1 L which is similar). My amp has a switch. The AB selector still gives you some decent class A power but it's much lower. I believe it's about 5 watts. The A selector allows for 60 watts of class A before transitioning to B. So what's nice is that one could flip between them to see directly if there is any difference.
(in non blind tests) The difference is very subtle. For instance, it would be impossible (even in sighted listening) to differentiate the sound if one monoblock was in class A and the other in class AB selection. That's how close it is.

But I end up preferring the class A. Over time, the sound overall is more natural and more full and overall feels more dimensional. But I couldn't point it out and say "oh here's where it is better". I have no trouble listening in class AB which also sounds pretty great.

 

[amirm]

There may not even be a consistent winner from the group. And if that is not the case, why not?

You can't ask that question until you put forward defensible data that audio science and engineering accepts as valid. Without it, you can't present it as a conflict with measurements. There is no contradiction in science that way right now. The contradiction comes form mixing audiophile ways of testing things, and trying to rationalize it with objective measurement and understanding of our perception.

Audio science has included testing many listeners in arriving at its strong stance. As I have. I have tested countless audiophiles and found them to not hear any of the artifacts that a) probably are there and b) I I and other trained listeners can hear. So the notion that these distortions are audibly good is not accepted as basis anyway. If these distortions were audible to audiophiles, they would be avoiding most if not all such products.

As to your taste buds, we know that 99% of the people like chocolate and ice cream. The notion that an ice cream store can go out of business because people are all different and may not like ice cream enough, is just not right. We know a lot about the general population because we have researched them. The industry wants to build the best audio product it can. That is why it tests consumers.

This is no different than what your doctor does. Each drug can have side effects and efficacy may not be 100%. If you have a throat infection, he will give you some antibiotic based on his experience of prescribing the same to hundreds of patients. You can't as a lay person challenge him by saying you are different so what is he going to do about that? His answer is to either show you the door or tell you to take that antibiotic. There is no third answer that takes your objection into account unless you bring solid data.

If you have such solid evidence including blind tests, please create a new thread and post how you tested and we can critique whether it was done correctly and what the conclusions mean. Until then, I can't, just like your doctor, put aside everything I know about your perception and take on a challenge that doesn't make sense.

 

[Xulonn]

On a contrarian note, it makes me almost curious to build the ACA myself,

With or without a $300 case?

 

[dtaylo1066]

Which ice cream is the most pure to the ice cream recipe? Which has the best or most expensive ingredients? Which is the best value? Which tastes the best and is most preferred?

One might expect this particular coffee to taste like ... well... total shit, but someone thinks it is grand. https://www.npr.org/sections/thesalt/2012/09/20/161478954/heres-the-scoop-on-cat-poop-coffee

Keep the faith, keep up the testing. It is all appreciated.

 

[simbloke]

Back when I was looking for an amp kit to build I didn't look for a £20 kit to put in a £300 case. I bought a kit for a few hundred and put it in a nasty £20 case.

Thing is, putting it in a nice expensive case makes you think you're getting something like the big Pass amps which also have nice expensive cases and the same name on the front.

 

[cjfrbw]

With or without a $300 case?

There are much less expensive kits up to full builds with cases and linear power supplies on ebay. ACA is open source, so I don't think I have to feel TOO guilty. I would want a linear power supply, anyway, rather than the wall wart.

 

[BDWoody]

Which ice cream is the most pure to the ice cream recipe? Which has the best or most expensive ingredients? Which is the best value? Which tastes the best and is most preferred?

One might expect this particular coffee to taste like ... well... total shit, but someone thinks it is grand. https://www.npr.org/sections/thesalt/2012/09/20/161478954/heres-the-scoop-on-cat-poop-coffee

Keep the faith, keep up the testing. It is all appreciated.

That would all be in the debate over recipes and chefs (music and artists)..hopefully not in the flavor imparted by the different compressors used in the ice cream freezer, or the power cord of my coffee maker.

These things aren't as magical as people seem to want them to be...