Pass ACA Class A Power Amplifier Review

[Deleted member 26571]

I took care in choosing my words, and I will respectfully disagree with your assertion.

 

[peng]

What I'm about to say will probably bring a lot of criticism down on my head, but ASR is not science. We do not listen with meters. Our brains do not objectively transcribe auditory stimuli into perceptual experiences. The idea that an amplifier should be a "straight wire with gain" is a defensible position to take, but it does not and cannot tell the whole story without taking into account the extraordinary perceptual processing that takes place in our brains. I find this forum useful because of the measurements, but the bulk of the discussion contributes nothing to the science behind audio engineering.

I think you are right for the most part, definitely the part "The idea that an amplifier should be a ..................................". However, the engineer/designer should still aim for accuracy in terms of trying to make the output signal as identical as the input signal, other than amplifying the magnitude in order to drive the load. All those other things that affect people's brain is none of the designer's business, even if it is, if they try to do things to cater for such things, they won't be able to please everyone anyway.

I find most subjective review magazines and websites to be nonsense. Sometimes I find myself getting bent out of shape when I see ads and reviews for snake oil products. But then I think about how much us-versus-them hatred there is in this world and I ask myself just how much I am really being harmed by snake oil audiophoolery. I am forced to conclude: not very much.

Good for you, but I believe those reviews must have led a lot of people to spend money unnecessarily. On the other hand, if they manage to influence those who spent the money to convince themselves they heard what they were told they would hear, then I guess it could be considered as mission accomplished, or at least no harm done especially when it did put smiles on faces.

By the way, I have actually used the 7 W version ACA amp to drive my KEF R900 with acceptable result at medium loud level (about 60-65 dB average iirc. Using the subwoofer does help a lot and is a must for me if I use it with the much smaller LS50. Would I use it as the default amp for those speakers, not really.

 

[peng]

I took care in choosing my words, and I will respectfully disagree with your assertion.

Disagree with his assertion on the point he made, or his interpretation of yours?

 

[RichB]

If this amp was from, Manford Quinn and not Nelson Pass, I don't think it would have the following.

It is a tribute to branding. If you like it, rock on.

- Rich

 

[rdenney]

Dude, this forum lives and lets live. Other forums provide all of what you think ASR lacks and then some, but they lack what ASR provides. Why don’t you go to those forums and ask them to live and let live? A member of one tribe can’t go to another tribe and complain about tribalism. Reducing tribalism requires polite tolerance, but tolerance does not imply affirmation.

...Objectively good in terms of the state of mind I experienced listening to it, for whatever reason....

This statement is a non-sequitur. I could not include a statement like this in any engineering work product and expect it to go unchallenged, or if damage resulted from it, expect it not to challenge my license to practice engineering. “This bridge beam will objectively support this load with acceptable deflection and resonant vibration because of the state of mind you will experience driving over it, for whatever reason.” “This traffic signal network timing plan objectively improved my state of mind when I imagined driving through it.”

Engineers are engineers because they don’t have to guess, while their clients would have to guess. This forum is dedicated to not guessing—to understanding why certain designs and measurements achieve specific outcomes repeatably.

Rick “expecting politeness is appropriate and even laudable; expecting affirmation of unevidenced subjective myths based on mere popularity is unreasonable” Denney

 

[amirm]

What I'm about to say will probably bring a lot of criticism down on my head, but ASR is not science. We do not listen with meters.

That would be like saying your doctor is not following science because he takes your blood pressure. He does this to gather clues as to what could be wrong with you. That is what measurements do for us. They provide important, objective, and repeatable clues as to what could be wrong with an audio design.

The idea that an amplifier should be a "straight wire with gain" is a defensible position to take, but it does not and cannot tell the whole story without taking into account the extraordinary perceptual processing that takes place in our brains.

This says the entire field of psychoacoustics is for not. That an MP3 is 1/8 the size of the original yet it sounds almost like it to vast majority of people must not exist. Of course it does. Our measurements analyze the performance of a device from multiple angles including full spectrum analysis using FFT that lets us apply psychoacoustics and figure out if something is adding audible distortion or not. Not much different than what doctor does above.

You say you are an EE. If your design doesn't work, you use scopes and meters to analyze what is going on. Surely someone could accuse you of the same argument and demand that you simply use your ears and eyes to troubleshoot the design.

But then I think about how much us-versus-them hatred there is in this world and I ask myself just how much I am really being harmed by snake oil audiophoolery. I am forced to conclude: not very much.

Live and let live, I say.

We do. We don't go crazy talking about this stuff. But sometimes members are interested in what is really going on in a device beyond the hype it has created. In this case one such member sent one in, we analyzed it and rendered an outcome. The believers could not believe their eyes because likely they thought their beloved device was surely much more performant than it really was. So arguments like yours started to create FUD around data. Our responsibility then, as in responding to you, is to set the record straight, not only as far as reliability of our measurements but also the applicability of science to their analysis. We cannot let such myths continue, resulting in people buying products on hype and fiction. It is what we do. It is why we are here. You can't ask us in good conscious to stay silent when it is core of our being to bring more understanding of science and engineering to audiophiles.

As for Nelson pass, he has allowed the reputation of this amp to travel well past the borders he created for it: that it was a simple amp to build and likely is a distortion generator. Many audiophiles are buying this, not because they have any interest in DIY electronics, but because they think it is a wonderful amplifier because Nelson designed it. His brand has become super powerful marketing tool, against principles of proper design and engineering in amplification.

 

[SIY]

That would be like saying your doctor is not following science because he takes your blood pressure. He does this to gather clues as to what could be wrong with you. That is what measurements do for us. They provide important, objective, and repeatable clues as to what could be wrong with an audio design.


This says the entire field of psychoacoustics is for not. That an MP3 is 1/8 the size of the original yet it sounds almost like it to vast majority of people must not exist. Of course it does. Our measurements analyze the performance of a device from multiple angles including full spectrum analysis using FFT that lets us apply psychoacoustics and figure out if something is adding audible distortion or not. Not much different than what doctor does above.

You say you are an EE. If your design doesn't work, you use scopes and meters to analyze what is going on. Surely someone could accuse you of the same argument and demand that you simply use your ears and eyes to troubleshoot the design.


We do. We don't go crazy talking about this stuff. But sometimes members are interested in what is really going on in a device beyond the hype it has created. In this case one such member sent one in, we analyzed it and rendered an outcome. The believers could not believe their eyes because likely they thought their beloved device was surely much more performant than it really was. So arguments like yours started to create FUD around data. Our responsibility then, as in responding to you, is to set the record straight, not only as far as reliability of our measurements but also the applicability of science to their analysis. We cannot let such myths continue, resulting in people buying products on hype and fiction. It is what we do. It is why we are here. You can't ask us in good conscious to stay silent when it is core of our being to bring more understanding of science and engineering to audiophiles.

As for Nelson pass, he has allowed the reputation of this amp to travel well past the borders he created for it: that it was a simple amp to build and likely is a distortion generator. Many audiophiles are buying this, not because they have any interest in DIY electronics, but because they think it is a wonderful amplifier because Nelson designed it. His brand has become super powerful marketing tool, against principles of proper design and engineering in amplification.

Honestly, I think some get confused by the conflation of "audio" and "high fidelity." The latter is a subset of the former. Nelson's Pass Labs stuff is audio but isn't (nor does it claim to be) high fidelity. So it needs to be judged by different criteria than something claiming to be high fidelity, much as we judge signal processing, effects boxes and plugins...

 

[pma]

He (NP) spreads confusion into many beginners who would like to learn about amplifiers. This is undefendable.

 

[Deleted member 26571]

Disagree with his assertion on the point he made, or his interpretation of yours?

The former. The purpose of an audio system is to create a sensory experience in the mind of the listener. The notion of using a set of microphones to capture sound pressure level measurements in real time, that are mixed down to two channels and played back via a pair of loudspeakers in a listening room is an engineering solution to that problem. It depends on the inability of our ears to discriminate a great deal of information in the sound field, which, if we could perceive it, would destroy the audio illusion. Defining the role of the amplifier as a perfectly linear voltage multiplier is a constraint that simplifies the solution and renders it workable in practice. It's reasonable to leave the tailoring of the sound to the recording engineers, who can work under the assumption of a standardized playback environment.

If you were to plot in real time the vector soundfield that exists in a hi-fi listening room and compare it to that at the original performance, certain parameters would correlate, but many would not even come close. It's both a limitation and a marvel that our ear/brain reconstructs the illusion of live sound in a 3D space from such wildly distorted perceptual stimuli. The real science behind audio engineering has to do with characterizing the response of listeners to these stimuli to optimize their subjective response in their minds. It is an assumption, though arguably not necessary nor sufficient, that the components in the signal chain are accurate with respect to linearity and frequency response. The perceptual processing that happens in our brains is phenomenally complex. To try to predict subjective experience from a few crude objective measurements is to narrow the scope of investigation to the point of triviality.

Nothing I'm saying is controversial. Hi-fi as we know it is a compromise. Certain distortions (whether linear or non-linear) will be picked up as cues by the ear/brain that enhance the subjective perception that the sound is "lifelike." Other distortions may render the sound less lifelike but more pleasing. The conventions we have for audio design are the product of decades of research into the best tradeoffs between perceived sound quality, practicality, and economics. The ACA, and Pass's designs in general, exist to explore the limits of these tradeoffs. It's no surprise they succeed on some levels and fail on others.

I observe a range of attitudes on this forum. It's ok to take interest in equipment that has state-of-the-art noise and distortion performance. It's also ok to poke fun at people who trick themselves into perceiving things they cannot possibly hear. Still, there is a subset of members who approach the subject with what strikes me as an overly narrow and dogmatic attitude. I find that counter-productive and needlessly contentious. Some people, not all of them, or maybe even most. I actually disagree in some areas with the overall tone and philosophy of this forum. But I respect the community. As a guest here, it would be inappropriate for me to troll, but I'm within my rights to offer polite criticism or to push back against views that I feel go too far to one extreme or another.

My main wish is to live in a world that is more tolerant of divergent views when such views do not cause harm. One could argue that audio subjectivism promotes deception on a wide enough scale that it's harmful. The world desperately, existentially, needs more scientific objectivism. But too often I see people on one side or another taking affront and coming to the discussion as though it's a battle.

The best forum, IMHO, for exploring the world of audio science, engineering, and DIY is diyaudio.com. I apologize, but ASR doesn't even come close. What this forum does best is compiling and publishing objective measurements of equipment performance. I do fear that this information gets picked up and interpreted in ways that aren't entirely justified. Most of you will probably disagree with me, and that's ok. You might even kick me out of the club, and that's ok too. I have my opinions and believe they are well-informed and thoughtful, but no one is obliged to agree with me.

My personal feeling about the ACA is that it's a cute little tech demonstrator that has garnered, maybe, more interest than it deserves. But I see a lot of good overall in the contribution Nelson Pass has made, and continues to make, to the DIY community. His design philosophy is controversial, but he's managed to keep his business going over the years, through many changes, while other very respectable manufacturers have gone by the wayside. He is clearly designing and selling products that challenge conventional wisdom. His amplifiers are arguably overpriced and underperforming when assessed against mainstream standards. But I've never seen Nelson Pass write disparagingly of conventional amplifier technology. He does his own thing, he seems like a content, generous, smart fellow, he's making a lot of people happy, and he's keeping the lights on and the doors open at Pass Labs. Unlike the folks selling ten thousand dollar power cords, he's not trying to trick anyone. He's completely transparent and unusually generous in sharing the detailed technical reasoning behind his designs, as well as his intellectual property. One could do much worse.

LOL, I know I've put myself on a soap box and am going on ten times longer than I need to for the sake of self-amusement. Still, I believe in what I'm saying. I beg your pardon. In the end, as Nelson Pass would say, it's just entertainment.

 

[Deleted member 26571]

He (NP) spreads confusion into many beginners who would like to learn about amplifiers. This is undefendable.

It's indefensible when judged against a standard that defines it as such. From my perspective as an advanced DIY audio hobbyist, Nelson Pass does far more good than harm to the community.

From the point of view of an environmentalist, NP promotes egregious waste of power and carbon pollution. I agree with that more. It all depends on your perspective.

 

[amirm]

The best forum, IMHO, for exploring the world of audio science, engineering, and DIY is diyaudio.com.

You can't be serious. When this review came out I went to defend our point of view there and I was just about hung in the public square. I had been a member for years and had not realized the massive shift they have taken in their approach to audio. For a site that is focused on engineering of audio, I thought for sure they would value measurements, instrumentation, etc. Sadly it was the exact opposite. And oh, not a fraction of the tolerance we show here was exhibited there.

 

[amirm]

I beg your pardon. In the end, as Nelson Pass would say, it's just entertainment.

It is and you should let the content do the talking instead of expecting the gear to intervene. If you want different tonality, spatiality, etc., change your speaker. Or add EQ. But let the art come through when it comes to amps.

 

[Deleted member 26571]

That's fine, Amir. You have a point of view. You're just as open and transparent about your philosophy as Nelson Pass is about his. I happen to disagree with certain aspects of your philosophy. I think you've defined your mission to be self-consistent within a narrow set of premises. I cannot refute the argument that an amplifier with 0.0001% IMD is more accurate than one with 0.001% IMD, all other things being equal. What I don't know at all is whether the former amplifier is better for its intended purpose than the latter.

“This bridge beam will objectively support this load with acceptable deflection and resonant vibration because of the state of mind you will experience driving over it, for whatever reason.” “This traffic signal network timing plan objectively improved my state of mind when I imagined driving through it.”

I can't agree with you. The purpose of a bridge is to support a load. The purpose of a traffic signal is to regulate traffic. These things can be measured objectively. Ah, but consider the following. Traffic jams are stressful. Stressed people become agitated and distracted and this leads to traffic accidents. Perhaps one measure of the utility of a traffic signal network is the extent to which drivers subjectively perceive traffic to be smooth-flowing, and whether or not the delays they encounter are fair. How do you measure that with a meter? This reminds me of the social engineering theme park designers do to make long waits in line tolerable to their guests.

The purpose of an amplifier is not to produce a chart on an audio analyzer. It is to render enjoyment to a listener. Your argument is akin to saying fashion design is a fraud because the color or shape of a garment has no bearing on the degree of weather protection it affords the wearer. If listening to an amplifier makes the user feel good about the experience, that is what matters. Engineering is about designing systems that meet specifications. Arguing that the design has failed because the subjective perception is at odds with measurements is not science. Understanding how a poor-measuring amplifier can produce a positive subjective outcome, and quantifying the ways that distortions are perceived as either good or bad is perceptual science.

I've often asked myself why audio is so contentious. I think it's because it lies at the intersection of art and engineering. It's pointless to try to coerce it into one domain or another. My philosophy is that subjective experience is personal and inarguable. I don't get my britches in a twist until someone tries to explain their perceptions in technical terms that violate physical reality as I understand it. Until someone can fully characterize the working of the brain and the nature of consciousness, we will never be able to answer this question. Even then, the answer would elude us. Think about it.

Yeah, yeah, yeah. I know from your point of view I'm just digging myself in deeper. I hope I come across as polite even if you don't agree with me. FWIW, I've been spending a lot of time lately reading some very technical papers about nonlinear distortions in solid-state power amplifiers. All things considered, I'm one of the good guys.

LOL, it's just a pastime. Something to keep us occupied in the short time we have in this universe.

 

[Purité Audio]

Creation is the art part, reproduction is the engineering part.
Keith

 

[Deleted member 26571]

Creation is the art part, reproduction is the engineering part.
Keith

Reproduce what? I've never once listened to a recording of a classical music concert in my living room and believed I was in a concert hall. I don't want to push back too hard because -- believe me -- I respect the social norms here. I'm just saying, it's not a problem that lends itself so readily to such simple truisms.

 

[Purité Audio]

You can only hope to reproduce the file accurately, if you want live then go to a concert.
Keith

 

[peng]

The former. The purpose of an audio system is to create a sensory experience in the mind of the listener. The notion of using a set of microphones to capture sound pressure level measurements in real time, that are mixed down to two channels and played back via a pair of loudspeakers in a listening room is an engineering solution to that problem. It depends on the inability of our ears to discriminate a great deal of information in the sound field, which, if we could perceive it, would destroy the audio illusion. Defining the role of the amplifier as a perfectly linear voltage multiplier is a constraint that simplifies the solution and renders it workable in practice. It's reasonable to leave the tailoring of the sound to the recording engineers, who can work under the assumption of a standardized playback environment.

If you were to plot in real time the vector soundfield that exists in a hi-fi listening room and compare it to that at the original performance, certain parameters would correlate, but many would not even come close. It's both a limitation and a marvel that our ear/brain reconstructs the illusion of live sound in a 3D space from such wildly distorted perceptual stimuli. The real science behind audio engineering has to do with characterizing the response of listeners to these stimuli to optimize their subjective response in their minds. It is an assumption, though arguably not necessary nor sufficient, that the components in the signal chain are accurate with respect to linearity and frequency response. The perceptual processing that happens in our brains is phenomenally complex. To try to predict subjective experience from a few crude objective measurements is to narrow the scope of investigation to the point of triviality.

Nothing I'm saying is controversial. Hi-fi as we know it is a compromise. Certain distortions (whether linear or non-linear) will be picked up as cues by the ear/brain that enhance the subjective perception that the sound is "lifelike." Other distortions may render the sound less lifelike but more pleasing. The conventions we have for audio design are the product of decades of research into the best tradeoffs between perceived sound quality, practicality, and economics. The ACA, and Pass's designs in general, exist to explore the limits of these tradeoffs. It's no surprise they succeed on some levels and fail on others.

I observe a range of attitudes on this forum. It's ok to take interest in equipment that has state-of-the-art noise and distortion performance. It's also ok to poke fun at people who trick themselves into perceiving things they cannot possibly hear. Still, there is a subset of members who approach the subject with what strikes me as an overly narrow and dogmatic attitude. I find that counter-productive and needlessly contentious. Some people, not all of them, or maybe even most. I actually disagree in some areas with the overall tone and philosophy of this forum. But I respect the community. As a guest here, it would be inappropriate for me to troll, but I'm within my rights to offer polite criticism or to push back against views that I feel go too far to one extreme or another.

My main wish is to live in a world that is more tolerant of divergent views when such views do not cause harm. One could argue that audio subjectivism promotes deception on a wide enough scale that it's harmful. The world desperately, existentially, needs more scientific objectivism. But too often I see people on one side or another taking affront and coming to the discussion as though it's a battle.

The best forum, IMHO, for exploring the world of audio science, engineering, and DIY is diyaudio.com. I apologize, but ASR doesn't even come close. What this forum does best is compiling and publishing objective measurements of equipment performance. I do fear that this information gets picked up and interpreted in ways that aren't entirely justified. Most of you will probably disagree with me, and that's ok. You might even kick me out of the club, and that's ok too. I have my opinions and believe they are well-informed and thoughtful, but no one is obliged to agree with me.

My personal feeling about the ACA is that it's a cute little tech demonstrator that has garnered, maybe, more interest than it deserves. But I see a lot of good overall in the contribution Nelson Pass has made, and continues to make, to the DIY community. His design philosophy is controversial, but he's managed to keep his business going over the years, through many changes, while other very respectable manufacturers have gone by the wayside. He is clearly designing and selling products that challenge conventional wisdom. His amplifiers are arguably overpriced and underperforming when assessed against mainstream standards. But I've never seen Nelson Pass write disparagingly of conventional amplifier technology. He does his own thing, he seems like a content, generous, smart fellow, he's making a lot of people happy, and he's keeping the lights on and the doors open at Pass Labs. Unlike the folks selling ten thousand dollar power cords, he's not trying to trick anyone. He's completely transparent and unusually generous in sharing the detailed technical reasoning behind his designs, as well as his intellectual property. One could do much worse.

LOL, I know I've put myself on a soap box and am going on ten times longer than I need to for the sake of self-amusement. Still, I believe in what I'm saying. I beg your pardon. In the end, as Nelson Pass would say, it's just entertainment.

Wow, in that case I misunderstood your original post. May be you were too careful with your wording or it was due to expectation bias on my part since I read that you were trained in EE. On re-reading the post I noticed you did say you didn't work in the field.

 

[Deleted member 26571]

Wow, in that case I misunderstood your original post. May be you were too careful with your wording or it was due to expectation bias on my part since I read that you were trained in EE. On re-reading the post I noticed you did say you didn't work in the field.

I don't work in the field, but I'm qualified in the subject matter.

 

[Deleted member 26571]

You can only hope to reproduce the file accurately, if you want live then go to a concert.
Keith

I can reproduce the file accurately any time I want:

$ cp foo.wav bar.wav

 

[dtaylo1066]

Thanks for the reply. I'm evidently not a long-time or frequent contributor to ASR, but I've been reading this forum daily for a few months now and I understand its purpose and culture. I see a range of comments in this thread, a fraction of which I would call genuinely dismissive or hostile to the amplifier, its designer, and/or its builders. Others seem to judge the amplifier against an objective standard that the it was never intended to meet.

I'm an electrical engineer by training (though I don't work in the field) and I daresay I know as much or more about audio amplifier design than the majority of ASR members. The ACA is not something I would build for myself personally. But I know exactly what Nelson Pass' design philosophy is and what the purpose of this amplifier is. Even though I'm not interested in building the amplifier myself, I find the concept interesting and it makes be happy to see people enjoying playing with this circuit.

The hardest part of any DIY project is building the box. The official ACA enclosure is a sweet-looking thing. I would have a hard time duplicating it for under two hundred dollars. If I had one of these amps, I would use it to drive horns or desktop speakers and have fun with it on its own terms.

I have even less interest in owning a Chinese Class D desktop amp than I have in the ACA. There are people who have built and listened to the ACA who are genuinely satisfied with its sonic performance. I feel no compulsion whatsoever to comment disparagingly on their subjective experiences.

Back in the day I had a chance to listen to a Pass Aleph 3 and I was astonished by how good it sounded. Objectively good in terms of the state of mind I experienced listening to it, for whatever reason. I don't think any of Pass' commercial products are particularly good deals and some of them are way over the top, evidently designed to appeal to the sort of buyers that make ASR readers roll their eyes... to put it politely. Personally, I don't feel the slightest outrage or indignation at the existence of this three hundred dollar oddity. It is what it is. Anyone who is looking for a state-of-the-art DIY power amplifier project would do quite well to check out the Wolverine thread that's currently underway at diyaudio.com.

What I'm about to say will probably bring a lot of criticism down on my head, but ASR is not science. We do not listen with meters. Our brains do not objectively transcribe auditory stimuli into perceptual experiences. The idea that an amplifier should be a "straight wire with gain" is a defensible position to take, but it does not and cannot tell the whole story without taking into account the extraordinary perceptual processing that takes place in our brains. I find this forum useful because of the measurements, but the bulk of the discussion contributes nothing to the science behind audio engineering.

I find most subjective review magazines and websites to be nonsense. Sometimes I find myself getting bent out of shape when I see ads and reviews for snake oil products. But then I think about how much us-versus-them hatred there is in this world and I ask myself just how much I am really being harmed by snake oil audiophoolery. I am forced to conclude: not very much.

Live and let live, I say.

Overall, well stated. We are all different and hear differently. If one likes the sound of something, that's a good thing for them. From a clear measurement standpoint, one cannot deny how accuratlety an amp or DAC reproduces the signal it is fed. To me that is one objective and verifieable performance evaluation. But the ultimate test comes down to one's ears. I have heard ultra expensive and impeccably designed products that produce fantastic numbers on the bench tests but I did not think they sounded very good. And I have heard mid-fi products that did not measure as well that I believe sounded fantastic.