Pass ACA Class A Power Amplifier Review

[scott wurcer]

You seem to have conflated my points and deduced (incorrectly) that I meant Nelson Pass makes misleading claims. I didn't say that and don't believe it. I hope this clears up any confusion.

Not my intent, I got the impression that the majority of the complaints attacked the product and not the person or maybe the large group of fans for these simple DIY amplifiers bothers folks.

I see no problem at all, if one is able, with exposing actual fraud in audio products (there is plenty of that).

 

[Deleted member 26571]

Did you ask to be defended against U-47700?
Jim

Yes, in fact I did. I wrote to my congressman and he kindly assigned a police detail to keep watch outside my residence during the U-47700 riots of 2032-33. It helps to have friends in high places. ;-)

 

[properoperation]

I know I'm late to the party and my apologies if this has already been asked. Would the fact that this amp is DIY affect the measurements? is it possible the workmanship isn't great (e.g cold solder joints) hence creating a noisy amp?

 

[AdamG]

Welcome Aboard @properoperation.

 

[amirm]

I know I'm late to the party and my apologies if this has already been asked. Would the fact that this amp is DIY affect the measurements? is it possible the workmanship isn't great (e.g cold solder joints) hence creating a noisy amp?

My measurements are similar to what Nelson himself published so there is nothing to fault in the unit. See this for example: https://www.audiosciencereview.com/...ass-a-power-amplifier-review.9741/post-261613

Shows the same extreme rise of distortion with output power.

 

[ta240]

Crazy I thought Pass Labs amplifiers were some of the best design!! I guess not

It depends on what you are looking for. Keep in mind that Pass Labs, First Watt and the ACA, while all from the mind of Nelson Pass all have different design purposes.

The ACA, according to Nelson Pass himself, wasn't designed to be the best sounding amp it was designed to be something a complete beginner could build in a day and that would not use dangerous mains level power. Having said that I do see the point of view from many here that having his name attached to it and selling the kit for a fair chunk of money does lead one to believe they are getting a high quality amplifier. Especially since it was a forum post where he mentioned that. As someone that has built the ACA (without the expensive case) and subsequently thrown it away I'm baffled when the fanbase trips over themselves raving about it.

Sadly, the site where the ACA is sold has these quotes, which completely counter the paragraphs above it that tell the story of what it is really designed for :
"Insane amount of detail, deep bass and crystal highs" - Starbender
"Great power, bass and massive detail" - Waam68
"Highly detailed, yet smooth and sweet" - Mitchba
"I am amazed at the sound quality" - Neoinc

The Akitika amp kit absolutely runs circles around the ACA for about the same price.

The rest of his out of production designs that he has released to the public have much higher performance but are also much more complex to build as well as having the danger of mains voltage at the amp. His design philosophy does not embrace the keep the signal pure and clean; he will alter it in ways that he finds pleasing for listening. He is open and up front about that and makes no claims about unlocking secret hidden sounds in the music. Although his fan base and reviewers make those claims for him.

For some reason the ACA reminds me of an old Saturday Night Live skit where Picasso is sneezing in a napkin and giving it out as art. https://www.nbc.com/saturday-night-live/video/portrait-of-the-artist-pablo-picasso/2868073

 

[welsh]

Thanks. I did not know about that type of tea. Will have to seek it out some day!

Why do Marxists drink Earl Grey? Because proper tea is theft!

 

[welsh]

Not really. The ACA was only ever intended as a low-complexity, beginner-friendly DIY power amp. There's only so much you can do with 4 transistors per channel on an unregulated power supply, even in Class A. The original JLH 10 W Class A design was intended to be used with capacitance multipliers for supply regulation at least, increasing transistor count by 50%. That one was supposed to post generally <=0.1% THD figures at 9 W across the audio band, which wasn't bad for a DIY amplifier in the late '60s.

Its low output noise and gain do make the ACA a good candidate for driving high-sensitivity horns, like @mitchco does. You could probably achieve the same with a more standard amplifier and an L-pad though, so whatever.

Back when I believed the hype, I (briefly) owned a Heed CanAmp headphone amplifier that used, if I remember correctly, just one transistor per channel. Output impedance around 120 Ohm... Predictably a darling of Head Fi et al. It did get rather warm. Messed with the FR of most of my ‘phones.

 

[Willem]

 

[solderdude]

the scientific method cannot prove the scientific method. This epistemic paradox was written about by the great empiricist david hume. To his credit, mr hume was not willing to ignore this metaphysical conundrum which is the modern academic solution to the problem to ignore metaphics entirely. there is a reality which cannot be measured on any ociloscope or spectral recorder.

But the actual electrical output signal going to speakers can be measured and fully characterized and even nulled.
Are you suggesting there 'might' be a yet unknown signal going along the electrical wires to the speakers that as of yet is unmeasurable and can pass through electrical cables and is only known to certain audio gurus that know which components make use of this signal ?

 

[ergre]

[PDF] http://www.firstwatt.com/pdf/art_amp_camp_1.pdf

View attachment 38102

It need loudspeakers at least with 98 dB/W/m. Really, more than 100 dB/W/m.

https://www.doctorproaudio.com/content.php?2273-calculators-proaudio-sound-dmx&langid=1#calc_spl

I want: 80 dBSPL continuous + DR15 = 95 dBSPL

* 98 dB/W/m, 2 watts, random phase, 3 m -> 94.5 dB


With 0.1 % -> near to 110 dB/W/m and... true 8 Ohms.


Therefore, some measurements made by amirm for this amplifier are inappropriate, which will only works properly with very few speakers. And the vast majority of those who have built these amplifiers do not use appropriate speakers, unless they listen in the near field and in low volumes, with commercial music with very/little dynamic range (DR < or << 10 dB).

Those speaker are going to be enormous or have very little bass. What would be the advantage of a setup like that?

 

[MediumRare]

But the actual electrical output signal going to speakers can be measured and fully characterized and even nulled.
Are you suggesting there 'might' be a yet unknown signal going along the electrical wires to the speakers that as of yet is unmeasurable and can pass through electrical cables and is only known to certain audio gurus that know which components make use of this signal ?

Whoops, where did it go?

 

[Goodman]

This is a review and detailed measurements of the Pass ACA Class A stereo power amplifier. It is a DIY design by the famous designer, Nelson Pass, meant to be starter amplifier for people new to audio electronics to build. There are many kits available. The one loaned to me from a long time friend of the forum. The kit seems to cost around US $320 or so.

The ACA sample I received comes in an attractive black case:

View attachment 38081​

The design is a Mosfet output stage with a single stage buffer. Very modest level of feedback (9 dB?) is provided to keep distortion somewhat under control. As you will see, the gain is very low requiring full 2 volt of DACs to drive it to max power.

Here is the back side:

View attachment 38082​

I did not mess with it but I am assuming the single XLR allows bridged mono operation.

Power is provided by a very beefy external switching power supply which outputs 24 volts with up to 5 amps or 120 watts. All of that is put to "use" in generating copious amount of heat. I was told by the owner to let it warm up for a few minutes. As you will see in the measurements, this actually made the performance worse, not better.

Overall, this is a very nice package if it performs.

Amplifier Audio Measurements
As usual, we start with our dashboard with 5 watt output into 4 ohm. Normally this is well below the max power but not in this case as we see later:

View attachment 38083

Gosh... What happened to aiming for high fidelity? This is a distortion-factory if there ever was one. The notion that it is mostly second harmonic does not hold true either. Welcome to the bottom of our amplifier ratings:
View attachment 38084

Let's dig into distortion spectrum at lower power level of just 1 watt:
View attachment 38085

Second harmonic is more dominant than at 5 watt but it doesn't stay there that long. Here it is at 2 watts:

View attachment 38086

Our 32-tone signal resembling "music" paints an ugly picture as a result:

View attachment 38087

I was pleasantly surprised how quiet the unit is though:
View attachment 38088

Frequency response is also very well extended:
View attachment 38089

You may be able to make an AM radio out of this thing.

Back to ugliness, here is our power versus distortion+noise into 8 ohm load:
View attachment 38091

Oh gosh again.... Only 7 watts of power with SINAD of 38 dB?

Things don't get much better with 4 ohm load:

View attachment 38092

We now have distortion climbing into near 10% category at max power of just 7.4 watts/channel.

There was little variation of response with respect to frequency:
View attachment 38093

Distortion level is so high that it is masking secondary sources of it.

Making sure this unit matches the reference, I compared them to what Nelson Pass has published:
View attachment 38094

We are getting very good agreement. So this is what the unit is supposed to do. Nelson stays with 1 watt output which shows the performance to be better.

EDIT: I noted on the dashboard that the gain was much lower than the spec. I am told that version 1.6 which I tested now has a spec of 10 dB for gain so we are good.

EDIT 2: See this link for bridged mode measurements: https://www.audiosciencereview.com/...ass-a-power-amplifier-review.9741/post-262036

Listening Tests
Is this thing on? That is the expression that immediately came to mind after I hooked up the ACA amp to my Sonus Faber Concertino bookshelf speaker. Granted, the Concertino is spec'ed at 86 dB efficiency but come on, I could barely hear it. There was no bass naturally. But not much above that either. And this was with my Topping DX3 Pro at full volume.

For grins, I plugged the US $74 tiny little SMSL SA100 Amplifier which I recently reviewed into the setup. Now the speakers came alive, with lots of bass, great dynamic range, and detail. Yes, the amp started to cry when I turned it up much but until that point, it left the ACA so far in the dust it was not even funny.

Conclusions
As a starter hobby amplifier to get some sound coming out of a speaker, with relatively few components, the Pass ACA Class A Amp does the job. Build it, have fun with it and then put it aside and get a proper amplifier. Amplifiers need to have a lot more power. And much less distortion. We are not even close here folks.

Do I need to tell you that I can't recommend the Pass ACA for hi-fi listening? I hope not.

------------
As always, questions, comments, recommendations, etc. are welcome.

I suspect the above review is going to make me many enemies in DIY audio circles. So I need hire a bodyguard and get some serious insurance for my bodily parts from Loyd's of London. Both cost good amount of money so please donate generously using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/

Criticizing Nelson Pass amplifiers may be considered Blasphemy. Although I don't have thousand of dollars invested in measuring equipment, but having been the owner of several amplifiers of the aforementioned I completely agree with your critic and assessment.

 

[tmtomh]

Criticizing Nelson Pass amplifiers may be considered Blasphemy. Although I don't have thousand of dollars invested in measuring equipment, but having been the owner of several amplifiers of the aforementioned I completely agree with your critic and assessment.

One great thing about ASR is that we have no sacred cows and reject appeals to authority - so criticizing Pass is totally fine.

That said, criticism does not have to be mean-spirited, and he is a giant in the history of amplifier design for good reason. This amp is not meant to be especially powerful or well-measuring - it's really a project amp and he is very clear about that.

Where I think he can be critiqued is that this amp has high distortion and low output not only because it's a project amp and he's made it simple to build - but also because he's bought into the notion that feedback is bad. This is a major reason that he sticks with inefficient (and noise-vulnerable) Class A topology, and why he leans towards lower power - pure Class A is hot and power-hungry, and his obsession with minimizing the number of transistors/gain stages. That, IMHO, is the core of where his design philosophy is problematic.

 

[Dilliw]

Where I think he can be critiqued is that this amp has high distortion and low output not only because it's a project amp and he's made it simple to build - but also because he's bought into the notion that feedback is bad. This is a major reason that he sticks with inefficient (and noise-vulnerable) Class A topology, and why he leans towards lower power - pure Class A is hot and power-hungry, and his obsession with minimizing the number of transistors/gain stages. That, IMHO, is the core of where his design philosophy is problematic.

His statement on his design philosophy leads me to a different place than obsession. From his 2017 Stereophile interview:

"The other fundamental thing—number 2—is that I am centrally aware that all this is just entertainment, mine and yours. The objective needs of amplifier users are largely solved on a practical level, and as [Marshall] McLuhan perceptively noted, when that happens, we turn our technology into art. For me, the art lies in making simple, unusual amplifiers that sound great and measure fairly well. They aren't for everyone, but if they appeal to even a narrow segment of audiophiles, I'm perfectly happy. I'm equally happy if they are reliable."

Nelson Pass Interview

 

[Goodman]

One great thing about ASR is that we have no sacred cows and reject appeals to authority - so criticizing Pass is totally fine.

That said, criticism does not have to be mean-spirited, and he is a giant in the history of amplifier design for good reason. This amp is not meant to be especially powerful or well-measuring - it's really a project amp and he is very clear about that.

Where I think he can be critiqued is that this amp has high distortion and low output not only because it's a project amp and he's made it simple to build - but also because he's bought into the notion that feedback is bad. This is a major reason that he sticks with inefficient (and noise-vulnerable) Class A topology, and why he leans towards lower power - pure Class A is hot and power-hungry, and his obsession with minimizing the number of transistors/gain stages. That, IMHO, is the core of where his design philosophy is problematic.

My first sentence was meant as a quip or a joke. Not mean spirited. Great accomplishments can be appreciated but also can be over-rated. More to the point using mosfets on the output stage has rarely, if at all, resulted in great sounding amps.

 

[Ron Texas]

This amplifier's measurements are so bad that they exceed loudspeaker distortion and most likely room noise floors. It takes some real tallent to screw up something this badly when there are all sorts of amplifiers on a chip which do much better.

 

[levimax]

More to the point using mosfets on the output stage has rarely, if at all, resulted in great sounding amps.

Now there is some real blasphemy .... there have been plenty of really good MOSFET amps over the years... high power, low distortion, and reliable. They have fallen out of favor due to lack of availability, cost, and lower efficiency.

 

[Goodman]

Now there is some real blasphemy .... there have been plenty of really good MOSFET amps over the years... high power, low distortion, and reliable. They have fallen out of favor due to lack of availability, cost, and lower efficiency.

Right, there have been some "good" MOSFET amps, but no outstanding or great mosfet amps. All depends on your expectations.

 

[levimax]

Right, there have been some "good" MOSFET amps, but no outstanding or great mosfet amps. All depends on your expectations.

There have been plenty of MOSFET amps that are transparent and quiet, not sure what the difference between a "good" amp and "great" amp are once they are transparent. Maybe @restorer-john knows of some "great" MOSFET amps and can chime in.