Pass ACA Class A Power Amplifier Review

[solderdude]

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

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).

Is this the idea behind building a 7W amplifier so one can only really use a questionable quality 0.1W and needs very special speakers ?
It's like building a 1000W amp which one can only use up to the first 15W.
Should I want a questionable quality 100W I would need to build a 7kW class-A amplifier.

Why not build a amplifier that delivers 7 useful Watts ?

 

[Ceburaska]

Excellent rejection of power supply mains frequency and its multiples should be mentioned. When I did several tests with mathematically added non-linear distortion with similar profile as of this Pass amplifier, almost nobody or just nobody was able to tell the difference in the ABX listening test. Correlation of SINAD with subjective sound evaluation is highly questionable.

Do you have any proof based on ABX tests with music samples (not single sine waves) with known and defined distortion, defined non-linear transfer function? If yes, please post it or give a link to a public source. Nothing else should be applied than a transfer function non-linearity, it means no changes of frequency response and residual noise would apply.

As you brought it up, how about you post or give a link to a public source of your ABX listening test?

 

[JPierre]

To be fair, most people who know anything about the ACA knows that it's not clean, neutral amp. You could almost place it in the same category as tube amps, and that's the reason they're still being bought. It's a subjective preference, but an objective nightmare.

 

[Shadrach]

Interesting comments when compared to the comments on threads of products from less guru like status manufacturers.
Of course, it's the great Nelson Pass......it's still a shameful offering from such a revered audio expert. Should be this have been designed by Joe Bloggs down the road it would get dismissed as a pile of crap.

 

[maty]

Is this the idea behind building a 7W amplifier so one can only really use a questionable quality 0.1W and needs very special speakers ?
It's like building a 1000W amp which one can only use up to the first 15W.
Should I want a questionable quality 100W I would need to build a 7kW class-A amplifier.

Why not build a amplifier that delivers 7 useful Watts ?

https://www.audiosciencereview.com/...ass-a-power-amplifier-review.9741/post-261651

Basically, the entire concept of building a little junk-box amplifier as an afternoon project for fun, has been bastardized by profiteering hangers-on and that's hardly NP's fault. His gifts to the DIY community are numerous, respected and sometimes polarizing, but universally appreciated.

We have the search engines to find information about the products, the problem is that many do not want to work on it. I would never buy the ACA.

 

[Eirikur]

Interesting comments when compared to the comments on threads of products from less guru like status manufacturers.
Of course, it's the great Nelson Pass......it's still a shameful offering from such a revered audio expert. Should be this have been designed by Joe Bloggs down the road it would get dismissed as a pile of crap.

Nothing shameful about this, he is perfectly upfront on the expected results as well as the purpose.
Amir does show that you should not invest $320 - stay close to the quoted $20 + heat-sinks/PCB.

 

[BDWoody]

Interesting comments when compared to the comments on threads of products from less guru like status manufacturers.
Of course, it's the great Nelson Pass......it's still a shameful offering from such a revered audio expert. Should be this have been designed by Joe Bloggs down the road it would get dismissed as a pile of crap.

It isn't unsafe, and he doesn't lie about what it is or what should be expected from it.

 

[FrantzM]

Let's admit people that this is a BS product not worth $320... Let's not beat around the bush. Let's call this, a turd.

 

[pma]

As you brought it up, how about you post or give a link to a public source of your ABX listening test?

https://www.diyaudio.com/forums/eve...be-distortion-listening-test-post5589641.html

 

[RichB]

From the product page:

"0.7% at 1 Watt (primarily second order harmonic which being even-order is described as "sweet, tubey" and being low-order as "benign/inaudible")"

The second order harmonic does not quite mask the 3'rd, 4'th, 5'th and 6'th harmonic.

$4000 buys a First Watt SIT-3 18 WPC class-A amplifier with about 10 dB better distortion performance.

- Rich

 

[anmpr1]

Whoopse! He needs a haircut.

Part of the mystique this designer has and exploits, turns on a counter-culture, hippy-Bohemian, "I'll do it my way, damned any convention", eccentric-genius front. The associated look is integral to what he's doing, and it's what his customers both want and expect. Some of it is just the aura of an era... an era that has passed, and is now passé. So I'll give Pass a pass on that (nice punning, eh?). I mean, even genius Brylcreemist pianist, Bill Evans, grew his hair out in the '70s. With men like Evans, one overlooks the look, being as they are in the presence of excellence. However, in Nelson's case...?

You sometimes encounter this 'counter-culture' fashion with other boutique 'hi-end' designers. For example, PS Audio guy, writing about his exploits, running from the Feds, smuggling contraband in Mexico, and so on and so on. That sort of hip literary fantasy should have ended shortly after Hunter Thompson shot himself, and then shot himself again, out of a canon, if for no other reason than out of respect for the true gonzo-master.

 

[anmpr1]

It's like building a 1000W amp which one can only use up to the first 15W. Why not build a amplifier that delivers 7 useful Watts ?

With average to low sensitivity speakers, you'll probably wind up using more than 15W. Not all the time, but sometimes. Because occasionally you'll want to turn it up, because it's clean power, and because you can. Your speakers will sound better with lower distortion, and the higher power will actually be safer for your speaker voice coils.

Back in the early '70s, when 60 watts/channel was a lot, and the then established Crown was huge (at 150 watts/ch), reviewers were reporting that in actual listening tests they were clipping Bob Carver's monster amp more than they ever did when using lower powered amplifiers. Why? Because it sounded so clean when they played at loud levels, they kept turning it up. Other, less powerful amps were immediately scaled back, because they just didn't have the huff to play at the higher SPLs, so no one wanted to listen to them at any more than average levels. Clipping the powerful amp more than the low powered amp was sort of an antinomy, or paradox that we sometimes take for granted today. But back then it was a new audio-oriented experience. In one of his reviews, Julian Hirsch (after living with the Phase Linear amp for a while) quipped that he wasn't sure that 350/ch was 'enough' power.

 

[KSTR]

This amp is clearly meant as a design and build exersize to understand discrete power amp working principles with a minimum amount of parts and still have reliable operation. It's all there: output bias current control loop, DC feedback and output quiescent voltage adjust, AC feedback, basic speaker protection via output cap, use of cheap and readily available 19V-SMPS.
Output quantity and quality never was a priority any higher than, say, "somewhat usable" and sure with the unavoidable specific voicing from such a distortion profile (which may or may not be audible in any given situation).
As such, it is a success and was well prepared and excecuted by Mr.Pass.
I'd like to suggest this thread might be moved to the DIY section which is more approbriate place for this kind of stuff, IMHO.

 

[Cahudson42]

Might be interesting to compare the ACA performance with something like this:
Kinter K3118 Texas Instruments TI Digital Hi-Fi Audio Mini Class D Home Auto DIY Arcade Stereo Amplifier with 12V 3A Power Supply Black https://www.amazon.com/dp/B0787BRQ2F/

$23. '15w' Has ASR already done a similar one and I missed it in the Index of Tests?

Seriously. Such a test could set a floor on what should be minimum performance from any amp today..

 

[MediumRare]

This amp is clearly meant as a design and build exersize to understand discrete power amp working principles with a minimum amount of parts and still have reliable operation. It's all there: output bias current control loop, DC feedback and output quiescent voltage adjust, AC feedback, basic speaker protection via output cap, use of cheap and readily available 19V-SMPS.

It seems to me this is an example of how NOT to design an amp. Aside from the obvious distortion, and amp with that pitiful level of gain that can't drive normal speakers to conversational level hardly qualifies as an amp. My son is an EE student and if he turned this in as a project surely he would get a failing grade.

Can anyone look at this design and point to one or two major faults that, if fixed, would improve the performance?

 

[ElNino]

I'm conflicted about Pass. I think he has done a lot to boost interest in DIY and emphasize the importance of performance at low power levels, but his obsession with amplifiers acting as boutique tone controls via high distortion levels is strange. He's actually shipping out a board for free right now that does nothing but intentionally inject distortion into signals. It's an odd approach.

 

[anmpr1]

1) and 2) This amp is clearly meant as a design and build exersize to understand discrete power amp working principles...

3) As such, it is a success and was well prepared and excecuted by Mr.Pass.

1) If that is so, then why design something that is so ridiculous, electrically, and then leave it at that? Why wouldn't he work it out better? And what was it that Pass was 'trying to understand' in this design? Was he investigating something that wasn't in the textbooks, or that couldn't be found out in already 'existing in the field' designs? What was it that he needed to figure out?

2) If it was 'clearly meant' to be a lab experiment and nothing more, how is it that others are selling it as a kit? Wouldn't that be something Mr. Pass would discourage, on the grounds of intellectual appropriation and fraud?

3) ...'well prepared and executed'? I really can't comment on that description, given what @amirm saw on his 'scope.

 

[KxDx]

This amp is clearly meant as a design and build exersize to understand discrete power amp working principles with a minimum amount of parts and still have reliable operation. It's all there: output bias current control loop, DC feedback and output quiescent voltage adjust, AC feedback, basic speaker protection via output cap, use of cheap and readily available 19V-SMPS.
Output quantity and quality never was a priority any higher than, say, "somewhat usable" and sure with the unavoidable specific voicing from such a distortion profile (which may or may not be audible in any given situation).
As such, it is a success and was well prepared and excecuted by Mr.Pass.
I'd like to suggest this thread might be moved to the DIY section which is more approbriate place for this kind of stuff, IMHO.

Yeah, isn't the point of "Amp Camp" more of a learning exercise? It actually measures pretty close to the listed specs, so you can't accuse Pass of false advertising.

I have a feeling any of JC's Parasound designs would measure a good bit better.

 

[Tks]

Rising distortion in a linear fashion with respect to the THD+N metrics? The heck?

Also, WORSE performance as the unit heats up? Maybe they can contact Sabaj, they can help them with a few heatsinks to at least give you a few minutes of enjoyment before you're forced into power cycling the unit until it cools back down again.

What an odd device, only made more peculiar seeing as how Pass make a device if I recall properly that injects distortions into the signal path..

 

[anmpr1]

Yeah, isn't the point of "Amp Camp" more of a learning exercise? It actually measures pretty close to the listed specs, so you can't accuse Pass of false advertising.

Do you have to pay to go to 'Amp Camp'? And is this an example what you 'learn'? If so, that would be like paying a personal gym trainer, and instead of exercising and working out on the machines, you sat around eating cheesecake and drinking Pepsi for your hour of 'training'.

I don't think anyone is accusing Pass of false advertising. Just false (or substandard) engineering--at least in this design. His other stuff might be different. I mean, how could they be worse?

In fine, you really can't put a positive spin on this sort of thing, no matter how much Kool-Aid you drink.