Pass ACA Class A Power Amplifier Review

[BillG]

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...

I've a $120USD TI based amp that measures far better with a lot more power as well, than the reviewed unit...

 

[maty]

It seems that some do not understand it or, worse still, do not want to understand it: it was an exercise to practice spending very little. And with a harmonic profile intended to emulate tube amplifiers.

It is not necessary to give more turns to the ACA.


NP has new commercial class AB very expensive and... with dominant H3. Stereophile graphs. I reported it some time ago in some threads of diyaudio.com. There the criticism fits.

 

[KxDx]

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.

It's the big-bucks audiophile paradox:

One wants to hear music "as it was intended" but not "as it was recorded."

Speakers should have flat response but sources should have audible distortions. Those distortions bring "warmth, realism, and life" into recordings. Low distortion, accurate sound reproduction is "cold, digital, lifeless."

Honestly, it's fine that everyone has a listening preference. Some prefer flat, accurate reproduction and some don't. Hey, some people like Kool-aid and some people like bottled water. My approach is get the best equipment I can afford, then sprinkle a little bit of EQ to get the right flavor for my ears. What makes me smile might make the next person jump from a window. who knows?

But the terms "inaccurate" and "distorted" aren't good selling points so those get replaced with lofty marketing terminology. It wouldn't kill people to just say equipment plays music "the way I want to hear it," instead of "the way the performers intended for me to hear it." But humans are too proud for that.

 

[solderdude]

It seems that some do not understand it or, worse still, do not want to understand it: it was an exercise to practice spending very little. And with a harmonic profile intended to emulate tube amplifiers.

It is not necessary to give more turns to the ACA.


NP has new commercial class AB very expensive and... with dominant H3. Stereophile graphs. I reported it some time ago in some threads of diyaudio.com. There the criticism fits.

So 'they' sell an exercise with a very nice cabinet and a not 'exercise alike' price that is just that. And NP thinks that is just fine and dandy because they say it is an excercise but gave it the looks of a finished design.

I haven't seen any tube amplifiers measure this bad but am sure some will exist.

 

[RichB]

So 'they' sell an exercise with a very nice cabinet and a not 'exercise alike' price that is just that. And NP thinks that is just fine and dandy because they say it is an excercise but gave it the looks of a finished design.

I haven't seen any tube amplifiers measure this bad but am sure some will exist.

This is the beauty of solid-state designs.
Finally, the technology exists to more accurately create the inaccuracy of tubes for the ultimate luxurious "tube" sound.

- Rich

 

[KxDx]

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.

From what I read, it's an assembly experience, not an engineering class. If people pay to socialize and assemble a cheap amplifier, then that's what they get. I don't have any skin in the game, nor do I drink Nelson Pass flavored Koolaid.

This forum doesn't need to be an "us vs. the world" platform. It's clear that a lot of people find source and amplifier distortions pleasing to their ears... and on the other hand a lot of people want the highest degree of accuracy they can afford. Some of these discussions feel like politics or religion... except no one's life is actually getting impacted by someone else's beliefs when it comes to audio!

 

[maty]

Exercise... with very high sensitivity speakers, that is the key.

 

[solderdude]

Exercise... with speakers with very high sensitivity, that is the key.

Then a well performing 0.5W amplifier would have sufficed when one should not use it above the 0.1W you mentioned.

 

[anmpr1]

From what I read, it's an assembly experience, not an engineering class. If people pay to socialize and assemble a cheap amplifier, then that's what they get.

So it's an 'assembly experience'? That's a new one on me. But if it makes someone happy hanging with their audio-guru, building substandard electrics, then I guess... I mean, we need more happiness in the world. Who can deny that? Of course, telling some jokes might make someone just as happy as building a joke amp.

 

[anmpr1]

It seems that some do not understand it or, worse still, do not want to understand it: it was an exercise to practice spending very little. And with a harmonic profile intended to emulate tube amplifiers.

Come on, maty. If it was just an exercise, why is it sold as an actual product? What's up with that? As far as emulating tubes? Even a 60 year old Dynakit specs out better than this thing. And is more practical to boot. Why not just go with the real old thing?

https://www.dynakitparts.com/

 

[ElNino]

It's the big-bucks audiophile paradox:

One wants to hear music "as it was intended" but not "as it was recorded."

Speakers should have flat response but sources should have audible distortions. Those distortions bring "warmth, realism, and life" into recordings. Low distortion, accurate sound reproduction is "cold, digital, lifeless."

Honestly, it's fine that everyone has a listening preference. Some prefer flat, accurate reproduction and some don't. Hey, some people like Kool-aid and some people like bottled water. My approach is get the best equipment I can afford, then sprinkle a little bit of EQ to get the right flavor for my ears. What makes me smile might make the next person jump from a window. who knows?

But the terms "inaccurate" and "distorted" aren't good selling points so those get replaced with lofty marketing terminology. It wouldn't kill people to just say equipment plays music "the way I want to hear it," instead of "the way the performers intended for me to hear it." But humans are too proud for that.

I know what you mean, and I used to think that way too. I thought I'd always have some form of tubes in my system, but as I've acquired more and more well-measuring gear (particularly well-measuring at lower power levels -- this was a major turning point for me), my desire to use tubes has died. It turns out that accurate, low-distortion playback is actually "warm" enough for me, and part of the motivation for using tubes was really to just cover up distortion elsewhere in my system.

 

[Midwest Blade]

In a previous life I bit on the ACA Pass, thought it would be a great little project to try and given the rave reviews, how could I go wrong. Well...it unfortunately sits in the closet and is somewhat unfunctional. I should have first bought the Emotiva A-100 which works like a charm.

 

[Soniclife]

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

Has he actually delivered any amps that are excellent at low power levels? The evidence in post 50 suggests he sells the story, but his expensive amps don't live up to it.

 

[ElNino]

Has he actually delivered any amps that are excellent at low power levels? The evidence in post 50 suggests he sells the story, but his expensive amps don't live up to it.

By low power, I mean under a watt, not ~5w. I don't think he's ever delivered an amp that measures competitively to even a simple LM3886 design at lower power, but some of his designs do outperform a typical class AB amp at that kind of flea power level -- see e.g., Stereophile's measurement of the First Watt J2; it does have a fairly atypical distortion curve at low power. That's not to say they're worth the money; there are far more cost-effective ways to get lower distortion at low power (LM3886 and modern class D, etc.), and without the same drawbacks as power rises.

 

[peng]

That ACA amp build was my first DIY project. Sounds the same as my other much more powerful amp when I use it with my R900 listening from about 8 ft at moderate level (probably around 70 dB average). It does sound neutral to me. I actually plotted FR graphs using REW vs my other amps, look almost exactly the same.

 

[Xulonn]

Sigh, you just cut short at least 10 pages of pinãta bashing including snarky comments on NP’s choice of clothes and hairstyle and his education and talking style, etc., etc by introducing a dose of reality. Taking all the fun out of the forum...

The post by @restorer-john indicates that the regulars here are generally astute enough to know who deserves snark/ridicule and who doesn't. Even though I participate in such activities upon occasion (but try to attack the words and not the person), I appreciate the contributions of Nelson Pass. And I do believe that comments about appearance (hair) are out of line.

 

[solderdude]

It does sound neutral to me. I actually plotted FR graphs using REW vs my other amps, look almost exactly the same.

That is to be expected as the harmonics are way too low and in level would 'drown' compared to the actual signal.
of course the rather high output R (around 3 or 4 Ohm) would show some variation in FR when tested using speakers but not when using a load resistor.

 

[gvl]

I have no issues with this Amp, works as advertised. There are many more worse ways to waste your $300.

 

[mitchco]

As a DIY amp with an adjustable bias control, makes me wonder if it was properly biased as that does change the distortion profile. The tell tale hint is that the distortion should go down as the amplifier warms up (for about an hour is the recommendation). Given that @amirm measured an increase in distortion with the amp warmed up, suggests the amp was not properly biased in the first place...

 

[ta240]

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?

The other amps that he releases the schematics on and people design boards for can be quite complex to build and setup properly. They require tracking down hard to find parts and are quite intimidating. This was a fun, inexpensive at the time build designed to be someones first attempt at soldering together an amplifier. At the time of the original 'camp' event most of the product costs were absorbed by Mr Pass. The basis of the 'event' was "you've never used a soldering iron? Well, lets build something and learn" An adult version of kids building bird houses at home depot. You don't end up with a priceless creation but you learn skills and gain confidence to try more complex.

I don't know what the original class cost but it was likely a tiny fraction of the current kit.

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?

Somewhere along the line the amp took on a life of its own. It is sold with his permission due to a very high demand. It started as the simplest design for a Class A amp that he could come up with and due to his name being associated with it and the hype online people have decided it is a high end amp. Even Mr Pass himself has said the main focus of the design was to be a Class A amp that a novice could complete in one sitting not on performance or sound quality.

The only failing of this design is that it has somehow become legendary for its performance rather than its simplicity and ease of build.

As restorer-john has pointed out you can make this amp for pocket change with a $10 ebay heatsink and a perf-board.

I built one on a cutting board, and probably used it for a total of 2 hours before parking it. It was fun to build but not the greatest sound. That being said I then went on to build his M2 amp and measurements or not any time I turned it on I'd find myself sitting there just listening to it for long periods, forgetting what I was supposed to be doing and closing my eyes and listening simply because it was enjoyable. And compared the the M2 a Volt+, icepower module and even LM3886 based amp never had that effect on me.

Many people that build it do go on to build the much more difficult amps that require a lot more skill, knowledge and that use mains voltage so it still does its job. A few people do praise it as an amazing amp and just build more and more of the same amp. Personally, I've been baffled by that take on the amp but if they enjoy it then more power to them.

I've a $120USD TI based amp that measures far better with a lot more power as well, than the reviewed unit...

And that is completely irrelevant unless you assembled the board, this isn't about price or performance it is about actually building it from components and a pcb into a completed amp. It is a class in DIY not a class in plug in an amp board you bought online. Yes, the price has grown to a point where it is absurd and the Akitika mentioned would likely be a much better investment; but even then I wouldn't want to try the Akitika as the first thing I ever soldered. There aren't a lot of build it yourself kits available that include everything you need, extensive documentation and follow-up support.

So it's an 'assembly experience'? That's a new one on me. But if it makes someone happy hanging with their audio-guru, building substandard electrics, then I guess... I mean, we need more happiness in the world. Who can deny that? Of course, telling some jokes might make someone just as happy as building a joke amp.

Not an 'assembly experience' but a learning experience. Its original incarnation was for people that had never soldered before. Something to get people into DIY in an age where DIY has become "I bought an amp module and hooked it up". Again it has taken on a life of its own far beyond the initial intention and in reality what should they do, ignore those rattling the door of the store going "more ACA kits!!!"?

Think a budget set of 50~100W IcePower modules would definitely measure better when put into that case.

Putting a module in a case is really not DIY. Again, don't buy the full $320 kit, buy the boards, buy the handfull of components and a heatsink off ebay and have some fun. Then go "hey, if I could do that maybe I can build something more complex"

Come on, maty. If it was just an exercise, why is it sold as an actual product? What's up with that?

Because customers asked for that. I don't know why, I wouldn't want to spend that much for it and it really does destroy the original intent but you can still buy just the boards for $15 and they are well made boards. Heck, people spend hundreds on DACs that don't have an audible improvement over their current DAC just because it makes them feel better that it so why slam people for paying $300 for a toy amp?

Whoopse! He needs a haircut.

I think maybe we need a call for civility when we start attacking people's appearance

I can put an amp together on a breadboard in 30 minutes and better this. As some parts of this amp are competently designed, the result is intentional, I have to ask.... why???

One more time, this wasn't designed for anyone that can breadboard an amp so you aren't the customer it was designed for. But hopefully some of the people that build it will move on to doing things like that.

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.

In terms of performance, it's a joke THD wise- we all know that, but if it cost just $30 for the parts you didn't have in stock, and the rest you pulled out of your junk bin or scavenged, it was no doubt a pile of fun.

I guess that really summarized my long winded reply.