Pass ACA Class A Power Amplifier Review

[solderdude]

That would depend on the type of music and output power as well as sensitivity of the speaker.
Probably anything between inaudible and loss of fidelity at higher levels.

 

[cistercian]

It makes it worse if you realized later, based on reliable objective data, that there was a much better option at lower cost, or with better performance so you did not have to hunt down special speakers to go with it.

The reactions in this thread (and what I read in private) ARE REAL. People are surprised because they had a different impression of this amplifier based on its reputation. I know I had. So we are entitled to complain how we got here.

This amplifier has gotten a life of its own way beyond its origins and intentions. We have brought some new data to that discussion which should give some people pause. That it doesn't give you that is odd but don't say the rest of us are wrong.

Good review. It made me a bit sad...because I am an older guy and remember when there were several brick and mortar stores
where I could buy parts for projects. No more. I used to cannibalize anything I could get my hands on for parts and made some
cool stuff out of junk. I find the price for this amp depressing to say the least. Dollar/watt ratio is criminal at the horrific distortion
levels seen. I know purists hate Crown XLS amps but at least the dollar/watt ratio is good!

I think everyone who has a passion for sound/electronics should be afforded the opportunity to do home building easily. Sadly, SMC devices
and other changes make serious hands on fun much harder...and the kits that are sold are very overpriced. I deplore this.

Thanks again Amirm. A good review that helped me feel my age...next week I am 58.

300 dollars+? That is just wrong...

 

[Azookey]

That would depend on the type of music and output power as well as sensitivity of the speaker.
Probably anything between inaudible and loss of fidelity at higher levels.

Loss of fidelity, like signal to noise? sounds/notes/intended material getting obscured by a static-like noise?

 

[mitchco]

Try https://quantasylum.com/blogs/news/abx-testing-and-distortion
Not exactly the same, as it is not the same type or spectra of distortion, but can give you an (audible) idea... and establish your threshold of audibility.
I could make a recording of an ACA driving a KEF LS50 maybe at 1 watt and then full blast. Could be fun, if I can find the time.

 

[dtaylo1066]

As a DIY project, is there certainty this unit was constructed and soldered properly, powered properly, and that all components were functioning properly?

 

[amirm]

As a DIY project, is there certainty this unit was constructed and soldered properly, powered properly, and that all components were functioning properly?

As I noted, my measurements are actually better than what Nelson had posted so I have to believe it is working well. I will try to tweak it and see how much better it can get.

 

[cistercian]

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.

I was using my stereo and turned it up pretty high. I was not that happy...it seemed the bass was falling off. I was pondering what was wrong
when my shelves began to shed items as they walked off due to vibration! My ears were in compression since the SPL was so high...and I mistook
the sound for a fault in my system...when it was my ears limiting! Crown XLS 2502 driving a pair of JBL SRX 835 Passive cabs in a small room.
I noted that at that SPL it was a good level throughout the house...with the door to the room closed!

Everyone needs problems like this. No distortion at all. It never sounded bad...I thought the bass was weak...then I noticed the chair I was in
was vibrating like mad...as I watched items walk off the shelves!!! 95Db/watt/meter speakers. A new experience for me. I have never had such
a powerful system before. And at that level, the -20DB lights were not even lighting. WOW.

There was no cue that the SPL was retarded too high...no audible distortion at all. Then stuff in the room began to move......

 

[Sal1950]

Back in the 70-80s I was doing pretty good with side jobs repairing old tube gear. New tubes were getting impossible to find but FREE tubes could be had every week just by cruising the alleys and scavaging all the old gear getting tossed out. Almost as good as panning for gold.

 

[cistercian]

Back in the 70-80s I was doing pretty good with side jobs repairing old tube gear. New tubes were getting impossible to find but FREE tubes could be had every week just by cruising the alleys and scavaging all the old gear getting tossed out. Almost as good as panning for gold.

I remember aligning and converging color TV chassis in that era. Tube based sets. The good old days. Back when it made sense to repair them!

 

[GradyBeach247]

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.

I recently built an ACA amp for fun. It powers my 1978 vintage Time Windows just fine near field in my office. The sound takes me back to my teenage years ( remember when 10 watts @ 0.5% THD was excellent performance? ). Yes, it is a somewhat expensive toy, but it is an easy build and has a nostalgic sound. It will join my collection of vintage gear.

 

[Azookey]

Try https://quantasylum.com/blogs/news/abx-testing-and-distortion
Not exactly the same, as it is not the same type or spectra of distortion, but can give you an (audible) idea... and establish your threshold of audibility.
I could make a recording of an ACA driving a KEF LS50 maybe at 1 watt and then full blast. Could be fun, if I can find the time.

I have an aca and I have ls50’s and to be honest I don’t hear anything I would consider “bad”. I will say there is a lack of low end with the aca vs some other amps I have used but in general it sounds fine. With a nice active preamp the low end comes back (as opposed to straight out of a dac as preamp).

I have a small room and don’t play too loud, maybe I just don’t crank it enough.

 

[dtaylo1066]

The DIYAudio web site states:

"The ACA is a minimalist, low power amp that's easy to build and the perfect choice for your first amp building project. It's the "fixie bike" of Class A amplifiers; no gears, no brakes; zen and the art of amplifier maintenance.
It likes efficient speakers that present an easy load to the amplifier. The right pairing will give you great sound full of audiophile buzz-words, and more than adequate volume for everyday listening or to annoy your neighbo'rs (8W with 91db speakers gives 93db at the couch). Double the power per channel with monoblocks."

I have not heard the ACA other than on you tube. Hardly a great listening environment. Many a DIYAudio builder is quite pleased with their ACA builds. It is a low power, single-end design, class A design. The main cost of the kit is the beefy enclosure. DIYAudio is a non-profit forum and is not operated by charlatans, as one poster indicated, but by passionate audio and DIY audio enthusiasts.

I am not going to take to task the design philosophy of Nelson Pass, nor praise it. I do not know the man, but he is very generous in providing the audio community with DIY ideas and projects. I am not too aware of other audio companies, or CEO's of companies, that offer that type of support or outreach. You may want to look at Nelson's interviews on you tube by the Audiophiliac. Nelson's designs have ardent followers, as seen by the folks who rave about Pass or First Watt products. He seems to pioneer certain types of circuit designs, often class A. As to this designs, I guess one can take it or leave it. Bench measurements are a tool, and a valuable tool. But I have heard many high power audio products that test remarkably well but do not sound particularly good to my ears. I have heard a few Pass products and my ears were pleased.

The ACA amp was created around a one day "amp camp," where folks could gather on some park benches and build a simple amplifier on a Saturday afternoon and in doing so enter the world of DIY and understand a simple circuit from a well-regarded designer.

This weekend in San Francisco Nelson Pass and others host an annual "Burning Amp Festival," where accomplished and newbie DIY enthusiasts strut their stuff. It is quite a hoot.

Several other of Nelson's designs are offered at the DIY Audio web page, designs that replicate (with his permission) some of his acclaimed OEM products from Pass Labs and First Watt.

I think criticism often demands some context, and I suggest forum readers may want to listen to a piece of equipment before ripping it, its designer, or its intent. Or check out the original design article.

Many consider the Miata to be an under-powered, little piece of shit of a car. But often, if you put a driving enthusiast or wanna be sports car owner behind the wheel, they are transformed by the experience and in love.

 

[cjfrbw]

Amir, making friends again: https://www.diyaudio.com/forums/pass-labs/215392-amp-camp-amp-aca-post5969463.html

Honestly, though, you need to respect Amir's mission and the work he does setting the record straight on these things.

People still have the freedom to follow their 'ears' and preferences.

 

[Thermal Alchemy]

I just came across this form and this review very interesting reading from a very uninformed perspective. Several years ago I came across this amp camp amp version 1.1 that only used a 19Vault power supply and only produced 4W or 5 W of power. Because it was advertised as class A is what caught my attention because I used to own a Tandberg amplifier that I bought used on a recommendation of a neighbor who is a high-end audio sales person I was only 22 not a lot of money and he told me it was only 10 W of class A and it was switchable to class AB at 40 W. I was used to class D amplifiers Sony Panasonic Kenwood Fisher Onkyo Denon Marantz that had 80 W or 150 W So I was not interested in a puny little amp that only was capable of 10 W of power or 40 W of power but I was ignorant and did not know what class A amplifiers can do and how they sounded.
Flash forward some 30 years and when reading forms I came across ACA (Amp Camp Amp) I was in college in electronics class EET 101 at the time And we were told to pick a graduation project. So at first I only purchased the transistors the PCB boards were out of stock so I built it on a breadboard. The resistors and diode’s and other little project parts were all free provided by the class I was in build a class a power supply from leftover scrap products I had from torn down scavenging.
I did read the details on this amplifier indicating they recommend higher 90s db efficient speakers 8 ohms to be exact. So I downloaded the Gerber files to build the Fostex BK-20 folded horn kit cabinet. To come over to the wood shop on the CNC machines used a few sheets of Baltic Birch plywood and constructed my speakers from scratch. Using the Fostex FE206EN drivers Rated at 98db Because I actually read the instructions and recommendations about this little amplifier I was about to build it’s called “Following Instructions!”. After listening to this working on the breadboard and it was absolutely outstanding I scavenged old aluminum computer case some heat sinks that I scavenged from a dead inverter board off an air conditioning unit part of my job I get for free. The PCB version 1.1 B became available and I bought them up assembled it Brought it to class with the speakers that weighed 80 pounds apiece and blew away the students and the teacher with both loudness and unbelievable sound they could not believe that was coming from 4 W. I got my A in class on this project. Sound source was a Microsoft Surface pro 4 laptop, Fed into a IFI BLACK LABEL, to the ACA.

Sony Panasonic Kenwood Fisher Onkyo Denon Marantz Are all gone in storage or used for reference testing as comparison. The Marantz was from 1972 I completely tore all the boards apart and restored it it resides at my mothers house. all the speakers that were with those different stereo systems I kept for testing reference in comparison.
The sound that comes from the Amp Camp Amp is definitely class A. Last year I purchased the expensive Italian aluminum cases and built the 15 W class A amp camp amp mono blocks. I’m now currently building Nelsons BA3 Front end preamp stage for my ACA For my Electronics class 102A project.
Next class project Nelson’s alpha J Amplifier.

It’s amazing how the stats and the numbers adding up to the best baseball team are supposed to be the winner but they don’t always win.
The NASCAR race car with the highest horsepower in the best driver who wins the best pole position and has the best statistics loses the race but he had the best numbers.
$3 billion in 500 years of combined knowledge of engineers send a satellite and with all the best equipment in the best math with numbers and it crashes.
And look at here again reading this form about the fancy equipment and the math and the readouts and Plotted charts. That come to a result put in black-and-white on paper. You could put the best tools and equipment into the hands of a person. And their results can show all the bells and whistle‘s and smoking glitter.
Well you can see where I’m going with this and I guess you know with all those numbers and readings mean.
Still to this day a quarter million dollars in equipment and not having the right person driving it cannot compare to what your ears hear. Numbers, math, fancy paper LOL

 

[amirm]

Still to this day a quarter million dollars in equipment and not having the right person driving it cannot compare to what your ears hear.

I must say, I did not see this snide remark coming after reading your nice story up to this point. You could have left this out and made a good impression.

Any way, nothing you stated is in conflict with my measurements or review. I did use my "ears" in my listening test. With the speakers I have on hand, another amp at 1/4 of the price runs circles around it. That amp takes fraction of the space, doesn't even get warm and produces far better bass, dynamics and volume on many speakers. If you had taken that amp to your class, you have gotten A+! Here it is: https://www.audiosciencereview.com/forum/index.php?threads/smsl-sa100-audio-amplifier-review.9706/

 

[Blumlein 88]

snippage..........

It’s amazing how the stats and the numbers adding up to the best baseball team are supposed to be the winner but they don’t always win.
snippage..........

One stat counts in baseball. Wins. Team with the most wins unsurprisingly perhaps wins the most games. So wins are the most important stat, and effectively the only one if you are interested in wins.

Audio isn't quite simple enough to say only one stat counts. The right ones always count the most however.

 

[solderdude]

Funny how people are drawn to 'class-A' and actually believe they get a glimpse of what class-A is all about.
They are not.
They get a glimpse of what amplification with a single FET (which has similarities to a tube) sounds like with minimal feedback.

One can build say a 50W class-A amp that has almost no distortion and would consider such an amp as what class-A is all about.

The fact that most owners find it wonderful sounding makes my point that one really isn't as sensitive to large amounts of distortion being bad.
I recommend the TotalDAC as a companion... it performs many factors better than this amplifier but then again will often reach 0dBFS which the amp in question may not always reach for sure.

As a technical person it is fun to measure for top performance which one can really only do with an excellent audio analyzer.
Myself I am not anal at all and do not believe for a minute that 0.00001% sounds any better nor worse than 0.01% but am interested to see how low one can get.
Transducers have a sound, a room has a sound, headphone positioning has a sound.
Electronics do (and should) not have to have a sound.

 

[Thermal Alchemy]

I must say, I did not see this snide remark coming after reading your nice story up to this point. You could have left this out and made a good impression.

Any way, nothing you stated is in conflict with my measurements or review. I did use my "ears" in my listening test. With the speakers I have on hand, another amp at 1/4 of the price runs circles around it. That amp takes fraction of the space, doesn't even get warm and produces far better bass, dynamics and volume on many speakers. If you had taken that amp to your class, you have gotten A+! Here it is: https://www.audiosciencereview.com/forum/index.php?threads/smsl-sa100-audio-amplifier-review.9706/

lower right with center button scavenged mini PC case with scavenged heat sinks from air-conditioning inverter board equipment original amp camp amp 4 WATTS.
Directly behind the old original Version 1.1 B prototype ACA. Eight broken S.M.S.L starting bottom to top. One S.M.S.L mini deck Dash scanskrit sixth anniversary version, Second from the bottom S.M.S.L SA– 50 PLUS, The next six amplifiers above that S.M.S.L SA-50, just happen to come in this week for repairs that I’m working on what a coincidence because the crappy prototype circuit boards they build them on of the lowest quality. With their fake knock off TI pretend chip amps. Garbage hung low who flung Pooh capacitors.
Hi Center if you look at my cheap Chinese manufactured DarkVoice 336 SE tube headphone amplifiers and with its high distortion measurements because it’s a tube amplifier we know it all must sound like garbage because of the readings taken off test measurement instruments say so. Sitting next to that DV SE 336 headphone amplifier you will see my chip amp class D reference system that I use for comparison of the lowest end. now look carefully zoom in and blow up the picture what is my chip amp class D reference system none other than, S.M.S.L SA-98E and S.M.S.L mini DAC-SANSKRIT 6th anniversary Edition.
Sitting dead center in the copper lined chassis on the table I’m in the process of constructing my Nelson Pass BA3 front in preamp.
To the right lower on the table you’ll see the Amp Camp Amp mono blocks experimenting voicing different components internally doing A/B test side-by-side comparison.
OPPSS

Yes the S.M.S.L just walk all over the ACA pushing low impedance 4 ohm low efficiency speakers And trying to thump some deep bass. at the same time providing high SPL levels. But at the same time I do kind of remember reading not recommending But at the same time I do kind of remember reading not recommending 4 Ohm speakers not recommending low efficiency speakers So why would anybody even think of testing that scenario so why would anybody even think of testing that scenario.

But I will admit when I strip down amplifier board in the S.M.S.L amplifier and replace the fake Texas instrument chip with a real one replace all the capacitors and rework the resistors to cut down the gain. And use a linear low noise high powered power supply but a little S.M.S.L amplifier actually sounds OK for a class D chip but it definitely has that class D signature sound that made me stop listening to CDs back in the early 90s and start listening to a.m. talk radio because of that sound. It’s all choppy like bits of information are missing when you’re playing sound through a fan in front of you.
When it comes to S.M.S.L audio equipment and you wanna tell me about comparisons you are definitely barking up the wrong tree.

 

[Wombat]

Performance is one thing, longevity is another. ASR does not have a crystal ball for predicting longevity.

You make some good points but a repair bench yells "hindsight".

 

[RayDunzl]

With their fake knock off TI pretend chip amps.

Maybe.

"Today, only 16 percent of the semiconductors used in China are produced in-country, and only half of these are made by Chinese firms. It is dependent on foreign suppliers for advanced chips."

https://www.csis.org/analysis/chinas-pursuit-semiconductor-independence