Pass ACA Class A Power Amplifier Review

[Killingbeans]

I know that ASR has taken an approach where "scientific" means measurements of electrical parameters, and the only test is ABX double blind. Yet "science" includes also other methods used in social sciences, psychoacoustics as psychology of sound can be wider than only ABX tests.

Absolutely. It doesn't have to be that complicated. Just volume match the outputs with a multimeter and let someone swap the amp connections at random without your knowledge. Then see whether the impressiveness still jumps at you.

Or just have fun with the normal component tasting audiophile procedure. If that's what gives you enjoyment, then who am I to judge? But I won't take it as proof of anything

 

[LTig]

I know that ASR has taken an approach where "scientific" means measurements of electrical parameters,

This is true in so far as there are no resources available at ASR to perform subjective listening tests with proper controls.

and the only test is ABX double blind.

Scientific research has shown that all other tests (sighted, not level matched) lead to unreliable results. (EDIT: added missing word )

Yet "science" includes also other methods used in social sciences, psychoacoustics as psychology of sound can be wider than only ABX tests.

Are these not the fields in science which are plagued by studies which cannot be repeated with identical results?

 

[SIY]

I have a feeling that ASR may be doomed if it further develops its fixation on perfectly repeatable measurements. I’ll try to explain:
People are ultimately voting with their money, and they are after certain goals.
For example, if I go to a cinema (in my case mainly in Brussels) and the sound there, supposedly done scientifically ‘by the book’, does not pull me in and immerse, it is not appealing, that casts doubt over the execution, but also on the validity of the method. May daughter says in Paris it is better, as the French take pride in their cinemas, she had objections to sound in Belgium, too (and she should know, she studies cinema, now in Masters).
As music goes, I like live performances (orchestra, opera), I do not recall a really disappointing one. On the other hand, amplified performances have been a hit and miss. Again, this casts doubt over execution, but also over the validity of the underlying methods used in some amplified performances.
Now, I try to bring some of the good experiences into my home. I need gear.
One can argue that if it measures fine, it should be accepted as “true”, whether I like it or not. But if it does not get closer to said live perfomance experience, but rather the opposite, where is the point?
I fully support the need for a solid scientific basis, basics should be right, Kellogg’s standard was too low, a minimum shold be at least Cocking’s, maybe better Williamson‘s, quantitative criteria. But once those boxes are checked, the benchmark is live performance experience. Once two amps measure as good as or better than Williamson’s, the better amp among the two for me is the one which gets me closer to the live performance experience, not the one which has 500W vs 300W power, or 110 dB over 95 dB SNR, or 0.01% instead of 0.1% THD, 800 vs 80 damping factor, etc. Once the measurements are good enogh, it is enough for me.
As our comparative religions professor once said, the biblical languages (hebrew and κοινή greek) for a clergyman are like underwear – one must have it, but one should not display it. The same could be said about Hi-Fi: basics should be fine, it should measure good enough, but that is just that, the basic precondition to be viable; past that, dwelling on measurements resembles some sort of a fetish, doesn’t it?

There's enough strawmen there to represent a fire hazard. Several stolen bases as well.

 

[Waxx]

Back at the ACA, i think most of you are missing the point of this amplifier. It's not ment as the most neutral clean amplifier, but as an class A amp with a lot of second harmonic colouration that is cheap and easy to build for diy builders without much experience with amp building. It's very coloured, and the harmonic distortion is high, but it does sound good to almost all who build it. It's ike the cheap version of the type of amps Nelson sells for a shitload of money to audiophiles who like that kind of colour (First Watt and Pass Labs brands). But it's still nice to see the objective measurements that confirm what i hear in mine (i own 2 of them, set up as monoblocks)

But if you need a clean neutral amp, this is not for you. Idem with higher power needs. It's a niche product, and it's very good in it's kind. It's the same with some other diy kits or boards he sells on diyaudio for little prices (like the H2 or nutube preamp). In that way it's more a solid state variation of a single ended triode tube amp, that have similar distortion and power figures, but still have a large following among diy builders who know.

 

[SIY]

It's very coloured, and the harmonic distortion is high, but it does sound good to almost all who build it. It's ike the cheap version of the type of amps Nelson sells for a shitload of money to audiophiles who like that kind of colour (First Watt and Pass Labs brands). But it's still nice to see the objective measurements that confirm what i hear in mine (i own 2 of them, set up as monoblocks)

I still have not seen even one person step up and show that the distortion here is audible. Just evidence-free claims. Part of Nelson's mystique, I guess.

 

[Goodman]

What if there is more to Nelson Pass designs than meets the meter?
I swapped ML 23 for an Pass Aleph 4 today. I had not used the Aleph with Schiit Freya S preamp before, and I was quite impressed (with some other preamps Aleph’s bass seemed too shy; but maybe the woofer is finally broken in after 2 1/2 years?).
I know that my CD and Schiit both measure exemplary, and so does ML 23. Aleph 4 measures worse, not as bad as ACA, but not nearly as good as a ML 23. So Pass Aleph 4 must be wrong, ‘cause it does not measure right; but probably it is wrong in the pleasant way?
I guess this makes me a subjectivist? To quote:
“Years ago my mother used to say to me, she'd say, "In this world, Elwood, you must be," — she always called me Elwood — "In this world, you must be oh so smart, or oh so pleasant." Well, for years I was smart. I recommend pleasant. You may quote me.”

I have owned several Nelson Pass amps and preamps 30 years ago, mostly Threshold brand. Excellent gear indeed. Since then, mister Pass has flogged a good dozen of différent amps, all selling for mega bucks. I wish he would explain what he knows now that he did not know then; or are there modern magical components that are now available and that have been kept secret? If that's the case, I will buy one of his latest tinkers even if outrageously expensive. It will improve my system by leaps and bounds, right? Enough snake oil and BS, Come clean now Mr. Pass.

 

[pma]

I have owned several Nelson Pass amps and preamps 30 years ago, mostly Threshold brand. Excellent gear indeed. Since then, mister Pass has flogged a good dozen of différent amps, all selling for mega bucks. I wish he would explain what he knows now that he did not know then; or are there modern magical components that are now available and that have been kept secret? If that's the case, I will buy one of his latest tinkers even if outrageously expensive. It will improve my system by leaps and bounds, right? Enough snake oil and BS, Come clean now Mr. Pass.

Why would he?
BTW you might have read a lot of his posts at diyaudio.com

 

[maltux]

Has anyone here seen this review?

Nelson Amp Camp Amp build and review

I have read all 50 pages and have decided to put in my email address so as to be notified when I can buy this kit. 25 pages earlier I decided not to build this. My brother-in-law keeps asking me if I build this kit and he is a NP fan. It costs me nothing to take the money from my wifes' purse and purchase it. If I don't like it; my brother-in-law gets a christmas present he will enjoy. I will post what I think of how it sounds and impressions. I know this topic has been chewed ad nauseam.

On another note I saw muddy water being discussed. I once asked my Scottish grandmother (over 20 years deceased now) why she drank Teatley Tea? Was it the best I thought? - No! she replied. It was the cheapest on the store shelf that day. What does that have to with the ACA? Absolutely nothing.

 

[enio nery]

ive had one of these. even with high efficiency speakers it sucked. it literally is a distortion generator.

 

[worktheweb]

I built one of the 1.6 versions and was not overwhelmed but finally got around to building another one I had bought during COVID. It was a 1.8 transition model so it had updated parts but wasn't drilled for a front power switch. I modified the chassis and built the new one to 1.8 spec and then modified the original chassis and rewired it to 1.8 spec. I have the 24V Meanwell power supplies on each unit. Ran them as bridged mono and it was way way way better than stereo but then switched to parallel mono and it's a night and day improvement. I have a pair of MB Quart 2000 tower speakers that are 4 ohm so they definitely benefitted from the increased current. I'm using the Nutube preamp from Diyaudio as well.

I don't have the greatest setup to evaluate and I will caveat this by saying that even if the setup sounded like total crap I'd wouldn't really care. I'd be disappointed but it wouldn't diminish the experience and the thrill of building my own gear. That's really what the ACA (and the Nutube) were all about. For even more DIY fun I assembled my own speaker wire using Canare 4S11 star quad wire.

That said, I think it sounds really good especially considering I assembled it. . When I swap my trusty ol' Proton AA-1150 in for the ACAs I'm shocked at how much more grunt the Proton has though. I previously had a pair of modded Carver m1.0t mkII in this system but those were just ridiculous. Most of my listening is done at moderate levels in a small room with a high angled ceiling. Speakers are about 9 feet from my listening area. I'm using a Technics SL-D3 turntable with a AT VM540 ML/H cart. Fosi phono box X2 with Riverstone 5654 tubes as a phono preamp as the Nutube just has 2 regular RCA inputs. I sometimes stream through my iPhone headphone jack or a RPi headphone jack. I listen to a wide variety of music - classical, jazz, electronic, rock & roll, folk, whatevs.

With this setup I can drive the Quarts to almost uncomfortable levels before the ACAs run out of steam. They have nice tight bass at all but the highest volume levels. Imaging is well defined although I don't get the sense that I can pick out individual instruments in orchestral pieces. I'm going to defer to the condition of my ears vs the electronics on that one though. I have a record titled "Stereo Action Unlimited!" (https://www.discogs.com/release/2946916-Various-Stereo-Action-Unlimited) that describes each song and where the instruments are "supposed" to be and it checks out. :>

My guess is most people that have formulated opinions on these in the past 50 pages of comments have never actually listened to them. Shocker, I know. Once again people need to consider what these things were designed for. The fact that Nelson Pass is associated with them is probably the ONLY reason this thread has 50 pages. If I had come up with the design and managed to put a kit out there would be no takers. Just wondering if any of the posters on this thread own companies selling amplifiers or have put their name on DIY kits?

 

[voodooless]

for. The fact that Nelson Pass is associated with them is probably the ONLY reason this thread has 50 pages. If I had come up with the design and managed to put a kit out there would be no takers.

Doesn’t that say it all…?

Just wondering if any of the posters on this thread own companies selling amplifiers or have put their name on DIY kits?

Why is that relevant?

 

[SIY]

My guess is most people that have formulated opinions on these in the past 50 pages of comments have never actually listened to them.

And more tellingly, no-one who built one and has it on hand has run an actual listening test to see if it actually sounds different from a similar amp with a low distortion design.

 

[MAB]

Just wondering if any of the posters on this thread own companies selling amplifiers or have put their name on DIY kits?

Likely next to zero.
And that would affect the performance of this amp in which way?
At least it isn't noisy.
And we now have an idea of the amp's actual power output ('punching above it's weight' is often stated unhelpfully).

To your point, do people need to own a company that sells DIY kits to matter? Lots of people build their own here, post about it, but don't sell anything, they do it to find out or to demonstrate how things work. Radio Shack used to sell education kits too, like an AM radio. Did the Amp Camp build also provide an exercise to evaluate the impact of Class A operation? How about negative feedback? If not, doesn't seem very competitive to the old Radio Shack educational builds...

Despite the fact that most amps are utterly transparent, this one with the 3 Ohm output impedance, the very low power, and the very high distortion might actually stand out from other amps a sounding different. At least it won't hiss too badly with high efficiency drivers due to the relatively low noise.

 

[manisandher]

And more tellingly, no-one who built one and has it on hand has run an actual listening test to see if it actually sounds different from a similar amp with a low distortion design.

I've got a pair of ACA monos that I've been meaning to compare with a pair of LA90 monos, both driving a pair of high-efficiency (97dB/W@1m) speakers. The intention is to capture level-matched outputs of the speakers with high-quality mics and preamps.

Will hopefully get around to this at some point in the near future, and will link to the audio files when I do.

Mani.

 

[voodooless]

Given the low output impedance (the 1.6 version should be similar), I'm sure it will give some audible differences, especially in the low-end. This gives the budget Class D amps some nice competition in this regard

 

[gvl]

And more tellingly, no-one who built one and has it on hand has run an actual listening test to see if it actually sounds different from a similar amp with a low distortion design.

I have, not a blind one however. It does sound different/warmer and less clean to my ears. That said, at lower volumes I can listen to it and it doesn’t bother me too much, it sounds kind of “cozy” actually.

 

[SIY]

I have, not a blind one however.

That’s why I said “an actual listening test.” AFAIK, nobody has done one.

I've got a pair of ACA monos that I've been meaning to compare with a pair of LA90 monos, both driving a pair of high-efficiency (97dB/W@1m) speakers. The intention is to capture level-matched outputs of the speakers with high-quality mics and preamps.

Will hopefully get around to this at some point in the near future, and will link to the audio files when I do.

Mani.

A much better test is recording the electrical signal at the speaker terminals. Far fewer confounding variables.

 

[617]

-30db for distortion is incredibly loud distortion for an amplifier, but it's still quite low in the grand scheme of things.

Imagine if you were listening to the output of an ideal amplifier and ideal speaker from a distance of 10', and then turned on another speaker that represented just the pure distortion, but placed it 320' away. Do you think you'd be able to notice the difference? That's what -30db is.

I doubt the difference between this and a -100db amplifier are noticeable unless you're really pushing it.

I am very disappointed in the participants of this discussion. The generic audio position is that 'you have to hear it', but the generic ASR position is not 'a performant amp sounds better' it's 'it sounds the same as any other amp'.

The proper critique of this amp is that it creates excessive heat, has no warranty and has limited resale value.

 

[manisandher]

A much better test is recording the electrical signal at the speaker terminals. Far fewer confounding variables.

Not much of "an actual listening test" then.

I'm personally interested in knowing how the ACA compares in SQ to the LA90 through high-efficiency speakers. I have everything to hand, so that's the test I'll perform. But sure, I can record the electrical signal from the speaker terminals too for extra data points.

 

[voodooless]

Not much of "an actual listening test" then.

You want to listen to the amp, not the speakers. So logically you record the amp.

At the few watts this thing brings, you better have high efficiency speakers, preferably 16 Ohm, to keep distortion lower.