Pass ACA Class A Power Amplifier Review

[LTig]

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 one may come close: Balanced Audio Technology VK-56SE

 

[Cahudson42]

Hopefully not too far off topic, but perhaps to add to inexpensive diy choices not more difficult to build than the ACA, and for low parts cost, here is a design similar to one I built from an ARRL Handbook schematic - late 60s maybe early 70s.

Using $2 2N3055s for the output stage. Mine used 2N2102s and it's complement for the drivers. Had a massive 70v 20,000uf supply. Nice stuff from Cortlandt Street in NYC. Drove AR3s to painful levels. Note the topology is not terribly different from the ACA.. No idea of distortion it produced back then 'but it sounded great'

 

[amirm]

As soon as I saw it was a DIY build, my first thought was 'has it been built and biased properly?' If not, then doing a test on the thing, and then blaming Pass for ripping people off with a terrible design is very unfair.

I showed the performance as measured by Nelson in one of my graphs. If anything, this sample performs better than whatever Nelson measured.

He is showing 0.7% THD+N whereas mine (in Blue) show 0.3% THD+N.

In that regard, you can throw out all of my measurements and go by Nelson's and see that the amp performs very poorly. That data has been there for a long time. It didn't get attention because it was not put in context as I did.

 

[GaryMnz]

Is it possible, being a diy product, that the builder has got something wrong? Nelson Pass is no slouch at audio design.

 

[amirm]

Is it possible, being a diy product, that the builder has got something wrong? Nelson Pass is no slouch at audio design.

See my post just above yours. The problem is in the design. It has too little gain. This means it can't have a lot of feedback to correct non-linearities. So we see a lot of distortion. His measurements show that. Mine do as well.

 

[anmpr1]

You can't use the L word any more, it has been redefined as racist.

No one wants to L-word Nelson. They just want him (or his proxies who are using his name but who he doesn't endorse-LOL) to build a decent amp for the dollars.

Really, if this was me, if it was my design, people might expect slop. I'm a nobody. But from a world class world famous designer? That's the amazing thing in all of this. You just don't expect this sort of performance, given the names involved. Again, would Marhall Leach or James Bongiorno (may they both RIP) allow this thing to carry their names?

 

[MRC01]

... The problem is in the design. It has too little gain. This means it can't have a lot of feedback to correct non-linearities. So we see a lot of distortion. ...

Ah, that's interesting. Normally, we expect low gain to have low distortion ("the more gain, the more pain"). But that's based on the assumption that low overall gain implies high levels of feedback. Like an opamp having infinite open loop gain so the closed loop gain is the ratio of the gain/feedback resistors, so low closed loop gain is achieved with high levels of negative feedback.
But that's not the case with this amp.

Since class A or AB solid state power amps have been around for decades, my DIY expectations would be much higher. I'd expect to see kits that are simple to build having exceptionally good sound quality. Not in this case, but maybe others?

 

[amirm]

Ah, that's interesting. Normally, we expect low gain to have low distortion ("the more gain, the more pain").

I was talking about how much gain there was to use for feedback. Not gain for the sake of amplification.

 

[Sal1950]

I do get the attraction of DIY, I am a model railway enthusiast and build models of trains from brass and white metal kits (and when I was at sea used to make models from scratch when I could spend time off duty in the workshops using the lathe, milling machine etc). For me the hobby of model making is about the pleasure of constructing something more than owning the end result and I understand why some take pleasure from building their own audio equipment. Even if performance isn't great it is about the journey of making something.

I would love to see some photos of your handywork. !!!

 

[peng]

I showed the performance as measured by Nelson in one of my graphs. If anything, this sample performs better than whatever Nelson measured.

He is showing 0.7% THD+N whereas mine (in Blue) show 0.3% THD+N.

In that regard, you can throw out all of my measurements and go by Nelson's and see that the amp performs very poorly. That data has been there for a long time. It didn't get attention because it was not put in context as I did.

I definitely did not hear distortions, but I listened to very moderate level from 8 ft and the output would typically cruise at around below 0.1 to 0.2 W average. At that level, the THD is likely much lower than o.3 W. Found the curve, looks like it would have been 0.08 to 0.13 % for my use.

By the way, the 0.7% spec appeared to be for the original 19 V 4.74 A power supply and you confirmed that you tested it with the 24 V 5 A power supply so I think your 0.3% at 1 W into 4 ohms is quite consistent with the graph on the DIY store site that shows about 0.27% into 8 ohms.

 

[MRC01]

I was talking about how much gain there was to use for feedback. Not gain for the sake of amplification.

Exactly. If I'm using the terms properly, the former is open-loop gain (before neg feedback applied), the latter is closed-loop gain (total gain after neg feedback).

 

[pkane]

What's wrong with this amp? Harmonic content just like a hybrid tube preamp I measured a while back, maybe even a little better, if you like that kind of thing

Pass ACA amp:

Presonus hybrid preamp:

 

[Sal1950]

People just seem to want an excuse to do mob lynching in this forum (and most reviews are red meat to this). Demands for civility and higher standards apparently only applies to people you don’t agree with in the forums and certainly not to ad hominems directed at people outside the forum.

I'm sorry but where do you see examples of "mob lynching"? Yes the design and it's results have been criticized but as I see it they are mostly deserved. I understand the points of the design being simply a assembly exersise but honestly, even then I don't get it. Wouldn't it have been possible to design something like this that would deliver reasonable performance at low power levels? As has been mentioned here the name PASS is pasted right on the front panel of a $300+ product, if it was me I would have written some bit of disclaimer.
I'm certainly not a Pass hater, I've purchased a bunch of Adcom amplifiers of his design over the years and currently have 5 of them in my multich system.
I find your post to be the attack on some of our members that you accuse them of doing to Nelson.
What would you have folks say after Amir published the bench results, Good Show Ole Man?

 

[audimus]

I'm sorry but where do you see examples of "mob lynching"? Yes the design and it's results have been criticized but as I see it they are mostly deserved. I understand the points of the design being simply a assembly exersise but honestly, even then I don't get it. Wouldn't it have been possible to design something like this that would deliver reasonable performance at low power levels? As has been mentioned here the name PASS is pasted right on the front panel of a $300+ product, if it was me I would have written some bit of disclaimer.
I'm certainly not a Pass hater, I've purchased a bunch of Adcom amplifiers of his design over the years and currently have 5 of them in my multich system.
I find your post to be the attack on some of our members that you accuse them of doing to Nelson.
What would you have folks say after Amir published the bench results, Good Show Ole Man?

I have been told the L word is taboo and racist. Nevertheless, the L mob rarely ever identify themselves as L mob or see what they are doing as L no?

Want to see examples of a L mob that go overboard? This very thread, the PS Audio thread, the NAD thread.

The problem isn’t criticism but rather allowing personal attacks or snarky comments on people outside that would never be allowed here, criticism devoid of context of a device like in this thread by setting up artificial goals for that type of device just to vent at it (this is like the stereotypical people at deal forums with the clichèd “no hdmi kills the deal” posts), automatic condemnation of a corporation as greedy or unethical or whatever with a confirmation bias of the company or the type of device, holding others to standards that one doesn’t hold itself, these are all symptoms of a group think mob because they mostly go unchallenged by the people who have the most influence to do so.

To me they are all indications of a form of tribalism and a form of group catharsism. The danger is that it invites more similar minded people to join feeding on itself and driving out people that don’t.

And pointing out this behavior is not equivalent to indulging in the behavior, it is to prevent the the activity which would make then make the former unnecessary.

 

[LTig]

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?

Bias setting could be wrong - but the results are close to the specs. More negative feedback would help to damp the high THD. It may be possible to put the amp into a feedback loop using a matching opamp.

FWIW: I just measured the line stage of my DIY preamp which also uses a MOSFET (influenced by NP) in the output but within the feedback loop of an opamp. Looks much better - but it's not a power amp, of course.

 

[captain paranoia]

The performance of the amp is deteremined by said PCB and parts which he has sanctioned and helped offer for sale

Okay, but sold to what end?

A functional, quality amplifier?

Or an educational exercise?

Having now read the original 2012 article, it is pretty clear to me that it was intended as an educational exercise. Unfortunately, it also makes claims about making a nice-sounding amp, and how DIY projects can surpass the performance of store-bought products (the opening paragraph, although it doesn't expressly refer to this design). Since he estimated the cost at about $15, I suppose all things are relative...

 

[amirm]

Okay, but sold to what end?

To this end from their website:

I suspect a lot more people buying this amp for its purported "sound" then any kind of need to get into electronics hobby.

Hopefully my measurements here bring an ounce of realism to counter the above mystical claims.

 

[Sal1950]

I have been told the L word is taboo and racist.

Just another example of the BS "politically correct" world we live in. Change the meaning of a word to fit a political agenda.
Lynching of a man is imposing a hanging death sentence on him without a fair trial, whoever that person may be, race doesn't enter into it.
What about cutting off his head, or shooting him in a firing squad, can't say those words either. BLAH
It's like the kids playing ball at school today but you can't keep score any more, sends them a bad message, BS. In the real world they'll grow into, success will come from the effort they put into it and rewards in the workplace gained from the measured success.
The only thing I've heard in this thread somewhat off topic is the making fun of his hair but seriously, can't you take a joke?
The criticism made against the performance of the ACA amp is well deserved and nothing like a ASR lynch mob.
Let's chill out a bit

 

[restorer-john]

I remember building my first amplifier.

It was around 0.5W output, all discrete TO92s and a few TO126s. Used my mum's baking paper to trace the art from an ETI (electronics today international) magazine with a 2B pencil and then rubbed it onto a scrap piece of blank copper PCB. The Dalo (etch resistant) pen and Ferric Chloride came next and considerable time was spent with a tiny hand-drill and a 0.8mm broken off drill bit.

I ran it off my model train transformer (higher voltage!) and into a full range car speakers I'd pulled out of car doors/parcel shelves at the wreckers. There was a lot of hum until I discovered giant capacitors, and consequently spot-welding. I used to run it hard until the transistors got too hot to touch. Those little Philips transistors took some abuse, let me tell you.

Pressed down into the shag-pile (it was the 70s!) carpet with the speaker magnet (no baffle- just free air) firmly attached to my desk's metal leg and playing radio Top 40 songs, I thought it was almost as good as my Dad's HiFi. I experimented with various desk legs, boxes and pillows to make it sound partially decent. Then came another channel, stereo FM, more speakers and cassette decks, turntables, you name it. I was hooked. Soon, in my early teens, I was repairing HiFi at a local pawnbroker and being paid actual money (which I spent on audio, test gear and components).

The rest is history for me, but it was that little discrete, hand built amplifier that pretty much started my journey. Prior to that it was egg-timers, alarms, and moisture meters. The ACA should be such an amplifier, but sadly, instead of 8 or 9 year old wide-eyed kids building them with their pocket money as a gateway into electronics, it's 40+ year olds desperately wanting and foolishly believing it to be something it was never designed to be. And paying absolute top dollar for the privilege.

The measurements serve to shine a spotlight on the folly of spending several hundred dollars on what is essentially a one project version of this:

PS I'd outgrown electronic kits by the time this one came to market in 1978/9, but I'd always secretly wanted one as the "laboratory manual" it came with was really good and full of ideas. Only a few years ago, I found this mint condition set in a thrift store for a few dollars- virtually unused with some 1970s batteries (not leaked). I had to have it.

 

[gvl]

As has been mentioned here the name PASS is pasted right on the front panel of a $300+ product, if it was me I would have written some bit of disclaimer.

In the PASS pricing, this amp performs right on the money.