Pass ACA Class A Power Amplifier Review

[BDWoody]

Back when I was looking for an amp kit to build I didn't look for a £20 kit to put in a £300 case. I bought a kit for a few hundred and put it in a nasty £20 case.

Thing is, putting it in a nice expensive case makes you think you're getting something like the big Pass amps which also have nice expensive cases and the same name on the front.

I got an old radio, thinking I would someday use it as a 'case' for some project...

Might not fit everywhere, but might be good for a retro look in the front and a party in the back kinda thing.

Cheap option...

 

[dtaylo1066]

I found this to be a mostly enjoyable and entertaining thread. But at this point, I think I have exhausted my commentary. I appreciate the many POV's. To all, enjoy the music you play. In the end, that is the reward.

 

[ryohnosuke]

So, my Lepy is a $25 piece of junk, but it actually has worked for some time - somewhat - and continues to function, unlike the two $105 Topping PA3 mini amplifiers that died on me.

How PA3 died? Both at the same time? I was thinking on buy one just because is cheap, hehe.

 

[tmtomh]

Which ice cream is the most pure to the ice cream recipe? Which has the best or most expensive ingredients? Which is the best value? Which tastes the best and is most preferred?

One might expect this particular coffee to taste like ... well... total shit, but someone thinks it is grand. https://www.npr.org/sections/thesalt/2012/09/20/161478954/heres-the-scoop-on-cat-poop-coffee

Keep the faith, keep up the testing. It is all appreciated.

With respect, I think your analogies are misleading.

It's not about what sounds good; it's about what sounds most like the original. So the question is not which ice cream is the most pure to the ice cream recipe - that analogy assumes there is no reference ice cream, only a recipe. With audio, there's always a reference, which is the original musical source. So it's about which ice cream, made from the recipe, will taste the most like the original ice cream that the recipe was created from.

IMHO this is the major problem: a confusion of pleasure with fidelity. If you prefer equipment that colors aka distorts the sound in some particular way, that's great - you should buy and use that equipment. But that is not even remotely the same thing as saying that this equipment has higher fidelity than other equipment; rather, it has lower fidelity. And that matters because fidelity rather than "pleasant distortion," is the only standard we can sensibly use to communicate quality and performance among different human beings with different perceptions and preferences.

 

[Xulonn]

How PA3 died? Both at the same time? I was thinking on buy one just because is cheap, hehe.

Although Topping has an excellent reputation for their DAC and headphone amp products, both of my PA3 amplifiers simply stopped playing. I don't have the special long torx (or whatever type of machine screw heads that they use) screwdrivers to open the case, and the case seems to be designed to discourage anyone from opening them. The first unit, (bought on sale on Amazon for $84) died after about 8-9 months of daily use, and the second one (bought from Amazon as used for $64), died after a couple of months.

Our mains voltage here in Western Panama runs typically from 125-128v, which is a bit on the high side of a nominal 120v, but I do not know if that is a factor. I use a UPS with AVR and surge protection, so our flakey power here with its frequent sags, surges and outages should not be an issue.

OTOH, for six-months so far, I have had a Topping DX7s DAC/HA, which is much larger and heavier, in my main system. However, Topping apparently does not offer heavier-duty amplifiers in a similar form factor as the DX7. (The $200 Topping TP-60, a Tripath based Class-D amplifier with an internal linear power supplty performed pretty miserably in Amir's testing, and was not recommended.) At this time, it looks like Topping's efforts towards improved technology and design have not yet been transferred to their amplifier products for passive loud speakers.

I am suspicious of the usefulness of Amazon reviews on the PA3, because after two years of being sold on Amazon, the three listings for that product show that only a total of 10 have been sold. Ratings are 5, 5, and 4.5 for the three listings. That cannot possibly be true, because I recall seeing more ratings in the past.

So I now have two Topping 24v power bricks without amplifiers, and the Fosi TPA3116 amp I was thinking of buying only comes with yet another 24v power brick. So I may try the below TPA3116 amp with tone controls ($42 from Amazon) for my Tuesday artisano mercado where I play international music at my table from a Cayin DAP. The no-name amp looks identical to the Fosi, but comes without a power brick. This would replace the Lepai amp with tone controls that has a very noise bass control. These components toys to me, and not for use in my main system.

I would really like to see Amir test one of these $42 amps as we search for decent el cheapo amplifiers. Interesting that it uses a socketed IC rather than a surface-mount type. (Although I assume that there could be a surface-mount IC under the heat sink.) The blank space on the PCB is used on otherwise identical amplifier boards that have Bluetooth capability, which I do not need or want. Boards like these seem to be commodities for Chinese audio "manufacturers" of mini, low-cost components. Simply buy the boards, buy some cases and connectors, drill, mount and label them, and boom - you have a "unique" product!

Same Board with Bluetooth:

 

[amirm]

I would really like to see Amir test one of these $42 amps as we search for decent el cheapo amplifiers.

Can you give me a link to Amazon for them?

 

[Xulonn]

Can you give me a link to Amazon for them?

At least the board looks well-designed, and the case looks good as well.

Amazon: Class D 250W AMP TPA3116 + LM1036 Digital Amplifier Adjustable Bass & Treble

 

[VeerK]

I had the opportunity to compare the ACA (bridged) to my Modulus-686 last summer at the Vancouver Island Audio Fest. Compared with a precise amp (the Modulus-686 provides 90 dB better THD than the ACA) ACA definitely compresses the sound stage and muddies up details. I know at least one participant in that Audio Fest who ended up getting one of my Modulus amps instead of the ACA.

Tom

I want to see the full fat, no compromises 686 here on ASR, too many DAC reviews recently

 

[amirm]

At least the board looks well-designed, and the case looks good as well.

Amazon: Class D 250W AMP TPA3116 + LM1036 Digital Amplifier Adjustable Bass & Treble

OK, I ordered it.

 

[RayDunzl]

OK, I ordered it.

I have a question:

The headline says 250W

(scroll down)

The specs say 2*50W

Answered:

https://www.ti.com/lit/ds/symlink/tpa3116d2.pdf

 

[amirm]

Yeh, I cracked up seeing that title too.

 

[RayDunzl]

Yeh, I cracked up seeing that title too.

I just accepted it as correct.

Figured it was a 250 watter (even if they were stretching it a little).

 

[cjfrbw]

I wonder what one of these would do: 280W@8 ohms class D (bridged??). Yamaha Pro, so will probably last forever:

https://www.ebay.com/itm/Yamaha-XMV...754328?hash=item2cec3987d8:g:rrYAAOSwqb5c20Dj

 

[tomchr]

How do they get bluetooth into the (presumably metal) chassis? Is one of the panels made of plastic?

Tom

 

[restorer-john]

How do they get bluetooth into the (presumably metal) chassis? Is one of the panels made of plastic?

It will work at close range.

 

[amirm]

How do they get bluetooth into the (presumably metal) chassis? Is one of the panels made of plastic?

Tom

The ones I have seen have external antennas.

 

[tomchr]

The ones I have seen have external antennas.

Looks rather internal to me.

It will work at close range.

Boy. I don't want to think about what the VSWR would be with the antenna inside a metal box.

Tom

 

[jan.didden]

This is a review and detailed measurements of the Pass ACA Class A stereo power amplifier. It is a DIY design by the famous designer, Nelson Pass, meant to be starter amplifier for people new to audio electronics to build. There are many kits available. The one loaned to me from a long time friend of the forum. The kit seems to cost around US $320 or so.

The ACA sample I received comes in an attractive black case:

View attachment 38081​

The design is a Mosfet output stage with a single stage buffer. Very modest level of feedback (9 dB?) is provided to keep distortion somewhat under control. As you will see, the gain is very low requiring full 2 volt of DACs to drive it to max power.

Here is the back side:

View attachment 38082​

I did not mess with it but I am assuming the single XLR allows bridged mono operation.

Power is provided by a very beefy external switching power supply which outputs 24 volts with up to 5 amps or 120 watts. All of that is put to "use" in generating copious amount of heat. I was told by the owner to let it warm up for a few minutes. As you will see in the measurements, this actually made the performance worse, not better.

Overall, this is a very nice package if it performs.

Amplifier Audio Measurements
As usual, we start with our dashboard with 5 watt output into 4 ohm. Normally this is well below the max power but not in this case as we see later:

View attachment 38083

Gosh... What happened to aiming for high fidelity? This is a distortion-factory if there ever was one. The notion that it is mostly second harmonic does not hold true either. Welcome to the bottom of our amplifier ratings:
View attachment 38084

Let's dig into distortion spectrum at lower power level of just 1 watt:
View attachment 38085

Second harmonic is more dominant than at 5 watt but it doesn't stay there that long. Here it is at 2 watts:

View attachment 38086

Our 32-tone signal resembling "music" paints an ugly picture as a result:

View attachment 38087

I was pleasantly surprised how quiet the unit is though:
View attachment 38088

Frequency response is also very well extended:
View attachment 38089

You may be able to make an AM radio out of this thing.

Back to ugliness, here is our power versus distortion+noise into 8 ohm load:
View attachment 38091

Oh gosh again.... Only 7 watts of power with SINAD of 38 dB?

Things don't get much better with 4 ohm load:

View attachment 38092

We now have distortion climbing into near 10% category at max power of just 7.4 watts/channel.

There was little variation of response with respect to frequency:
View attachment 38093

Distortion level is so high that it is masking secondary sources of it.

Making sure this unit matches the reference, I compared them to what Nelson Pass has published:
View attachment 38094

We are getting very good agreement. So this is what the unit is supposed to do. Nelson stays with 1 watt output which shows the performance to be better.

EDIT: I noted on the dashboard that the gain was much lower than the spec. I am told that version 1.6 which I tested now has a spec of 10 dB for gain so we are good.

EDIT 2: See this link for bridged mode measurements: https://www.audiosciencereview.com/...ass-a-power-amplifier-review.9741/post-262036

Listening Tests
Is this thing on? That is the expression that immediately came to mind after I hooked up the ACA amp to my Sonus Faber Concertino bookshelf speaker. Granted, the Concertino is spec'ed at 86 dB efficiency but come on, I could barely hear it. There was no bass naturally. But not much above that either. And this was with my Topping DX3 Pro at full volume.

For grins, I plugged the US $74 tiny little SMSL SA100 Amplifier which I recently reviewed into the setup. Now the speakers came alive, with lots of bass, great dynamic range, and detail. Yes, the amp started to cry when I turned it up much but until that point, it left the ACA so far in the dust it was not even funny.

Conclusions
As a starter hobby amplifier to get some sound coming out of a speaker, with relatively few components, the Pass ACA Class A Amp does the job. Build it, have fun with it and then put it aside and get a proper amplifier. Amplifiers need to have a lot more power. And much less distortion. We are not even close here folks.

Do I need to tell you that I can't recommend the Pass ACA for hi-fi listening? I hope not.

------------
As always, questions, comments, recommendations, etc. are welcome.

I suspect the above review is going to make me many enemies in DIY audio circles. So I need hire a bodyguard and get some serious insurance for my bodily parts from Loyd's of London. Both cost good amount of money so please donate generously using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/

Amir, just read the diyaudio ACA builders thread, someone apparently picked up on your test. Predictably, emotions run very high, ad hominen attacks popping up, factual arguments being absent. You carried your self, and how you sit in this, well. Not that it makes a difference with them, of course ;-)

 

[Wombat]

Looks rather internal to me.
View attachment 38575


Boy. I don't want to think about what the VSWR would be with the antenna inside a metal box.

Tom

I read a report that one of those unfortunate Vietnamese individuals recently found dead in a refrigerated shipping container ln the UK had 'phoned her parents to tell them she couldn't breath and would probably die. The call was received. Terrible.

I had a similar thought re Faraday cage.

 

[despoiler]

Oh? You mean other than the fact that it doesn't have enough power to power vast majority of speakers in the market? Did you read the review and my listening tests? This thing is like trying to read under candle light. It can be done, but there is no reason to because amplifier power is so easily to buy now. There is absolutely no bass whatsoever with many speakers. That, I call coloration to the extreme given how important bass is.

Listening level isn't only determined by speakers and amp though. The speaker placement in regards to the room and the listener factor in.

I actually have the ACA, but the newest 1.6 version. My wife bought it for my birthday because she wanted to give me an interesting project to do. The only reason she knew to do it was because I was researching DIY amps. I have a pair of Quad S-2 speakers. 87db efficient and our listening room is small. It's only 7.5 feet from speaker to listener. Speakers are 3.5ft from the front wall. You can see the max level works out to 91.8db. That hits the 90db threshold for loud sustained sessions if the amp is flat out. Normal listening is usually in the 70 to low 80 db for me.

http://myhometheater.homestead.com/splcalculator.html

Just out of curiosity amim how far is your listening position from your speakers? Where are your speakers in the room? I don't remember seeing any numbers to give context to some of your subjective review.

I have an Emotiva A-150 I compared the ACA against. The A-150 is a $299 (now $319) class AB 75w per channel stereo power amplifier. We are in the price ballpark. Again, my ACA is the latest 1.6 version with the 24v supply. I level matched both. The A-150 has a more substantial presentation. There is more weight behind everything so it sounds fuller. It has after all far more power and better damping factor. It's audibly obvious, but by no means a gulf. The A-150 has better controlled bass, but again they are more close than dissimilar. I would not say the ACA has no bass. Negatively, the A-150 has a veil that covers everything. Detail is being crushed. It makes the music sound dulled. The ACA sounds more clean and a bit more detailed comparatively. It's more obvious than the power factor differences, but still more close than dissimilar. So what about that ACA distortion from the measurements? Shouldn't it sound horrible? It's tube distortion in transistor form. The ACA has the audibly obvious holographic top end presentation that tubes are known for. It's clear what the amp's design has achieved.

Between these who wins? For me the ACA is staying and the Emotiva being sold to fund my next DIY amp project. Not that I could sell my birthday present. To recap. Emotiva is much louder, a bit fuller sounding, has a better controlled bass, but overall sounds a little dead because of the veil. The ACA sounds clearer and more detailed when compared. It puts the tube sound into play and everything sounds more more convincing in terms of reproduction as counter intuitive as that might seem. It sounds good. I know, I know. There are some of you are gnashing your teeth and clenching your fists that something that measures so abysmal can actually sound good, but the science shows a preference to certain harmonics and harmonic structures. That's what is in play with ACA. It's using psychoacoustics to overcome the measurement shortcomings. For an experiment and for brand recognition, it's very, very clever. Pass says somewhere in the project something to the effect "that you will never look at measurements the same again." I do think it was, ahem, ear opening to hear what he did. Unfortunately, for anyone that has more experience there are many amplifiers that don't have to rely so heavily, if at all, on psychoacoustics to deliver superior performance.

For what it's worth I'm going to be building Tom's Modulus-86 next.