Pass ACA Class A Power Amplifier Review

[amadeuswus]

Now if someone would be brave enough to send a 'First Watt' amp to Amir for testing we can seen what Pass really has up his sleeve.

For a while I was tempted to get a used First Watt F3, which (going by the manufacturer's measurements) does surprisingly well at low levels. Probably cleaner than the SIT-3 that Stereophile tested.

http://www.firstwatt.com/f3.html

 

[solderdude]

I hope you are here to tell us you didn't do that in the end.

Why would one want to buy an amplifier that performs relatively better at lower output powers than at higher output powers when distortion becomes more audible (peeps out above audible thresholds) at higher listening SPL ?

Arguably any design with much lower distortion at all levels would be technically better.

 

[MediumRare]

So to me THD < 0.01% is 'transparent enough' which doesn't mean that technically a device with distortion products below -120dB is not better.

So, in principle, I agree. BUT (as I know you know): 0.01% is equal to -80 dB. The best this amp does with 0.1 W into 8 ohms is -61dB (0.09%), nearly an order of magnitude worse than your standard. At 1 W it's -50dB (0.31%).

Yes, it makes sounds resembling the source. It will even be enjoyable compared to my bedside clock radio. But I don't believe we can use the term "transparent" in any sense.

 

[amadeuswus]

I hope you are here to tell us you didn't do that in the end.

Why would one want to buy an amplifier that performs relatively better at lower output powers than at higher output powers when distortion becomes more audible (peeps out above audible thresholds) at higher listening SPL ?

Arguably any design with much lower distortion at all levels would be technically better.

Hi Solderdude
I fought off the urge. It made me a better person (or at least less impoverished). My next self-improvement project is resisting buying a used Quicksilver integrated amp or Quicksilver Horn Monos (the latter measure pretty well, according to Stereophile). Fortunately these don't come up often!

 

[solderdude]

I

So, in principle, I agree. BUT (as I know you know): 0.01% is equal to -80 dB. The best this amp does with 0.1 W into 8 ohms is -61dB (0.09%), nearly an order of magnitude worse than your standard. At 1 W it's -50dB (0.31%).

Yes, it makes sounds resembling the source. It will even be enjoyable compared to my bedside clock radio. But I don't believe we can use the term "transparent" in any sense.

I will let you in on a secret.
I can't hear anything below -70dB (0.03%) myself even when playing very loud as the dynamic range of my hearing tops out at 70dB.
Maty had measurements of an ACA that had 3x lower distortion than the one measured by Amir. Possibly the bias was not set properly or the used parts were 'less' in performance / quality.
At low levels (50mW) the distortion was 0.02% so below my audible thresholds.
The discussion was that when it was used below 0.1W on very high efficiency speakers (horns etc) that it performed audibly well enough.

That said... I find the design technically very poor and such poor performance is not needed for any amp.

When one were to use this particular amplifier as one would normally use this with 'normal to somewhat higher sensitivity' speakers this amplifier certainly won't be audibly transparent.

 

[Sal1950]

I still have one of those. Haven't tried it in some time, but last time I did (2010'ish) it worked still.

You see the prices those are getting on ebay!

 

[Blumlein 88]

You see the prices those are getting on ebay!

No. Maybe I should look.

I was surprised to see some eye watering prices on some CD's I own.

Okay, good thing I looked. I need to retrieve it from storage and clean it up. I think I still have one of the HF20's, but it needed some caps or something as I recall. Ditto for the mono tuner.

I also have one of the Stromberg-Carlson ASR 433's. Seems it worked, but had some weak 7199's you couldn't get anymore. But I believe the Chinese started making those again. Looks like I need to pull it out too. Woops, maybe they stopped the 7199 again. I forget there were one or two tube types that were the same, but different pin-out. Guess I'll have to look those up.

 

[ta240]

Pass is fully aware of this and give his permission. The fact that he sells amps that are close in distortion numbers tells me he has a different approach to what is good enough and or 'good sounding' than that of most ASR members. Pass states specifications and has explained his builds often.

Those wanting perfect measurements simply should look elsewhere. The facts about this amp are known.

Now if someone would be brave enough to send a 'First Watt' amp to Amir for testing we can seen what Pass really has up his sleeve.

I seem to remember reading or seeing in a video somewhere that Nelson often listens to one at his bench. If he enjoys it then is there anything disingenuous about allowing/backing the sale? And many of the people that build them really enjoy them. To each their own. They aren't pushing the amp as being low distortion at full power so why should I be angry with them for offering it for sale? Why would it possibly annoy me that people buy it and enjoy it or even end up putting it in a closet? As I said before many people that build it go on to building much more complex amps with what they learned from it. Meanwhile people buy their third headphone amp that has a tiny fraction of a percent less distortion than their prior two and we go "yeah! that is awesome enjoy that!" ??

I have a friend that insists on trying to make his car with a tiny engine 'fast' rather than just buying a car with a bigger engine. I don't get it but if he enjoys it then why not? Same thing with finding super efficient speakers to go with low power amps for low volume listening; how does it make my life worse if someone enjoys that? If they want to see what they can pull out of 1 watt or even half a watt then they should enjoy their hobby.

It is interesting to me how the world has gone from "I don't like that" to "Why would anyone like that" and finally to "That is just stupid to like that" about everything.

Why bother testing amps that are openly stated as being designed for people that like a particular sound rather than specs? It is a bit like having a movie review site that is all about comedies and then reviewing dramas and saying "there just weren't enough laughs". Sure, everyone can circle around and get all excited about how un-funny the movie was but what was anyone expecting? It just seems like an absurd crusade.

 

[solderdude]

Yep ... lets call it human behavior.

Regardless of human behavior... this amp in particular doesn't measure well but receives positive reviews.
I see no reason NOT to review it and hold it against other amplifiers in the same, higher or lower price class.
Non ASR readers may not be interested but are pointed to it.
ASR members do like to see the numbers.

It does not matter what they like or prefer nor the reasons.
The review is what it is... measurements of a often well reviewed amp that shows some similarities with other 'high-end' and 'audiophile' DACs and amps that also measure poorly.
Also some cheap and expensive gear has been measured that does perform admirably so it's not that it can't be done.

What this means is that either audiophiles enjoying those expensive and poorly performing devices don't even hear the poor performance
or those that are seeking the lowest numbers are delusional and don't need to be so anal below certain numbers.

That's all very fine but unless all these devices aren't measured properly we don't know and would have to rely on very flawed listening impressions of people that may or may not be actual experts.

Audio is madness with extremists at both end of the spectrum and lots of happy people in between that enjoy music through their own systems.
Those in-betweeners seem to be the happiest ones.

 

[amirm]

Why would it possibly annoy me that people buy it and enjoy it?

I used to eat at this Thai restaurant near work that I really liked. Then I read the health department report that showed it to be in top 3 worst restaurants in the area! It had every kind of sanitation failure from rodent to food that was kept in unsafe conditioned. I did not go back to that place anymore. Would you have continued to eat there?

 

[amirm]

Why bother testing amps that are openly stated as being designed for people that like a particular sound rather than specs?

It is a myth that these amps are designed to have a certain sound. They are either transparent or sound distorted. So that won't be a basis for me not testing something.

As to distortion, the information provided is rarely the same way I measure them so that we can properly compare. It is that clear comparison that has been enlightening to myself and membership at large.

This is a popular amplifier and deserved to be tested just like I test everything else so that people know how it stands relative to other choices.

 

[amirm]

Same thing with finding super efficient speakers to go with low power amps for low volume listening; how does it make my life worse if someone enjoys that?

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.

 

[mitchco]

To this end from their website:

View attachment 38198

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.

Yep, that is me (Mitchba) with the quote. I wish they had included that I use them on 113 dB @ 2.83v/meter horns (ooops I mean waveguides ;-) When I bought the kits in 2012 (13 can't remember), there was not too much to choose from and my main criteria was low noise into a highly efficient compression driver/waveguide combo. As Amir measured, they are very low noise. Putting my ear right up to the waveguide I can only hear the slightest hiss.

I am quite sure, even at really loud levels, I have never exceeded 1 watt or maybe even 1/2 watt (1/4 watt? didn't do the math). At reference level (83 dB SPL) at the LP some 9ft away, to my ears, they do sound good. I have some acoustic distortion measurements around here somewhere and I think if I remember, the distortion was below 0.5% with both the amp and compression driver across the CD's bandpass. I don't hear any distortion and I have tried a variety of amps with these horns, and subjectively, these sound best to my ears.

I knew well before hand what "Papas" designs are all about and so do most people in his "camp". He uses various distortion profiles to colour the sound. It is as simple as that. If you don't like, don't buy it. The first watt F5 is perhaps the only exception and noted for it's "dry and analytical" sound, which if you look at the distortion profile, means it is a (much) lower distortion amp ;-)

@amirm if you still have the amp, it would be interesting to pop the lid and adjust the bias pot to see where the null is for the lowest distortion. After the hour warm up, it would take about 10 minutes to find the null for the lowest level distortion...

Perhaps another way to look at this is to realise that our ears are not that sensitive to distortion and is frequency dependant (i.e. even in the double digits distortion at low frequencies from loudspeakers, most cannot tell). I would love to see some blind tests where folks can determine what level of distortion can really be heard. Until then, enjoy the music.

 

[amirm]

@amirm if you still have the amp, it would be interesting to pop the lid and adjust the bias pot to see where the null is for the lowest distortion. After the hour warm up, it would take about 10 minutes to find the null for the lowest level distortion...

I still have it. I will have to ask the owner to make sure he is OK with it as there is always some risk of damage.

 

[maty]

Yep, that is me (Mitchba) with the quote. I wish they had included that I use them on 113 dB @ 2.83v/meter horns (ooops I mean waveguides ;-)

It is evident that you did read the PDF and you noticed the THD vs Power graphs.

 

[audimus]

I used to eat at this Thai restaurant near work that I really liked. Then I read the health department report that showed it to be in top 3 worst restaurants in the area! It had every kind of sanitation failure from rodent to food that was kept in unsafe conditioned. I did not go back to that place anymore. Would you have continued to eat there?

Wholly wrong analogy. The above analogy would be the equivalent of saying that I was listening to an amp that I liked and later found out that unknown to me the amp was feeding power back into the source connectors with potential for damage to the connected components (there are actually some switching devices on Amazon that do this incorrectly). Would you have continued to use that amp?

A more justifiable analogy is I used to eat at a restaurant that I liked the taste of very much and I found out later that an analysis showed that this restaurant had the highest non-organic ingredients of any restaurant in the area. Would you still continue to eat there? Depends. Are non-organic ingredients of concern to me? While comparably high, as long as there is no provable detriment relative to rest of my diet and the frequency with which I eat there, am I ok with it or should I change because some ideological group believes I should only ever eat organic food even if I don’t like restaurants that are wholly organic? Would I switch to another restaurant if it had as good tasting food that had less non-organic content but required a longer drive? How much less? Etc. This is the reality in the practical world. Not so black and white.

The above applies to all reviews on this site. And often the reactions are overblown independent of the context and purpose.

It is not the measurements that are at fault (that is where science is). It is the interpretation of the measurements that are subject to human weaknesses - biases, prejudices, ideological affinity, etc. The latter is not an objective activity as much as people want to believe it is by the self-serving label of objectivists.

 

[restorer-john]

If nothing, this review has been entertaining. Found a listing for another Pass amplifier board on eBay with this in the description:

Made me smile.

 

[mitchco]

I still have it. I will have to ask the owner to make sure he is OK with it as there is always some risk of damage.

Thanks! I hope you get the go-ahead.

Not to make it more arduous, but it would be good to know the bias voltage that goes with the lowest distortion (ha ha) profile. See @RayDunzl post: https://www.audiosciencereview.com/...ower-amplifier-review.9741/page-5#post-261995

Heads up the bias pot is finicky, a small movement makes a big voltage change.

 

[m8o]

Is the distortion audible?

In that abundance at those levels? Absolutely.

 

[Azookey]

In that abundance at those levels? Absolutely.

Serious questions, what does the distortion sound like?