Pass ACA Class A Power Amplifier Review

[manisandher]

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

I want to listen to the amp driving speakers.

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

As already stated, the speakers are 97dB/W@1m.

 

[voodooless]

I want to listen to the amp driving speakers

Exactly. You do that by recording at the speaker terminals.

 

[manisandher]

Exactly. You do that by recording at the speaker terminals.

And what information would this provide over and above Amir's measurements?

 

[voodooless]

And what information would this provide over and above Amir's measurements?

It will confirm the load dependence that Amir didn’t test for. It will also provide some test tracks to ABX against for those interested. It would be interesting to remove the load dependencies and the redo the ABX. I bet it will be much harder with only the distortion. Doing all this by microphone wil just introduce too many variables into the recording.

 

[SIY]

And what information would this provide over and above Amir's measurements?

The speaker responds to the electrical signals at its terminals. No more, no less. So if the electrical signals (the amp driving the speakers) are captured as files, you can easily do a DBT. If you can hear a difference, great, if you can't, also great.

If it were me and I could hear a difference, I'd compensate for the source impedance by putting a series resistor between the competent amp and the speaker (the source impedance will change the frequency response) to simulate the Pass amp's source impedance, record and test again. If I can still hear a difference, it's not frequency response and the point is proved. If now I can't, it's just a frequency response deviation and that is trivially implemented (and switchable on and off!) in the digital domain.

 

[manisandher]

It will confirm the load dependence that Amir didn’t test for. It will also provide some test tracks to ABX against for those interested. It would be interesting to remove the load dependencies and the redo the ABX. I bet it will be much harder with only the distortion. Doing all this by microphone wil just introduce too many variables into the recording.

The speaker responds to the electrical signals at its terminals. No more, no less. So if the electrical signals (the amp driving the speakers) are captured as files, you can easily do a DBT. If you can hear a difference, great, if you can't, also great.

I hear what you're both saying. And sure, I can and will record at the speaker terminals.

But my real interest is in validating what Amir wrote:

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.

I'm interested in knowing how different mono ACAs sound to mono LA90s driving high-efficiency speakers, when the former are well-below their clipping point. What difference does a SINAD of 32 vs. 120 make when combined with the speaker's own distortion?

Just think it would be interesting...

 

[voodooless]

What difference does a SINAD of 32 vs. 120 make when combined with the speaker's own distortion?

The problem is: your not just testing for that. If you hear a difference, you still don’t know why.

 

[ta240]

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.

Is that 97 just claimed or actually verified efficiency?

 

[mhardy6647]

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

"Send me in, Coach!"


There actually is an ACA here -- sort of.
Imagine that?

It's a semi custom variant of the original morph (I believe) which was very kindly passed along to me by someone on the Polk forums.
Thus, unfortunately, I didn't build it, so I am a bit hazy on the exact details.

 

[tmtomh]

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. [...]

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. .

Enjoying the sound of gear you assembled yourself must be very satisfying - congrats, and enjoy!

 

[solderdude]

I hear what you're both saying. And sure, I can and will record at the speaker terminals.

But my real interest is in validating what Amir wrote:



I'm interested in knowing how different mono ACAs sound to mono LA90s driving high-efficiency speakers, when the former are well-below their clipping point. What difference does a SINAD of 32 vs. 120 make when combined with the speaker's own distortion?

Just think it would be interesting...

As @voodooless already mentioned you will hear a combination of distortion profile, high output resistance (DF 2 in 4ohm load), the damping properties of your speaker itself might also be a thing (not only voltage division aspect).
The distortion profile changes with volume and with high efficiency high impedance speakers at low listening levels it may just sound 'fine'.

But... you will not know what of the above described 'effects' of this 'start your own simple build' amp you are actually listening to and it might be a different sound than what others may hear depending on the used speaker and listening level.

Your test thus will let you hear the amp + interaction with your speaker.

MEASUREMENTS: Pass Amp Camp Amp (ACA) 1.1. The crossroads between objectivism and subjectivism, and reconsidering von Recklinghausen.

A blog for audiophiles about more objective topics. Measurements of audio gear. Reasonable, realistic, no snakeoil assessment of sound, and equipment.

archimago.blogspot.com

 

[manisandher]

Your test thus will let you hear the amp + interaction with your speaker.

Sure.

And it'll be interesting to hear how close they sound to the LA90 monos through the same speakers.

 

[solderdude]

Sure.

And it'll be interesting to hear how close they sound to the LA90 monos through the same speakers.

yep, no distortion, low output R and more power vs everything LA90 is not.

Wouldnt be surprised if you enjoyed the CA# over the LA a bit more.

# CA stands for Crappy Amp.

 

[manisandher]

Wouldnt be surprised if you enjoyed the CA# over the LA a bit more.

# CA stands for Crappy Amp.

Why do you think that?

 

[MAB]

Why do you think that?

Amps are hard to tell apart.
When there are audible differences, we tend to think different = better.
Even if there are no actual differences in performance, just the inkling in the customer's head that they heard something, the brain interprets better. I hope you understand this is the odd human response that is used to sell ineffective things like cables, tubes, op amps for rolling, etc.

The same trick causes us to assign different = better when there are small differences. This amp, of all tested amps, likely would be audibly different in a proper blind test. The frequency response alterations with low impedance speakers would be one way to spot this amp's differences. Plus the distortion is so high it will have the illusion of playing at higher volume. Most people don't like the sound of distortion once they can identify it specifically though. So, yeah, it will likely sound better.

 

[manisandher]

When there are audible differences, we tend to think different = better.

This is non-sensical.

There is no reference. So, if there is a difference, I could just as well think the LA90 is better.

 

[voodooless]

This is non-sensical.

There is no reference. So, if there is a difference, I could just as well think the LA90 is better.

Of course there is a reference. You already ingested all kinds of stories about these products. In your head they are subconscious swirling around and creating some kind of bias and expectation. And there is absolutely nothing you can do about this.

 

[manisandher]

Of course there is a reference. You already ingested all kinds of stories about these products. In your head they are subconscious swirling around and creating some kind of bias and expectation. And there is absolutely nothing you can do about this.

Huh?

What kinds of stories have I already ingested about these products?

 

[voodooless]

Huh?

What kinds of stories have I already ingested about these products?

That’s for you to know Anything counts: facts, opinions, impressions, videos, images, you felt them, maybe assembles them. It all counts.

 

[SIY]

Huh?

What kinds of stories have I already ingested about these products?

Then no problem, compare things using ears-only basic controls.