Audio Science Review = "The better the measurement, the better the sound" philosophy

 

 

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@crymeanaudioriver , there you go!

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I see it differently. Sure, we learned quite a bit since these articles were written. Yet, the time during which they were published marked presence of many amplifiers designers and manufacturers in North America. They knew what they were talking about.

It was a comment about how amplifier design and speaker design has progressed. To your point, we have learned a lot.

 

A typical “moving the goalposts” / “straw-man” rhetorical manipulation. That discussion of IM correlation with THD is still conducted in the context of the amp’s linearity under best conditions, not in the context of the linearity of system comprised of amp+speaker.

Pointing out that this whole discussion (not you specifically) seems to be about picking your facts, or what you think they are. I spent more time learning about the math today. One of the joys of being somewhat retired. I don't have the engineering chops, but I took more than a few nasty math courses on the way to my PhD. By testing down to 2 ohms, where only the rare speaker reaches, there is significant exploration of the vast majority of speakers impact on stability.

 

Current limiting occurs in amps without specific current limiting circuits too. As an example, a heating up power transformer coil may in effect serve as a current limiter. Another example is insufficiently sized capacitive power bank.

The article, nor I differentiated what was current limiting, however, I believe the two examples you gave are not. This comes back to the math above.  Some EE's could probably jump in on that.

 

Look at the curves of THD vs output power characteristics of amplifiers, and you’ll see that typically, there is a rise in THD (and by extension in IM) long before the amp clips. The degree of such deterioration is typically frequency-dependent too.

 

Even I know that is about how the amplifier is designed and feedback. The feedback goes down as the frequency goes up. Going back to the math I learned today, as the feedback goes down, the stability will improve.

 

This is indeed one of the mechanisms explaining the phenomenon of some of the amps distorting significantly more while they are connected to a speaker compared to when they are connected to a dummy resistive load.

 

I think you made that up. That does not make sense.

 

Yes, occurrences when a commercially sold amplifier becomes unstable and turns into a generator while connected to a specific speaker are rare. Even though, the thread referred below has a description of a surprisingly common-case instance of that.

If you mean tube amps, I noted that, and that ASR rarely tests them.

 

However, just like with the discussion of THD and IM, we need to take into account that the amp-speaker system can “ring” for some time, instead of turning into a self-supporting generator. Some of the replies in the thread below describe precisely such occurrences.

Which brings us back to the 99.9% of the time it does not happen. "Ring for some time"? You mean unstable. Again, even I know that. Perhaps you should not be the person trying to lecture me on this.

 

However, some of the replies highlight the fact that in some other  market segments, including that of affluent audiophiles, larger speakers employing exotic transducers and much more sophisticated crossovers are more prevalent, and thus the events of ringing and self-generation are much more probable.

 

This is conjecture on your part. Fortunately, we can test this theory as Stereophile tests a lot of expensive speakers. Wilson Alexx5? No issue. Sabrina? No problem. Sasha, Alexia? No problem. Magico, 4 models, worst was 3 ohm, -60 degrees, not extreme by PowerCube article. B&W 801 - no issues. Big Magnepan? Child's play.  Soundlab?  Normally fine, but you can make the Brightness control nasty. Infinity IRS, etc. not as bad as many make out to be. Saw a note that tube amplifiers were considered in their design.

I will stick with my 99.99% and that seems to extend well into audiophile speakers. The corollary is no amp vendor provides any detailed measurements and you are hoping you detect this fault in a listening test.

 

There is no such consensus in that discussion. Interested readers can go there and see for themselves. I would roughly split the multitude of members posted there onto three categories:

1. Designers and restorers of amps from Western countries. They are for comprehensive testing with non-purely-resistive loads.

Restorer John is the only "restorer" and he does not come across as technical as others. More a tinkerer.  As the conversation progressed as noted, the consensus (always detractors) was low resistance for sufficient for goals.

 

1. Designers of amps from China. They are for limited testing with non-purely-resistive loads.

All 1 of them?

 

1. Vendors selling amps made from pre-built blocks, Amir, and some of Amir’s followers. They maintain that testing on purely-resistive load is not ideal, yet good enough for predicting amp’s performance in 99.99% of cases.

By far the most popular being Bruno Putzey designs and B&O. Nice thing given these are modules, they only need to be tested once to cover all units that will use them. 

 

Behavior of most amplifiers, including tube ones, does depend on the value of purely-resistive load, yet the change in behavior is much more predictable with the change in just the resistance value.

That is not a logical sentence.

 

Thus, while testing on 2 ohm has its merit, it appears from the discussion that testing on non-purely-resistive loads is of more interest to people with practical experience in designing and repairing amps.

 

I went back and saw that only one designer felt very strongly about it and he designed car audio amplifiers. That is not surprising given what car audio people will connect.  Going back to the math I learned today, if the designer knows the transfer function, they can estimate with high probability if the amp will be unstable.  You may want to read this:

https://d1.amobbs.com/bbs_upload782111/files_28/ourdev_548669.pdf

I had to read it 3 times, but this is very interesting too.
https://linearaudio.net/sites/linearaudio.net/files/volume1bp.pdf     That pokes holes in all the so called arguments about feedback.

 

Unless a member is an unscrupulous dealer pushing some version of snake oil, most of the “banter” deserves consideration, in my opinion. I give full credit to Amir for filtering out vast majority of such snake oil salesmen. However, the story doesn’t end there.

No it does not, no more than the banter on "medicine" by people with limited knowledge of what they are talking about. It is akin to people here talking about feedback, even a 100 of them, compared to Bruno and his article.

 

Similarly to doctors giving zero credence to patients describing their symptoms, and only relying on the results of locally available objective tests, amp designers and restorers only relying on simple measurements aren’t likely to keep their clientele for long.

Good marketing often wins over good product, and audio is no different. The first thing doctors often, if not normally do after hearing about the symptoms is to run tests. However, unlike audio, where only the external presentation matters, the patient describing symptoms are often the only measurements we may have of what is happening internally.  The issue presents not when the patient describes their symptoms, but when the patient tries to force their diagnosis on the doctor, even when the tests indicate otherwise. The test is almost always correct. The listening part criticality is to ensure there are not multiple issues at play. The communication part is letting the patient think they had influence when they had none.

 

Ignoring your not so subtle attempt of slighting me, a person whom you obviously don’t know much about.

Once again, interested readers can go there and see for themselves. They’ll find the traces of reductions exactly as I described them.

I will use this definition of redacted as I think it is appropriate:

to edit (text) so as to remove or hide confidential or sensitive information:

There is no evidence of this. When someone quotes another poster, it is normal, as one would also do here, to cut out what the replyer considers extraneous content w.r.t. their reply. That is not redacting, that is editing.  I have not seen nor experienced ASR "redact" anything, though they will suggest less harsh language.

 

Indeed, ASR runs on a more modern discussion platform than Audiogon. The “reply system” in practice also includes so-called moderation subsystem, or, in simpler terms, censorship features.

All forums, with few exceptions, have moderation, including this one. If you label ASR censorship, then you have to label all others.

 

Yet a discussion site with overreaching moderation generates its share of issues, both for regular members and site owners. I maintain that the ASR moderation has been such since about 2021.

I only passively use ASR these days, so I will not comment on this. That was not my experience as of 2 years ago when I was still active, though I did find it at times toxic. I think alternate means, i.e. a sand-box for people who don't fit in would be more appropriate. However, I go back to my comment, that many of the commenters here would effectively be flat earth followers on a science site given what they write. Hence, I am not surprised by how ASR treated them and I don't think it is unwarranted.

 

Nope, topic at hand is relevance of the testing Amir does on specifically power amplifiers to the subjective perception of audible distortions contributed by amp A vs amp B when connected to a specific speaker.

Except we come back to 99.9% or more of amplifier / speaker combinations will not have stability problems, ASR does not test tube amplifiers often, and based on my research, however, limited, that even audiophile speakers do not commonly have extreme characteristics, then there will be no change, at the amplifier level, with almost all speakers.

 

My position, as is the position of majority of ASR members with practical experience in designing and repairing power amplifiers who cared to express their opinions, is that the testing Amir has been conducting is marketed as more definitive than it shall be based on scientific understanding of the limited nature of the tests.

I will only state that you have no provided any concrete examples of where this is the case, not even strong potential examples, though I have accepted tube amplifiers could be most at risk here.

 

OK, let me give you another analogy. Imagine if we assigned championship titles in boxing based on tests involving a boxer and a punching bag.

The geometry of the bag and the rope it is suspended on would now influence the system dynamics stronger. An audiophile analogy would be thermally induced deviations of the load resistance value, and parasitic capacitance of the cable leading to the resistive load.

Thermally induced variations in the load have no impact on the amplifier performance as you admitted it is effectively time invariant. Cable capacitance is very low, and with the rare exception, far in the past (was that Naim) has not been a real issue. One could argue there is no reason to have that low of capacitance.

 

The goal of the boxer remains the same: the center of the mass of the bag he is punching must exhibit a specific pattern of acceleration. Only now the bag is also pushed and pulled by other swinging bags via the springs and the ropes threaded through friction pads and pulleys. This is analogous to how an amp must work when attached to a practical speaker.

 

You are pushing me out of my comfort zone, but I will respond with what I know, what I read, and my newfound knowledge of the math of stability and feedback. Looks like those math courses were not single minded!  I read in one technical article that the electrical simulation models using resistors, inductors, and capacitors are both realistic and valid models of real speakers including the movement of the cones.  As these are all linear elements, at least for the purposes of our discussion, then they can be simplified to magnitude and phase.  Hence we are right back to our stability discussion and 99.9% it does not matter. Audiophiles may be interested. It does not mean their interest is relevant.

 

Indeed, thermal drift of a transducer coil resistance value due, to ,say, a loud music passage, is a factor that a good amplifier must somehow compensate for. Yet even if we remove the friction pads in the system of bags described above, its behavior will remain pretty sophisticated, and very different from a behavior of a single heavy punching bag.

This is not the purpose of the amplifier. How would it know it was the transducer coil, and not some other element.  This would be the job of the speaker designer to compensate for.

 

One of the cases is simply running our of amp’s power supply current capacity. A music passage may be such that at some point all the bags will be moving towards the boxer, overwhelming him with the combined impulse.

Testing power output at different frequencies and 8,4,2 ohms would cover your argument. Again, even I know what. Using a reactive load that is not the same as your speaker is not going to provide easy guidance.

 

This type of deficient behavior may be exhibited on some music passages by certain class A, A/B, and especially class D amplifiers, with their open loop bandwidth insufficient to deal with such combination of the speaker and music passage.

I feel this statement is made up. I don't think it based in theory or reality.

 

Audibly, such deficiency may manifest itself as a lack of transparency, and timing errors, especially in music produced by dozens of instruments playing at once.

But we go back to testing at 2,4,8 ohms, which is sufficient for 99.9% or more of speakers will provide all information needed on where the amplifier will clip, and testing with a reactive load will provide no additional information. How would you even related it to your speaker that you intend to use?

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This is just me, learning as I am going, finding it easy to counter your arguments, some of which I have a high confidence are flawed.

 

Audio Science Review. Haha, I love that name. It's funny.

 

@crymeanaudioriver

You have neither proven the uselessness of ASR testing, nor have you provided alternatives

Sir you have been here rambling on for awhile now.

There has been NO ONE who says that measurements, ASR measurements or any measurements are useless!

Most seem to be like my view. They aid in the evaluation of equipment, but I do not rely on them alone.

What has been the issue from the 1st page is the culture of ASR!

And that to that, you keep reiterating their point again and again..ad nauseam.

For a man who flaunts his education, I would think you’d get it by now.

 

Amir don’t need a fanboy, fighting his battles.

@henry99 , if you didn't have only 9 posts, all in this topic, all effectively band-handed attacks on ASR, then I may take your post seriously. However, as you do have only 9 posts, all in this topic, I see no need to take you seriously. That would create an account, solely for the purpose of attacking ASR is definitely a point of interest. Education is important if you want to understand certain topics.