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AES Paper Digest: Audio Capacitors. Myth or reality?

amirm

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#1
Thanks to Michael for bringing my attention to this paper. I had it in my library already but I did not remember it :).

Audio Capacitors. Myth or reality?
P. J. Duncan , P. S. Dodds , and N. Williams University of Salford, Greater Manchester, M5 4WT, UK. [email protected]
ICW Ltd, Wrexham, Wales, LL12 0PJ, UK. [email protected]
ICW Ltd, Wrexham, Wales, LL12 0PJ, UK. [email protected]


The paper hypothesizes the mechanical resonance of a film resistor which occurs in the audible band between 10 Khz and 30 Khz could have deleterious effect on audio when used in crossover circuits in a speaker:

ABSTRACT
This paper gives an account of work carried out to assess the effects of metallised film polypropylene crossover capacitors on key sonic attributes of reproduced sound. The capacitors under investigation were found to be mechanically resonant within the audio frequency band, and results obtained from subjective listening tests have shown this to have a measurable effect on audio delivery. The listening test methodology employed in this study evolved from initial ABX type tests with set program material to the final A/B tests where trained test subjects used program material that they were familiar with. The main findings were that capacitors used in crossover circuitry can exhibit mechanical resonance, and that maximizing the listenerís control over the listening situation and minimizing stress to the listener were necessary to obtain meaningful subjective test results.


Seems like a great study! Let's dig in to see if they deliver on that promise.

Let's focus on the listening part of the test. Here is the test setup:

upload_2016-3-20_15-48-26.png


Speakers in use are B&W 802. The woofer is directly driven but the tweeter goes through an AB switch of two different capacitors. They say they use the "bi-wire" connection of the B&W speakers which to me sounds like its crossover remained in the path. So guessing, the capacitor under test would be in series with that. This doesn't simulate what would happen if you changed a cap in the crossover. It simply adds another cap to the audio path which would likely increase the filter slope of the tweeter.

They start off with an ABX test. As you may know with ABX you can hear capacitor A versus capacitor B and then a random A or B is played ("X") and you have to identify it. Initially they do a comparison of an electrolytic capacitor against film (both 4.7 microfarads). They used this for "training" which means it should be easy for listeners to tell them apart. They then followed that by testing one ordinary film capacitor against a fancier one:

"The initial ABX tests involved a panel of fourteen volunteer listeners comprising students and staff in the University. The listeners generally found the tests to be very difficult and quite tiring, and in general, the results from the initial ABX tests gave no usable outcome.

One surprising feature of the tests was that during the initial training part of the tests where the electrolytic capacitor was compared with a film capacitor, and the difference was fairly audible, many of the listeners were unable to correctly identify A or B. In the second part of the tests, where the differences in audio delivery from the two film capacitors was much more subtle, the panel was unable to correctly identify A or B and no useful results were obtained.

Translating that into English, the "easy" case turned out not to be so. What was obvious to the people who created the test, was not to the people taking the test blind. Difficulty of telling the caps apart went through the roof when the two capacitors were of the same kind, i.e. purpose of this test. The testers failed to tell the capacitors apart in double blind ABX test.

They regroup and declare two things:

1. Maybe some of the testers don't have proper training. This is a valid criticism and you do want to use the training phase to weed out people who are lost and can't tell obvious differences. The trick however is to know for sure the training outcome was predictable. That is, we want to have proof that audible difference exists and based on that filter out testers.

As a way of example, if I want to test CD against high-res, one "control" for training would be to filter out the CD down to 12 Khz. If someone can't tell that from the CD, they sure as heck are not qualified to test CD against high-res.

The problem here is that we don't know that an electrolytic cap is indeed this bad or obvious to tell apart. The testers just assume so without any objective data or proof otherwise.

2. The experimenters blame ABX test itself faulty because the difference they could hear sighted, could not be replicated blind using other testers. Well duh! That is why we do ABX tests: to see if sighted results can be replicated when knowledge is removed. If ABX tests don't agree with sighted, the arrow does NOT automatically point to ABX tests being wrong. It as a minimum says that any audible difference must be very small. At worst it says audible difference does not exist.

Sadly organizers move on to a preference test of one capacitor against another. For their defense they site ITU BS-1116 which is a highly recommended set of standards to determine fidelity differences between samples. BS-1116 was designed for testing lossy audio codecs where we "know" sonic differences exist. That is, we don't need to first use ABX testing to know there is a difference. That is a given.

Using that logic is a mistake in this context. It is not given at all that the caps of any sort sound different. They cannot as such skip the ABX phase and go to preference.

The problem with doing that is that you get false data. I can test two cans of identical soda and ask people which one tastes better and one may come out ahead of the other. That alone means nothing. It is vagaries of human testers and too few trials.

The incredulity here for fancy caps versus standard is one where we don't believe there is any audible difference. That is the hypothesis that had to be proven or disproven. You can't assume an outcome of audible difference and then go and grade them as they did. Indeed their ABX testing showed that such reliable difference did NOT exist!

But they proceed with the AB preference test. Here, they use a reference capacitor and compare it to three others:

1. Elec is the electrolytic capacitor that is supposed to sound different.
2. Cap 1 is the same as reference so should sound identical.
3. Cap 2 is the fancy film capacitor which they hope sound better

Here are the results
upload_2016-3-20_16-11-2.png


Let's parse this as it is not easy.

Electrolytic capacitor was picked by 14 out of 14 testers/test to sound duller. And 13 out of 14 to have worse spatial information.

Cap 1 which is identical to its alternative in the AB test should have generated 14 out of 14 people saying it is the same in both categories of preference. As is commonly the case, it did NOT generate that result. In the detail front, only 8 people thought it sounded the same as its identical twin. Three (3) people said it sounded brighter, and the same number (3) thought it sounded duller. Identical results were generated in the Spatial category with just 8 people correctly identifying it as sounding the same.

You now see the problem I mentioned. Two identical caps being compared, yet people randomly give preference to one or the other.

The testers declare victory based on the outcome for the fancier film capacitor's results. There, six (6) people in both preference tests thought the caps sounded the same. Two less than the generic cap. One of those two people said the sound was duller. One said it was brighter. In other words the people who shifted their votes did not agree on what they heard!

The spatial results did better. Here two people shifted their votes (presumably) from being the same to being better when comparing cap 1 to cap 2.

What does this mean? Nothing! A statistical analysis must be made to see if the results of either outcome, cap 1 or cap 2 is due to chance or not. None were provided in the paper. This is like doing a math homework in high-school and not showing your work!

That slight shift in preference does not speak at all to validity of the outcome.

The last part of the paper which I won't go into confuses again the science of blind testing. It takes research that shows trained listeners are equiv to 7 non-trained people, and performs some vague computation to exaggerate the preference ratings for the fancy cap.

Conclusion
My personal conclusion is that they proved one and only one thing: that when blind tested properly using ABX listeners failed to hear a difference between generic and fancy film capacitor. Maybe a different test would be more revealing. But for now, that is the only valid part of the paper. Preference tests are not appropriate and defensible when not backed by statistical analysis that can demonstrate if listeners were randomly voting or not.
 
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dallasjustice

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#2
Thanks Amir. I got started on this subject because there are many "experts"pleading with me to replace the horrible caps and other crappy parts of my JBL 4367 crossovers.
http://www.whatsbestforum.com/showthread.php?18741-Jbl-4367&p=386929#post386929

This study is the only one I could find which seeks to find out the extent to which folks can really hear differences in caps.

The problem I have is that there's no measurements made of the various caps under test. We don't know if our test subjects are supposed to be identifying IM distortions or totally different frequency responses of the various caps.
 
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Mivera

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#3
If would help if they shared the caps that were used in the test. Also the audience of listeners matters as well. Some people can't tell the difference between the Mark Levinson No 53 amp and a $49 best electronics class D amp.
 

Mivera

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#4
Thanks Amir. I got started on this subject because there many "experts"pleading with me to replace the horrible caps and other crappy parts of my JBL 4367 crossovers.
http://www.whatsbestforum.com/showthread.php?18741-Jbl-4367&p=386929#post386929

This study is the only one I could find which seeks to find out the extent to which folks can really hear differences in caps.

The problem I have is that there's no measurements made of the various caps under test. We don't know if our test subjects are supposed to be identifying IM distortions or totally different frequency responses of the various caps.
One thing that is an option is just trying it for yourself. I find hands on the the best way to discover new things. There's a reason for the variety in this box. If the cheapest ones sounded the same I'll assure you I wouldn't be wasting my money on the better ones. It's strange that the naysayers are always the folks who refuse to try things.

IMG_4134.jpg
 

AJ Soundfield

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#5
4 posts and no one has blamed the switch box yet? Tsk, tsk.

I got started on this subject because there are many "experts"pleading with me to replace the horrible caps and other crappy parts of my JBL 4367 crossovers.
Now that's just silly. Everyone know resistors are by far the biggest culprit.
 

Mivera

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#6
4 posts and no one has blamed the switch box yet? Tsk, tsk.


Now that's just silly. Everyone know resistors are by far the biggest culprit.
The switch box is before the caps so I suppose it's equal for all anyways. The fader was probably a realistic from 1972 with a S/N ratio of 20dB :)

$_35.JPG



What do you use for caps, inductor and resistors in your speakers?
 
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AJ Soundfield

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#7

Mivera

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#8
You're saying the expensive ones are "better" than the cheap ones? Hmmm, you might be on to something.:)


How many ESR measurements for equivalence confirmation, have you done prior to your blind listening tests?

cheers,

AJ
It's not the price that makes them better. I prefer to test them with humans. When they decide to buy them after the audition, I consider the parts worthy. So far this method works for me as I don't have any old speakers in my inventory.
 

AJ Soundfield

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#9
The fader was probably a realistic from 1972 with a S/N ratio of 20dB :)

View attachment 610
Nice. Haven't seen one of those in a while.


What do you use for caps, inductor and resistors in your speakers?
None!...with the actives. Ok, maybe a blocking cap on the tweeter. Film of course no 'lytics.:)
Passives: Clarity, Jantzen and Mundorfs.

cheers,

AJ
 

tomelex

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#12
They have to measure differently in a way that is audible on the music being played. Believe me, I have swapped in and out enough caps for folks and depending on where the cap is in the circuit, even though they are quite different types, it is not easy to tell, I imagine in series with a tweeter, it would be harder than usual.

http://www.keith-snook.info/wireles...Proof for the golden ears hypothesis.pdf?.pdf
 

amirm

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#13
I don't think they were electrically different. Here is the text:

"The ESR value for all capacitors tested varied between 5 and 15 mΩ which in the context of a typical loudspeaker crossover and wiring is negligible and there were no observable electrical anomalies over the audio frequency range. To all intents and purposes, the capacitors that were tested were electrically identical, exhibited no electrical anomalies and showed no significant variation with aspect ratio, film thickness or cost per unit."
 

AJ Soundfield

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#14
Thank you for linking here Michael.

Perhaps the question is if the capacitors clearly measure differently, why was it hard for people to hear in this test?
Well, those aren't the caps in the AES listening test. Those don't look much like speaker (audio frequency) caps at all.
The test Amir linked looks more like an electrolytic vs film. Basically a worse/pathological case, as even many fairly inexpensive speakers use film caps in their highpass section, due to the small values.
Yet still no one in the test could reliably pick out the dreaded electrolytic.

cheers

AJ
 

tomelex

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#15
I don't think they were electrically different. Here is the text:

"The ESR value for all capacitors tested varied between 5 and 15 mΩ which in the context of a typical loudspeaker crossover and wiring is negligible and there were no observable electrical anomalies over the audio frequency range. To all intents and purposes, the capacitors that were tested were electrically identical, exhibited no electrical anomalies and showed no significant variation with aspect ratio, film thickness or cost per unit."
Right, but look at the link I posted just above, esr is not everthing, as usual, harmonic spray is the fingerprint.
 

amirm

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#16
The test is three-way, electrolytic versus two types of film. The latter were two kinds, one with mechanical resonances in audible band and the other with less or none of it.
 

tomelex

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Well, those aren't the caps in the AES listening test. Those don't look much like speaker (audio frequency) caps at all.
The test Amir linked looks more like an electrolytic vs film. Basically a worse/pathological case, as even many fairly inexpensive speakers use film caps in their highpass section, due to the small values.
Yet still no one in the test could reliably pick out the dreaded electrolytic.

cheers

AJ
See my link above, an electrolytic can sound good too.
 
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#18
Well, those aren't the caps in the AES listening test. Those don't look much like speaker (audio frequency) caps at all.

cheers

AJ

They are not all necessarily crossover capacitors, but all of the material types are common in audiophile gear. And most of them you can get in a crossover sized cap.
 

AJ Soundfield

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#19
They are not all necessarily crossover capacitors
Right, that's what I said. They have zero bearing on audio bandwidth loudspeaker caps as in this paper.

but all of the material types are common in audiophile gear. And most of them you can get in a crossover sized cap.
Once more, whether they can be found in "audiophile gear" is irrelevant. This paper deals with (real) audibility of loudspeaker audio bandwidth capacitors, not MHz performance and such. As one would expect from soundwaves>ear physical reality, no reliable audible differences were found.

Actually I want to thank Mike for that WBF link with all the "expert EE advice", that is one of the funniest threads I have read recently, you can't make that kind of stuff up, it's the real deal.:)

cheers,

AJ
 
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#20
AJ, audio gear doesn't play by your rules, that you're trying to simplify everything with.

Four of the capacitor materials tested are common in speaker crossovers. You didn't look close because all the bridge residual testing was done in the audio spectrum. Besides I proposed the question why you can't hear the differences, supposedly, based on the paper.

I'm not feeling any cheers, as you seem to be combative without looking at what I posted.
 
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