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-   -   Another cap comparison (http://www.diyaudio.com/forums/parts/100383-another-cap-comparison.html)

Conrad Hoffman 20th April 2007 03:45 AM

Another cap comparison
 
As Rod Serling used to say on The Twilight Zone, "Offered for your consideration"... are two capacitors. One, a 3 uF polypropylene, with 0.02% dielectric absorption, and a dissipation factor of about 0.0013. The other, a 3 uF polyester (Mylar), with 0.17% dielectric absorption, eight times worse, and a dissipation factor of 0.019, more than ten times worse.

To make the values exactly equal, and this is very important, a small polyester was paralleled with the large polyester. The above measurements included this.

One end of each cap was connected to a GR 1390B noise source, and the other end of each cap terminated to ground with a 1% metal film 20K resistor. The common lead of the caps was driven with 2 VRMS of random noise covering the entire audio band.

A high gain differential amplifier (Tek 1A7A) compared the signals across each 20K resistor. If there was any difference at all between the signals passed by these two caps, it would show up. The results? The residual common mode of the amplifier with both probes connected to the same cap was about 100uVRMS, plus a trace of 60hz pickup.

Measuring between the caps gave the same 100uVRMS signal, plus the same trace of 60hz pickup.

That means that any difference between the two caps due to DA and DF was less than 0.005%. Considering the CMRR of the amplifier, a lot less. It also means that any difference from unknown mechanisms is also less than that, because the test doesn't care where the difference comes from. IMO, this test is pretty representative of how coupling caps are used.

Now, maybe there's some condition of non-linear source or load impedance where a larger difference would show up (I gotta give people something to hold on to), but logic says if the signals are the same, the sound has to be the same. IMO, if less than 0.005% doesn't qualify as "the same", and you can hear it, caps are the least of your worries.

If you want to see larger differences, just mismatch the cap values a bit. The low frequency difference will skyrocket simply because the RC constant is slightly different. Very small mismatches shouldn't affect the sound, but it's a variable that has to be considered when caps appear to sound different. Another way to see larger differences is to test against an electrolytic or a ceramic!

Ok, did I listen to them? Not yet- too much junk on the test bench from running the test. But I will. The difference is that now I don't expect to hear any difference, unlike the frame of mind I'd be in if I had great expectations due to the DA and DF differences.

No doubt some will consider this test bogus and useless- my flameproof underwear is on, so give me your logic as to why. :devilr:

Pano 20th April 2007 08:44 AM

Cool test. I like the idea of the differential signal measurement. I proposed doing a similar test on speaker cabales in another thread.

If there are differences, your test should show them. And a random noise would seem a good source. How about trying it with music? That's what it would be used for anyhow, right? A musical signal would show differences too, if there are any.

The only "flaw" in the test I can think of is that since the caps were tied together on one end, any distortion would be transmitted from one cap to the other and also reflected back to the source. Possible? I don't know - maybe not, but it's worth thinking about.

Hartono 17th May 2007 05:28 PM

try testing into very low impedance load, near the voltage rating of the capacitor at the same time.

Hornlover 18th May 2007 04:42 PM

This illustrates my experiences over the years. I have been involved in a lot of blind listening tests involving caps, and what we found in every case was that polyester caps perform well for most audio applications. I know that there will be many that will flame me because of this, but this is what we have found. We assembled a group of experienced listeners, using a variety of systems (at different times, usually), and the results have always been the same. Most listeners can hear the difference between an electrolytic and a film cap, but once film caps are compared, no reliable identification could be made. We compared generic polyester and ploypropylene caps to the very expensive 'audiophile' caps, and no consistant identification could be made. For this reason, I feel that if you are using (even generic) film caps in your system, dont waste money on expensive varieties unless the rest of your system is already optimised. Put your money where it will really pay off, such as bi-amping.

Hartono 18th May 2007 05:56 PM

Hi Hornlover,

I wouldn't recall my experience to be the same (was selling "audiophile" caps for 3 years), caps do differ in their construction, and while I agree that trying to do A/B test to identify which one is which maybe a little hard,especially at different listening sessions on differing system.

But there is identifiable audible difference, switching capacitor into circuit shows this more clearly.

Many generic cap do perform well, but the more sophisticated construction/material caps also have their use in more critical application. (more expensive is not necessarily better constructed/material ) some generic one might do well, but the manufacturer wouldn't guarantee that, they can change the dielectric to same material but lower quality ones without warning. We can not expect the metalization for 5 USD caps and 50 Cents one to be of same quality.

The problem with the so-called "audiophile" caps is that the manufacturer who really make the good ones do not want to reveal what they did to improve the caps, and do not break in ( burn in is better term here ) the caps before shipping them, even worse is that they do not tell people how to use them and in what circuit.

So the user have no idea which one did which things better, many only to be confused after their expensive purchase of "some" famous brand that in fact no better than generic ones.

Yes, it would be good to spend money on Bi-amping , but for people who already have Bi-amp-ed system, or want to improve other part of their system would benefit from better quality caps.

Speaking of Bi-amping for example, using metalized polyester inside active crossover is a very bad idea. generic one tend to develop short/noisy that can't "self-heal" in low power/voltage circuit. film and foil type is best in this case.

Now I invite people to see the differences in caps, instead of listening :D , try changing your DVD player video coupling caps. Make sure the caps have been properly broken in, or it might lead to sub-optimal comparison. Yes the size of the caps in this application is big, yes it is expensive, yes the result is different.

If anybody have physically tiny metalized polyester/polypropylene in their amp zobel network, do yourself a favor and change them to more appropriate ones, this one is critical, and also that tiny resistor.

I'm not defending "audiophile" caps, I am defending "good quality caps", I only say that cap have differences, some is better quality and constructed than others, and must be used on the correct application. Vishay make caps and that's not "audiophile" brand, unfortunately that doesn't make their caps cheap :rolleyes:


Now I wouldn't comment on resistor differences :headshot: better ask knowledgeable RF and digital people about this :cheers:

john curl 18th May 2007 10:39 PM

Conrad, in all fairness, you don't know how to measure caps. We have already done, and you can too, IF you follow directions.

FastEddy 18th May 2007 10:50 PM

john curl: " ... We have already done, and you can too, IF you follow directions. ..."

... got links to your comparisons? Appreciate it ... :smash:

Conrad Hoffman 19th May 2007 12:17 AM

Yeah John, how 'bout expanding on that remarkably enlightening post. You might be surprised what I know about caps, or I might be surprised at how little, but unless you offer some direction, we'll never find out :cool:

john curl 19th May 2007 01:42 AM

Go to Walt Jung's website. Find CLASSIC articles, Find article about caps written by Walt and me (John Curl) in 1985 or so. Read article.

Conrad Hoffman 19th May 2007 03:12 AM

Well John, I'm a bit baffled. I read that article when it was first published, and have read it again. It's basically the test I'm doing. The only significant difference is that I haven't varied the impedance, though I did use various sources. There's a rule of testing that says "test it the way you use it", which is why I chose the values I did. I can certainly force a difference to be seen, but I could just as well do that by using a standard (traditional) 4-arm bridge, and looking at the residual on a scope instead of a tuned null detector once the bridge is supposedly "balanced". They never are, you know. I don't think there's any reasonable disagreement as to what's going on, just with my opinion that the differences are below any possible audible threshold if they're buried in the noise floor. Now, if the caps were used at a low impedance level, say a crossover network, the results might be different. I didn't test that, and have no opinion on the matter (though I avoid electrolytics in crossovers if at all feasible).

Regards,
Conrad


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