Non-Polar Electrolytic vs. Polypropylene Film Capacitors - Page 3 - diyAudio
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Old 22nd February 2008, 04:47 AM   #21
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This link seems a bit sluggish, but IMO is essential reading for anyone confused by the conventional explanation and description of capacitors. The related articles are valuable as well.
http://amasci.com/emotor/cap1.html
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Old 22nd February 2008, 03:51 PM   #22
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Quote:
When you give a value of <0.1ohm for instance, what do you mean? Is 0.1ohm the limit of measurement for that test condition?
Yes, measurements with a < sign are below the reliable limits of measurement for the test setup. I could probably improve the the quality of those measurements to get down past that resolution with a proper test fixture and more time to tweak the settings on the impedance analyzer.

This project started as a "lunch break" project. This is my daily time where I can borrow the lab tools for a few minutes to perform some personal tests. Given the time rush, it is difficult to optimize the settings properly, and I had to settle with "good enough for my purposes". Now that more interest has been expressed, I might revisit these tests with more capacitors and a proper test jig.


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The esr of capacitors isn't so critical because their reactance is so high at low frequency. For instance 3.3uF at 100Hz has a reactance of 482 ohms so an ohm of ESR doesn't really make much difference. Even at 10KHz, the reactance is 4.8ohms. Not to say it's not important, it needs to be taken in context and of course, it's the opposite for inductors, ESR is the essence of inductance.
This is a good point, and one to which I will have to give some thought. The whole motive behind these tests was to find out whether it would be preferable to replace some rather large 92.5uf value electrolytics with film capacitors in the NHT M3.3. These capacitors are in series with the mid-bass drivers, so it seemed intuitive to use the lowest ESR available. What I did not consider, as you have pointed out, is that the ESR is a fraction of the reactance at the frequencies of interest (100-300 Hz).

Impedance = Resistance + Reactance

Reactance of Capacitor = 1/(2*pi*frequency*capacitance)

Measured Impedance of 96.9uF Capacitor at 100Hz = 16.96 Ohms

Calculated Reactance of 96.9uF Capacitor at 100Hz = 16.42 Ohms

Equivalent Series Resistance = Impedance - Reactance

Equivalent Series Resistance of 96.6uF Capacitor at 100Hz = 0.54 Ohms

Impedance difference due to Equivalent Series Resistance = 100*(1-(16.42/16.96)) = 3.18%

Not as small of a difference as I had originally suspected, but still gives validity to your point.

I would like to simulate this non linear resistance in a crossover schematic, to see how much error the ESR of a capacitor could introduce in the frequency response of the filter.

Good day all,
David
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Old 22nd February 2008, 04:12 PM   #23
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David, have you downloaded the impedance converter I wrote and posted a while back in a similar thread?

converter
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Old 22nd February 2008, 04:25 PM   #24
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Conrad,

Hadn't seen that program before, thanks!

I also enjoyed the link you posted above, I think that is a much better mental model than the one my physics book used.

Regards,
David
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Old 23rd April 2011, 04:33 PM   #25
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Default Capacitor SnakeOil and Misinformation

Old post I know- but it makes a great point that well designed crossover takes in to account the type and characterstics of capacitors-such as these 'lowly" bipolar electrolytics.
Zobel networks are a classic case-I would leave stock as stock.
Amazes me- people laud about the sound-then when replacing them comment on how much better -for example poly's are.
Old electrolytic caps drift, and are a maintenance item- but I too am in the school of sticking with the stock types in a well designed speaker.
A lot of the so called superior sound in caps is hog wash IMHO-and more of a justification of spending $40 on a cap- when the stock $1.25 would have worked fine, and actually worked better with the stock crossover, because it was taken into account when the speaker and crossover were designed.
Of course if you rengineer the crossover to poly/film types-then your fine.

Quote:
Originally Posted by gtforme00 View Post
Would a moderator please make the above correction?

An update to my inquiry on capacitor ESR and crossover design.

First, I was a bit redundant in specifying both ESR and DF, as they are inextricably linked for a given frequency. DF = 2*PI*F*ESR*C*(100 for percent). However, it is nice to see the ESR to get a grasp of the resistance each capacitor represents.

Second, in private conversations with a loudspeaker engineer (The above mentioned Jack Hidley from NHT) he confirmed that an experienced designer will take into account the ESR of both capacitors and inductors. Furthermore he stated that the ESR was taken into account in the design I am cloning (NHT M3.3) and that I should try to match the Dissipation Factors given in the specifications. This was good for me to discover, as I had purchased at least one film capacitor to replace an electrolytic, a substitution which I will no longer be using.

Now, it is very common to hear discussion of Inductor ESR, but this is the first confirmation I have seen that capacitor ESR should be considered when designing loudspeakers. This does NOT automatically invalidate all designs which have not taken this into account. The differences among the ESRs of various types of film capacitors are fairly small, however there is a significant difference between the ESR of film type capacitors and that of electrolytics. The practice of swapping electrolytics for film capacitors to "improve" a design is immediately called into question.

There is no paradigm breaking news here, and to some this may even be old news. This is just one more small block of knowledge to add to the vast amount of information available on this forum.

So what practical lessons should we take away from this experiment? If one is designing a crossover with electrolytics, one should be aware that the ESR of the capacitor will have a small effect on the drivers actual response, and that some software is not capable of modeling this (in my experience Speaker Workshop). Similarly, if an existing design has an electrolytic, be aware that there will be a deviation from the design if a film type capacitor is substituted. Whether this deviation will be perceived subjectively as an improvement is outside the scope of this discussion.

I hope that this discussion has been as informative for the readers as it has been for the author.

Regards,
David

P.S. Next up, can I measure "voltage coefficient of capacitance", and how do different types of capacitors compare?
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