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In Reply to: Bypass caps, a little more information posted by erik_squires@hotmail.com on March 23, 2007 at 12:43:50:
In your last paragraph you state "If you are really interested in reducing time smear, according to the way this article models capacitors, you are going to be much better off using parallel capacitors instead. That is, if you need a coupling cap of 4uF, use 4 x 1uF capacitors or 2 x 2uF. This should reduce the memory effects substantially." This is not the conclusion I come to.DA is an inherent property of the dielectric. If you look at the model, reducing DA would require either greatly increasing the Rs or reducing the Cs of the "parasitic" network. I don't see how parallelling "macro" caps would accomplish this.
Pease really only covers two means of getting around the issue: 1. create a compensating network and, in effect insert it into a corrective feedback loop. 2. Use better quality caps to begin with. Number 1 will be difficult to accomplish in an audio circuit. To my way of thinking, you'd be better off designing a direct coupled circuit not requiring caps. Number 2 is kind of obvious. What might be surprising to some is that the NPO/COG ceramic cap is pretty darn good at least WRT DA. This also is consistent with my expereince where NPO ceramics are sometimes found in good sounding vintage audio equipment.
Note that the article is written in the context of instrumentation and D to A apps where timers and sample & holds are involved. Presumably while in the hold mode, the cap is being "measured" by an amp with very high input impedance. In audio coupling applications the relative impedance will be much lower and the memory effect much less pronounced. Also, there are other characteristics of real world caps that may have a greater affect on what we hear. Bottom line for me is simple: use quality PP caps wherever possible and don't worry about DA.
As always, YMMV. And if you think my interpretation of the article is flawed feel free to point it out.
Follow Ups:
DA is an inherent property of the dielectric AND construction.As i understand it, if you took the capacitor and unrolled it, what you would end up with is are two long strips, separated by the dielectric, with the electrodes connected either at one end, or the middle.
So, the problem with DA really has to do with the fact that some of the material is physically and electrically closer to the electrodes than the rest. You can eliminate DA by making all of the dielectric be equally spaced from the conductor. This is unfortunately, impossible.
So, one way to come closer to this is to parallel multiple caps. For instance, a 4uF cap may have a 4" wide strip (I'm making this up). With the maximum distance from the electrode being 2".
By using 2uF caps instead, you get the same capacitance as before, but the film strips are only 2" wide for each capacitor, in effect getting closer to this ideal.
But you know, this is all highly theoretical. I wish I had the time to construct Pease's test harness, I would love to try this with some inexpensive Solen's and see if there is any merit to what I'm saying or not.
Regards,
"DA is an inherent property of the dielectric AND construction."
Construction plays little role in DA. Refer to the articles by Pease and now Sencore courtesy of Jim McShane. Nowhere do they consider ANYTHING except dielectric materials as the primary factor in DA."As i understand it, if you took the capacitor and unrolled it, what you would end up with is are two long strips, separated by the dielectric, with the electrodes connected either at one end, or the middle."
Although many caps consist of "rolled" construction, end terminations are almost never done as you describe: inductance would be problematic."So, the problem with DA really has to do with the fact that some of the material is physically and electrically closer to the electrodes than the rest. You can eliminate DA by making all of the dielectric be equally spaced from the conductor. This is unfortunately, impossible."
If I understand you assertion correctly (and I may not) caps employing your "ideal" construction exist and are readily available and still have DA issues. One term for them is "stacked film" IIRC. In some respects these caps could be considered a physically large # of small caps in parallel...and they still possess significant DA if made of something like polyester. So much for parallelling to get rid of DA.Capacitors have been around a very long time and like most objects of human endeavor, have reached a very high level of development. If parallelling caps could reduce DA, commercial designs would have incorporated some version of it decades ago...but they haven't and DA is still with us.
Well, actually the Multicap line does exactly this, so there are in fact commercial designs which use this principle. However, whether they intended for it to reduce DA or inductance I do not know.Regards,
"So, the problem with DA really has to do with the fact that some of the material is physically and electrically closer to the electrodes than the rest. You can eliminate DA by making all of the dielectric be equally spaced from the conductor."The geometry is not the only factor, in fact it's not even the primary factor. Otherwise why would there be a large difference between various materials used as the dielectric?
Here's an excellent definition of dielectric absorption:
"That property of an imperfect dielectric whereby there is an accumulation of electric charges within the body of the material when it is placed in an electric field." (my italics)
And if you want to know more try the link below. It is a great bit of work.
I'm not denying that the materials matter, at all.I'm just saying that given the same material, having a capacitor which uses geometry with there is less distance between the furthest reaches of the film and the electrodes, you would get less soakage.
I will read your link, perhaps I'm not understanding it well enough yet. :)
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