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Re: Bypass caps, a little more information...You sure about that?...

Posted by Steve O (A) on March 24, 2007 at 18:00:50

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.

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