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Parallel Capacitors-Theoretical Question

I want to replace the capacitors in my speaker's crossover networks. My Large Advent's came stock with 13uf NPE's which I would like to replace with PIO caps. Unfortunately, PIO caps are unavailable in that value. What's the best way to get to 13uf, (12uf + 1uf) or (6uf + 7uf)?
This is a purely theoretical question and I realize either method would probably sound the same, but I'm wondering if there is a reason why one method would be preferred over the other. Are there phase, impedance or other issues associated with either method. I realize of course that I could simply use 12uf or 15uf and probably never notice anything given that the original 13uf NPE had a 20% tolerance, but in theory, what's the best way and why?
heyraz

Showing 2 responses by almarg

almarg's avatar
almarg
9,670 posts
 
Given that the paralleled caps would presumably be similar in design, I'm not sure that using dissimilar values would make any difference compared to using values that are close to each other. But my instinct would be to use values that are as close as possible, because doing that keeps the higher of the two values as low as possible. Everything else being equal, a cap having higher capacitance can be expected to have higher stray inductance, which would be undesirable if the difference were significant in degree.
In case of parallel caps you'll simply have larger tolerance.
I respectfully disagree. For example, 6uf +/-10% in parallel with 7uf +/-10% = 13uf +/-10%.

Regards,
-- Al
almarg's avatar
almarg
9,670 posts
 
03-22-11: Paulsax
Shouldn't the tolerances for cap add in quadrature? (0.1^2 + 0.1^2)^0.5 = 0.14 or 14%?
Hi Paul,

You ask a good question, as usual.

If the two tolerances are the same in percentage terms, then as I stated the tolerance of the parallel combination will be that same percentage. That can be seen by calculating the worst case values. For example, if a 10uF 10% capacitor is paralleled with a 5uF 10% capacitor, the minimum possible value of the combination is 9uF + 4.5uF = 13.5uF. The maximum possible value of the combination is 11uF + 5.5uF = 16.5uF. In both cases the deviation from the nominal value of the combination (15uF), is 1.5uF, or 10%.

My statistics courses are now a (very) distant memory, but I believe that combining inaccuracies on an rss (root sum square) basis such as you described would be applicable to standard deviation and related calculations, that involve the PROBABILITY that a combined inaccuracy will fall within limits that are NARROWER than the worst-case limits.

That in turn would typically involve situations where tolerances are being combined that act on the same nominal value, not on nominal values that sum together.

Best regards,
-- Al
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