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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
almarg's avatar
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almarg
8,673 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
zoya
119 posts
 
Theoretical answer to the theoretical qustion is 2 + 2 = 3 + 1 = 3.5 + 0.5 = 4.

But in physical situation 12 uf + 1 uf will give an added benefit of a small value capacitor added in parallel to the larger value capacitor because small value capacitors are better for filtering higher frequencies. The rolloff of the filter will be the same but you will also filter out high order harmonics that will pass through higher value capacitors.
4est
507 posts
 
"The rolloff of the filter will be the same but you will also filter out high order harmonics that will pass through higher value capacitors."

What? They are in parallel...

As usual, Almarg is the voice of reason. The only real reason I can see using say a 12uf and a 1 uf is if already had the part on hand or if I wanted to "bypass" the larger one with one of a different type. Bypassing is a mixed bag IMO, but some people like it...
zoya
119 posts
 
"The rolloff of the filter will be the same but you will also filter out high order harmonics that will pass through higher value capacitors."

What? They are in parallel...

__________________________
Sorry, I meant to say - high order harmonics that will NOT pass through higher value capacitors, but will pass through the parallel lower value capacitor.
jburidan
672 posts
 
Since the engineers at Advent used a single 13uF cap, my guess is that they didn't want the high order harmonics. Using 6.2uF & 6.8uF would alter the harmonic structure somewhat; and using 12uF & 1uF would alter it more. I guess the choice would depend on what kind of tweeter is in the Advent, e.g., silk dome or metal.
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