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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
zoya
120 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
120 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
803 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.
heyraz OP
177 posts
 
Jburidan-very good insight. I think the Original and New Large Advent tweeters were more silk than metal and were described as the "fried egg" tweeters. They had their own unique sound which I think defined the character of the speaker. The crossover was a simple 6db first order type. Advent later changed the tweeter to a black poly unit and used a different crossover (3rd order), which in my opinion ruined everything.
Can anyone explain how adding a bypass capacitor would affect the sound? Is that what the old "Vitamin Q" was about? Why was it described as a mixed bag earlier in this thread?
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