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Old 28th December 2007, 02:50 AM   #1
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Question about capacitor values

heres my situation....
for my capacitor on a 1st order butterworth i need a uf value of 15.9.....and I cant find an exact match...can i parallel two caps together to make a value of say 16uf with a 15 uf cap and then a 1 uf cap?????
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Old 28th December 2007, 03:32 AM   #2
jnb is offline jnb  Australia
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Yes you can, but unless your crossover was designed with direct measurements of your drivers, or specified in a pre-existing design, then 15uF will probably be OK on its own.
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Old 28th December 2007, 03:19 PM   #3
jdlech is offline jdlech  United States
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If you absolutely insist, yes, a 15 + a 1uF cap in parallel gives 16uF assuming you keep the leds short. Also, three 47uF caps wired in series will give the equvalence of 15.666...uF. But I think I would take jnb's advice first.
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Old 28th December 2007, 03:40 PM   #4
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Is there any reason why paralleling is bad ? I've heard people RECOMMENDING either bypassing with a smaller capacitor(in the hundreds nF range) or paralleling larger values with very high quality smalle ones (1uF)
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Old 28th December 2007, 05:07 PM   #5
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Quote:
Originally posted by bzfcocon
Is there any reason why paralleling is bad ? I've heard people RECOMMENDING either bypassing with a smaller capacitor(in the hundreds nF range) or paralleling larger values with very high quality smalle ones (1uF)
Paralleing is fine to achieve the required value.

By-passing large caps with small ones maybe useful in chip supply decoupling.

In crossovers and any kind of filter network in the audio band it is not relevant and its advocates do not understand what they are doing!
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Old 28th December 2007, 05:39 PM   #6
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I used the crossover calculator avalible on lalena dot com. It dsn't ask for specific driver values other than the drivers impedance (one scan speak revelator-mid @4ohm and scan speak 7100 tweeter @4ohm) and it also asked for the x-over freq which was 2500hz. it gave me a cap at 15.9 and an inductor at .25 i'm basically making a "clone" of the sonus faber cremona auditor using the 1st order crossover and scan speak components. but using the dayton audio curved box design that is maybe 1 inch off size from the original cab design.
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Old 28th December 2007, 06:08 PM   #7
DcibeL is offline DcibeL  Canada
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Sounds like you are going to pair a couple of the worlds finest drivers with the worlds poorest crossover.

At least if you must use that useless online calculator, you must enter the impedance at the frequency you are crossing! Impedance plots are included in the datasheet for the drivers.

I would highly recommend using a proven design designed by one of the several diy experts around the net.
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Old 28th December 2007, 06:08 PM   #8
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Those impedance values are nominal. It is necessary to find the actual impedance at the intended XO point.

I may be wrong but I have always believed that if you are paralleling caps try to use two the same. ie: it is better to use two 4.4's to get 8.8 than it is to use a 6.8 and 2.0 Not sure if that's correct though.

EDIT: Posted at the same time as DcibeL
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Old 28th December 2007, 07:08 PM   #9
Nordic is offline Nordic  South Africa
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Cal you may be thinking in the right direction... 2uf cap will have higher ESR (generaly) so even though you reach the same capacitance, it is at a higher impendance. Sometimes however we jsut need to get to a value quickly and anything will do...

Hope you have some way of measuring the cap....

The 15uf cap could have a physical value from 13.5 to 16.5uf if it is common 20% tolerance models...
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Old 28th December 2007, 07:12 PM   #10
hermanv is offline hermanv  United States
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Capacitors in these value ranges are usually 10% or 20% accurate, for real money you can get 5%. If you parallel a 15 and a 1 or 2 x 8 you get 16 uF an error 0f 0.6%.

I tend to favor two capacitors of 1/2 value each, based on personal experience one of the two should be connected anti-parallel i.e. backwards from the other (assumes a non-polar design).
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