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Old 16th November 2005, 11:39 AM   #1
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Location: Paris
Default 1st order low-pass filter, which cap ?

Hi all,

I'm about to modifiy my active crossover's output stage. As I need "cleaning" the signal at the very end of the output stage I shall install an RC 1st order low-pass filter to achieve a gentle slope down from above 40Khz.

Any suggestions for best capacitor type to use for C ? Silvered mica ? MKP ? MKS ? other ? or really doesn't matter ?

Sorry if question is unclear, stupid or incomplete, but I didn't find any answer in existing threads

Thanks
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Old 17th November 2005, 12:33 AM   #2
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Try polypropylene, just make sure you know what the voltage rating of the capacitor should be.

==> Do not use electrolytic (even non-polar), as sonic quality is generally considered not so good.
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Old 17th November 2005, 05:44 AM   #3
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Thanks !
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Old 17th November 2005, 08:29 AM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
you need a cap that has lo inductance to ensure that the filter slope keeps falling almost all the way to infinity.
Your best chance to achieve this is a ceramic.
I would suggest that theoretically the cap to ground is not in the audio path so slight variations in capacitance as the output changes will have no sound effect.
If you want to play safe then use propylene film or styrene foil and add a small ceramic in parallel.
Third choice would be stacked film, ester or carbonate (now almost obsolete). Stacked should avoid the need for //ceramic.
There should be no need to go expensive here.

Do you need to alter any other slopes to compensate for the extra slope you propose to add?
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regards Andrew T.
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Old 17th November 2005, 08:37 AM   #5
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Hi !

Thanks for the input, very instructive

Actually I just need to get rid of unwanted noises above 45 Khz.

This takes places into a circuitry modification project right after a DA converter chip.
Then no, I don't need to alter the rest of the audio band, but I definitely need something as transparent as possible.

Cheers
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