hp/lp filters..

[TrlcKed_OuT 10+ year member]

i just went to go get some hp filters/capacitors and i need like 2 7.9(µfd) for 2.5khz fequency well when i was up at radio shack all they have are µf..not µfd whats the difference!?!?

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[sumone 10+ year member]

µfd = µf

...which means I'm saying that fd = f = Farad.

 

[TrlcKed_OuT 10+ year member]

ok well i bought a couple and the numbers dont add up to what the charts on the12volt.com so thats why im concerd...

 

[sumone 10+ year member]

What numbers?

 

[TrlcKed_OuT 10+ year member]

well the ones i got at radio shack are 10µf and 4.7µf both for 8ohm at 4khz..so that doesnt make any sense there and if i look at 4khz at the12volt.com i need a 4.9µf....(not that im going that high but just for comparison..)

 

[sumone 10+ year member]

You pretty much need to be exact with cap values. If you use 4.7uF as opposed to 4.9uF the cutoff frequency is around 8.4khz (with an 8ohm impedance) [using the formula 1/(pi*cutoff_freq*impendance)]. But capacitors in series add up. You can break the final value down to multiple capacitors to reach the equivalent capacitance..just make sure they're in series.

 

[helotaxi 5,000+ posts]

Caps in parallel add, caps in series don't. In series it is the inverse of the sum of the inverses.

Also, last time I was at RS there was nothing there that I would use for an audio filter. Use the tables on Parts Express and order audio grade caps from there. You should be able to get the exact sizes you need from them.

 

[TrlcKed_OuT 10+ year member]

ok thanks where on there site would i find these charts?!

 

[TrlcKed_OuT 10+ year member]

im looking at partsexpress now and using the forumula 1/pieXcuttoffXohm...they dont match for instance...using the forumula..2.5khz@8ohm is 1.59uF i would beleive..and 2.5khz on partsepress says 7.96uF at a 6db/octave...is the forumula u gave me for a diff slope or wha?!? im tryin to learn but if someone who knows alot about this stuff could u tell me what cap i will need to cross my tweets at 2.5khz-3khz and lowpass my mids at 1.5khz....thanks!

 

[helotaxi 5,000+ posts]

The correct formula is 1/(2*pi*cutoff*imp).

 

[TrlcKed_OuT 10+ year member]

is that for a 6db/octave slope?!

 

[helotaxi 5,000+ posts]

yeah, 6dB per octave. The correct formulas are listed on the Parts Express ite on the "caps and coils" page.

 

[TrlcKed_OuT 10+ year member]

ok cool thank u for the help!!