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High Pass Filter


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#1 Andre Trollip

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Posted 04 August 2014 - 03:35 PM

Hi Guys,

 

Anyone have an idea if you could use a 8pF Ceramic Capacitor as a high-pass filter in a 6Ohm speaker system?

 

I tried to Google it, but I'm afraid I just don't know enough about capacitance to understand the articles.

 

Thanks in advance.



#2 Mario Vernari

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Posted 04 August 2014 - 04:29 PM

Strange question...not so related to the Netduino forum.

However, here is my answer...

 

A 8pF capacitor is a drop in the ocean: simply you won't hear anything.

Since you should place the capacitor in *series* to the speaker, a *very rough* formula is given by the impedance (read as "resistance") of the capacitor at the desired cut-off frequency.

Supposing the speaker as a tweeter, that is a typical cut-off of 3kHz, you might consider the capacitor impedance as "greater enough" than the speaker itself. Might we say 5 times more?...Thus, 30 Ohms

Since the cut-off frequency of a C-R high-pass filter is:

f = 1 / (2*PI*R*C)

then:

C = 1 / (2*PI*R*f)

 

If you use the given values, you'll have about 8uF, which is one million times more than the 8pF one!

 

Anyway, that's a very rough way to calculate and filter a speaker. A bad calculation/adaption of the speaker impedance will stress the amplifier, and you could even damage it.

 

Good luck


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#3 Andre Trollip

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Posted 04 August 2014 - 06:28 PM

Hi Mario,

 

Thanks very much for this.

 

Yes I know it's not really subject appropriate, but I find I get more concise and quicker answers on electronic questions than on other electronics forums.

 

Thanks again.






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