Extremely Basic Low Pass Filter Question - diyAudio
 Extremely Basic Low Pass Filter Question
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 Multi-Way Conventional loudspeakers with crossovers

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 11th May 2013, 01:04 PM #1 miragem3i   diyAudio Member   Join Date: Jul 2010 Location: Western China Extremely Basic Low Pass Filter Question I want to create a low pass filter and just understand this point better. Reactive Capacitance is: XC= 1 / ( 2πfc) Where c is in farads and f is in hertz. I want to calculate the rolloff and cutoff frequency for a given value of capacitance - let's assume 4.7uf - at a given frequency. How do I calculate this roll-off/cutoff frequency of a low pass filter with only one capacitor in the circuit? Example a woofer connected to a tweeter with a 4.7uf wired across the woofer terminals. What's the rolloff/crossover frequency? TIA!
 11th May 2013, 06:20 PM #2 jjrenman   diyAudio Member   Join Date: Mar 2013 First, lets go over some basics. A single capacitor in series with the tweeter is a 6db/oct high pass. A single inductor in series with the woofer is a 6db/oct low pass. I don't know of a situation that you would put a lone capacitor across the woofer. Second, your formula does not look correct. For speaker applications the formula will take into account the impedence of the driver.
miragem3i
diyAudio Member

Join Date: Jul 2010
Location: Western China
Quote:
 Originally Posted by jjrenman I don't know of a situation that you would put a lone capacitor across the woofer..
My mistake.
So how to calculate the xo frequency for that 6dB/octave high pass filter using only one capacitor?
I obtained that formula from several difference sites. What is the correct formula.

 12th May 2013, 01:54 AM #4 sofaspud   diyAudio Member     Join Date: Nov 2005 Location: San Antonio Think of it a different way... imagine the capacitor as a (frequency-dependent) resistor. Picture the circuit with that resistor, rather than a capacitor. And keep in mind that current flows through the path of least resistance. __________________ It is error only, and not truth, that shrinks from enquiry. - Thomas Paine
 12th May 2013, 02:05 AM #5 miragem3i   diyAudio Member   Join Date: Jul 2010 Location: Western China Right, that is how I think of it. So given that it is a frequency dependent resistor, what would be the way to calculate the crossover response of said capacitor with a given driver? I wish to calculate where to cut off the high pass at a given frequency. "For speaker applications the formula will take into account the impedance of the driver. " Ok. So this formula would be...what? Last edited by miragem3i; 12th May 2013 at 02:09 AM.
 12th May 2013, 02:23 AM #6 Jerrym303   diyAudio Member   Join Date: Apr 2011 Not an expert or even experienced, but these pages might help: Passive Crossover Networks Passive Crossovers, Capacitor and Coil Calculator
 12th May 2013, 02:33 AM #7 counter culture   diyAudio Member   Join Date: Jun 2011 The 3dB (half power) point (generally quoted as the nominal frequency of the network) of a 2 component network is where Z1 = Z2. So in the case of an RC network (one resistor and one capacitor), the power will be split 50/50 across the 2 components when R = Xc, or R = 1/(2*pi()*f*C). You can jiggle this around to get f = 1/(2*pi()*R*C). __________________ Now I am Become Death, the Destroyer of Words
 12th May 2013, 02:34 AM #8 sofaspud   diyAudio Member     Join Date: Nov 2005 Location: San Antonio C = 1/2πfR where R is the driver impedance or f = 1/2πCR __________________ It is error only, and not truth, that shrinks from enquiry. - Thomas Paine
Richard Ellis
diyAudio Member

Join Date: Sep 2006
Location: Mar del Plata, a BIG seasonal getaway city, can see the Ocean from our residence.
First Order Formulas

Note, top of both formulas, the 159,155 has a comma, the second one is a decimal point.

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----------------------------------------------------Rick........
Attached Images
 CrossOver.JPG (26.8 KB, 176 views)

Last edited by Richard Ellis; 12th May 2013 at 02:55 AM. Reason: Maybe can't see it.

 12th May 2013, 11:50 AM #10 Pano   diyAudio Moderator     Join Date: Oct 2004 Location: SW Florida This calculator is also very good and easy to use. 2-Way Crossover Designer / Calculator Instructions are here: 3 Way Crossover __________________ Take the Speaker Voltage Test! planet10 needs your help: Let's help Ruth and Dave

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