
Home  Forums  Rules  Articles  diyAudio Store  Gallery  Wiki  Blogs  Register  Donations  FAQ  Calendar  Search  Today's Posts  Mark Forums Read  Search 
MultiWay Conventional loudspeakers with crossovers 

Please consider donating to help us continue to serve you.
Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving 

Thread Tools  Search this Thread 
11th May 2013, 02:04 PM  #1 
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 rolloff/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, 07:20 PM  #2 
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. 
12th May 2013, 02:40 AM  #3  
diyAudio Member
Join Date: Jul 2010
Location: Western China

Quote:
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, 02:54 AM  #4 
diyAudio Member
Join Date: Nov 2005
Location: San Antonio

Think of it a different way... imagine the capacitor as a (frequencydependent) 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, 03:05 AM  #5 
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 03:09 AM. 
12th May 2013, 03:23 AM  #6 
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, 03:33 AM  #7 
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, 03:34 AM  #8 
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 
12th May 2013, 03:50 AM  #9 
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.  Rick........ Last edited by Richard Ellis; 12th May 2013 at 03:55 AM. Reason: Maybe can't see it. 
12th May 2013, 12:50 PM  #10 
diyAudio Moderator

This calculator is also very good and easy to use.
2Way Crossover Designer / Calculator Instructions are here: 3 Way Crossover
__________________
Take the Speaker Voltage Test! 
Thread Tools  Search this Thread 


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
simple low pass RC filter noob question  mr.duck  Parts  18  18th October 2015 11:37 AM 
multiple feedback low pass filter question  graygem  Analog Line Level  20  14th July 2011 07:01 PM 
DIY One fader mixer with Filter Low Pass, Hi Pass, Notch Pass Filter PGM  stevep314  Analog Line Level  0  10th August 2010 10:15 PM 
Extremely basic  ward cleaver  Construction Tips  3  30th December 2009 12:16 AM 
Low Pass Filter question  synthius  Chip Amps  18  28th February 2008 01:13 PM 
New To Site?  Need Help? 