Resistor/ cap question
06-30-2004, 11:35 PM
#1
ok. I am using passive X-overs in my van. I would like to run a pair of Peerless 8's I have at home out of a set of older home speakers. But I only have a 2 way crossover for my comp set....is there a resistor/cap I can use so that my 8's run from 250Hz down?
If so, would it be possible also, to change the impedence of the drivers from 8ohm to 4ohm without changing the frequency that the 8's would be playing? I don't want them playing much over 250Hz as it's not needed.
Any help would be appreciated.
If so, would it be possible also, to change the impedence of the drivers from 8ohm to 4ohm without changing the frequency that the 8's would be playing? I don't want them playing much over 250Hz as it's not needed.
Any help would be appreciated.
06-30-2004, 11:58 PM
#2
1000 Watt CAFz'r
Join Date: Oct 2003
Posts: 1,783
Buy this:
www.solen.ca/ldc.htm
Read the cross-over section.
To cut to the chase, you need an inductor.
Adam
www.solen.ca/ldc.htm
Read the cross-over section.
To cut to the chase, you need an inductor.
Adam
07-01-2004, 06:21 AM
#5
50 Watt CAFz'r
Join Date: Apr 2004
Posts: 194
Isn't there an equation to make a passive crossover, ie: Inductor = R/(6.28*Freq)
The Inductor would be in Henries, the Resistance is in Ohms and Freq is in Hertz, but don't quote the exact number for the formula (6.28). I believe those are for 1st order passive so its a 6 dB/octave slope...right?
To change the impedence, forgive me if I'm wrong but wouldn't the only way be to like cut out half the VC
? Help guys....?
The Inductor would be in Henries, the Resistance is in Ohms and Freq is in Hertz, but don't quote the exact number for the formula (6.28). I believe those are for 1st order passive so its a 6 dB/octave slope...right?
To change the impedence, forgive me if I'm wrong but wouldn't the only way be to like cut out half the VC
? Help guys....?
07-01-2004, 06:32 AM
#6
Hey man, PEI330CI is right you can buy an inductor to help fix your problem. I've also had to make a low pass filter out of nothing before. I bought a pair from a local shop here cause i didn't like the high off the 6x9's in the rear interfering with the fronts. There were too many conflicts with the highs in general.
07-01-2004, 08:04 AM
#7
1000 Watt CAFz'r
Join Date: Oct 2003
Posts: 1,783
Alright, I give up. I'm going start spewing forth my great knowledge. Just don't expect me to do this every time someone has a question about passive X.O.s.
First, you will need to find the impedence of the driver at 250hz. To do this you will need a tone generator, and amplifier, a DMM, and a 10 ohm 5% resistor. Connect some speaker wire to the amplifier terminals and connect the other end to the 10 ohm resistor.(the resistor runs across the positive and neg. lead the same as a speaker) Now connect your DMM to the positive and neg. side of the resistor and set your meter to measure A.C. volts. Next, turn on your tone generator and set it to 250hz, then ajust the gain on the amp until your DMM reads exactly 1.000 volts. Now repeat the above with your speaker replacing the resistor and take note of the voltage reading on the DMM. Using ohms law (and some simple math) you will see that 1 volt= 10 ohms, 0.8 volts= 8 ohm, and 0.4 volts= 4ohms. So if your reading is 0.755 volts, you have an impedence of 7.55 ohms at 250hz. If you want, you can continue to graph the entire impedence of the driver by varying frequency on the tone generator. Of note, this isn't a totally accurate way to measure impedence. As impedence rises, the voltage value on the volt meter will become increasingly inaccurate by a small percentage. It works well for values below 20 ohms though.
Now that we've got our impendence (imaginary 7.55 ohms) we can calculate the values of components for our cross over. Considering your application Brandon, I would use at least a 12 db/oct. slope for that 8. This will require an inductor in series with the speaker, and a capacitor in parallel with the speaker to achieve a 12db/oct roll off. (The inductor provides the first 6db, the capacitor provides the second 6db.) The formulas:
Inductor: multiply impedence by 0.3183, and then take this number and devide it by frequency. For example, 7.55 by 0.3183 = 2.4031, now devide 2.4031 by 250 = 0.0096124. This value is in Henries, but most values are expressed as "mH" or millihenries. So you need a 9.61mH Inductor.
Capacitor: multiply impedence by frequency, and then take 0.0796 and devide it by the previous sum. For example, 7.55 by 250 = 1887.5, now 0.0796 devided by 1887.5 = 0.00004217. This value is in Farads, but most values are expressed as "uF" or microfarads. So you need a 42.17 uF Capacitor.
So now that you know what you need, you can call up Solen and place your order. While your at it, you may as well order up that book I linked to above; The Loudspeaker Design Cookbook. Everything I described above is in it, as well as a whole lot more. Worth the 2 week wait in my opinion.
Just to really mess with people....voice coils heat up when power is applied. This increases impedence as temperature increases...and you guessed it, so does your cross over point. Just something to think about when your blaring you system full blast with passive cross overs. Of course....if you have the right drivers playing at reasonable levels this doesn't happen. But, then again, car audio people aren't reasonable....
I'll shut my trap now.
Adam
First, you will need to find the impedence of the driver at 250hz. To do this you will need a tone generator, and amplifier, a DMM, and a 10 ohm 5% resistor. Connect some speaker wire to the amplifier terminals and connect the other end to the 10 ohm resistor.(the resistor runs across the positive and neg. lead the same as a speaker) Now connect your DMM to the positive and neg. side of the resistor and set your meter to measure A.C. volts. Next, turn on your tone generator and set it to 250hz, then ajust the gain on the amp until your DMM reads exactly 1.000 volts. Now repeat the above with your speaker replacing the resistor and take note of the voltage reading on the DMM. Using ohms law (and some simple math) you will see that 1 volt= 10 ohms, 0.8 volts= 8 ohm, and 0.4 volts= 4ohms. So if your reading is 0.755 volts, you have an impedence of 7.55 ohms at 250hz. If you want, you can continue to graph the entire impedence of the driver by varying frequency on the tone generator. Of note, this isn't a totally accurate way to measure impedence. As impedence rises, the voltage value on the volt meter will become increasingly inaccurate by a small percentage. It works well for values below 20 ohms though.
Now that we've got our impendence (imaginary 7.55 ohms) we can calculate the values of components for our cross over. Considering your application Brandon, I would use at least a 12 db/oct. slope for that 8. This will require an inductor in series with the speaker, and a capacitor in parallel with the speaker to achieve a 12db/oct roll off. (The inductor provides the first 6db, the capacitor provides the second 6db.) The formulas:
Inductor: multiply impedence by 0.3183, and then take this number and devide it by frequency. For example, 7.55 by 0.3183 = 2.4031, now devide 2.4031 by 250 = 0.0096124. This value is in Henries, but most values are expressed as "mH" or millihenries. So you need a 9.61mH Inductor.
Capacitor: multiply impedence by frequency, and then take 0.0796 and devide it by the previous sum. For example, 7.55 by 250 = 1887.5, now 0.0796 devided by 1887.5 = 0.00004217. This value is in Farads, but most values are expressed as "uF" or microfarads. So you need a 42.17 uF Capacitor.
So now that you know what you need, you can call up Solen and place your order. While your at it, you may as well order up that book I linked to above; The Loudspeaker Design Cookbook. Everything I described above is in it, as well as a whole lot more. Worth the 2 week wait in my opinion.
Just to really mess with people....voice coils heat up when power is applied. This increases impedence as temperature increases...and you guessed it, so does your cross over point. Just something to think about when your blaring you system full blast with passive cross overs. Of course....if you have the right drivers playing at reasonable levels this doesn't happen. But, then again, car audio people aren't reasonable....
I'll shut my trap now.
Adam
07-01-2004, 09:00 AM
#8
http://ccs.exl.info/calc_cr.html#first
Choose 1st/2nd/3rd/4th order, plug in your impedance, away you go.
Choose 1st/2nd/3rd/4th order, plug in your impedance, away you go.
07-01-2004, 09:32 AM
#9
Guest
Posts: n/a
Theory http://www.passivecrossovers.com/
Quick answers: http://www.partsexpress.com/resources/xover.html
Quick answers: http://www.partsexpress.com/resources/xover.html
