8 ohm to 4ohm-does the crossover need to change much?

mclsound · 28th July 2008, 12:22 AM

hi guys,
i had Solen build me a 2-way(mid/hi(atc 75/150 non S & SS 9700)) crossover,at 8ohms.
this wknd i played around and put both top cabinets together for a MTTM and it sounded spectacular. i ran it off a Classe CA-400.
if i put 2 mids and 2 hi's on each original crossover do i have to change much?
they are a 24db at 380 and 3800hz
thanks John

PeteMcK · 28th July 2008, 01:12 AM

caps double in value & inductors halve; having two tweeters may not be so good, google 'diffraction effects'

BlueWizard · 28th July 2008, 05:52 AM

It depends on the impedance of the individual speakers and how those speakers are wired.

If you have 4 ohm speakers wired in series or 16 ohm speakers wired in parallel, then you are OK.

But of course, the Nominal Impedance means nothing, you want the impedance of the speaker at the crossover point.

I was recently playing around with a crossover designer. I calculated the crossover for an 8 ohms 2-way at 1000hz, then determined at what frequency I got the same capacitor and inductor values but for a 6 ohms speaker system. SIX ohms being far more likely at 1000hz than the nominal 8 ohms.

It turns out to get the same Inductor, I had to drop down to 750hz and to get the same capacitor, I had to move up to 1350hz. That's how far off a crossover is when it is designed to a nominal 8 ohms but the speakers are really 6 ohms at the crossover point.

So, if you go from 8 ohms to 4 ohms, that gap at the assumed crossover point is going to be even bigger.

As a point of illustration -

8 ohms, 3000hz, 12db second order Linkwitz-Riley

C1 = 3.32 µF
L1 = 0.85 mH

C2 = 3.32 µF
L2 =0.85 mH

4 ohms, 3000hz, 12db second order Linkwitz-Riley

C1 = 6.6 µF
L1 = 0.42 mH

C2 = 6.6 µF
L2 = 0.42 mH

http://www.apicsllc.com/apics/Misc/filter2.html

Using 8 ohm values to design a 3000hz crossover but using 4 ohms speakers in the final construction, the coil begins to crossover at 1500hz and the capacitor begins to crossover at 6000hz. That's how far off your crossover is.

For what it's worth.

Steve/bluewizard

28th July 2008, 12:22 AM	#1
mclsound diyAudio Member Join Date: Sep 2007	8 ohm to 4ohm-does the crossover need to change much? hi guys, i had Solen build me a 2-way(mid/hi(atc 75/150 non S & SS 9700)) crossover,at 8ohms. this wknd i played around and put both top cabinets together for a MTTM and it sounded spectacular. i ran it off a Classe CA-400. if i put 2 mids and 2 hi's on each original crossover do i have to change much? they are a 24db at 380 and 3800hz thanks John __________________ thanks John

28th July 2008, 01:12 AM	#2
PeteMcK diyAudio Member Join Date: Feb 2002 Location: Western Sydney	caps double in value & inductors halve; having two tweeters may not be so good, google 'diffraction effects' __________________ Impedance varies with frequency, use impedance plots of your drivers and make crossover calculations using the actual impedance of the driver at the crossover frequency

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28th July 2008, 05:52 AM	#3
BlueWizard diyAudio Member Join Date: Jun 2007	It depends on the impedance of the individual speakers and how those speakers are wired. If you have 4 ohm speakers wired in series or 16 ohm speakers wired in parallel, then you are OK. But of course, the Nominal Impedance means nothing, you want the impedance of the speaker at the crossover point. I was recently playing around with a crossover designer. I calculated the crossover for an 8 ohms 2-way at 1000hz, then determined at what frequency I got the same capacitor and inductor values but for a 6 ohms speaker system. SIX ohms being far more likely at 1000hz than the nominal 8 ohms. It turns out to get the same Inductor, I had to drop down to 750hz and to get the same capacitor, I had to move up to 1350hz. That's how far off a crossover is when it is designed to a nominal 8 ohms but the speakers are really 6 ohms at the crossover point. So, if you go from 8 ohms to 4 ohms, that gap at the assumed crossover point is going to be even bigger. As a point of illustration - 8 ohms, 3000hz, 12db second order Linkwitz-Riley C1 = 3.32 µF L1 = 0.85 mH C2 = 3.32 µF L2 =0.85 mH 4 ohms, 3000hz, 12db second order Linkwitz-Riley C1 = 6.6 µF L1 = 0.42 mH C2 = 6.6 µF L2 = 0.42 mH http://www.apicsllc.com/apics/Misc/filter2.html Using 8 ohm values to design a 3000hz crossover but using 4 ohms speakers in the final construction, the coil begins to crossover at 1500hz and the capacitor begins to crossover at 6000hz. That's how far off your crossover is. For what it's worth. Steve/bluewizard