# 4-ohm passive xover with 8-ohm drivers...



## typericey (May 13, 2006)

Got a pair of PolkMOMO xovers collecting dust and thinking of using it in the fiance's car. It's got some "pretty good" specs: 12db/octave, 2.6khz cross point, and I don't know how, but it's bi-amp capable. Got a short list of DIY drivers to match with it but my main challenge is that DIY drivers are mostly of the 8-ohm variety. This is a temporary setup until I can financially afford to go full active.

Will there be issues using 8-ohm drivers with a 4-ohm passive crossovers?


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## yermolovd (Oct 10, 2005)

i want to click you should have searched, but im watching hockey and am lazy, so why don't you search.


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## Lothar34 (Oct 6, 2006)

I'll do it.


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## Dangerranger (Apr 12, 2006)

typericey said:


> Got a pair of PolkMOMO xovers collecting dust and thinking of using it in the fiance's car. It's got some "pretty good" specs: 12db/octave, 2.6khz cross point, and I don't know how, but it's bi-amp capable. Got a short list of DIY drivers to match with it but my main challenge is that DIY drivers are mostly of the 8-ohm variety. This is a temporary setup until I can financially afford to go full active.
> 
> Will there be issues using 8-ohm drivers with a 4-ohm passive crossovers?


The crossover frequency will effectively double. Aka instead of 2.6khz, you'll have a 5.2khz crossover point. Won't be optimal, but should still work decent as a tie-over in a car door since off axis rolloff will kill the top end response of the woofer regardless.

The tweeters are both likely 8 ohm, but if its make for 4 ohm tweeters and you put 8 ohm tweeters on there, the result is the same, x/o frequency doubles. If they're both 8 ohm, you'll have a 5.2khz woofer cross point combined with a 2.6khz cross at the tweeter, which isn't ideal but as said this is temporary anyway.


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## TXwrxWagon (Sep 26, 2008)

Dangerranger said:


> The crossover frequency will effectively double. Aka instead of 2.6khz, you'll have a 5.2khz crossover point. Won't be optimal, but should still work decent as a tie-over in a car door since off axis rolloff will kill the top end response of the woofer regardless.
> 
> The tweeters are both likely 8 ohm, but if its make for 4 ohm tweeters and you put 8 ohm tweeters on there, the result is the same, x/o frequency doubles. If they're both 8 ohm, you'll have a 5.2khz woofer cross point combined with a 2.6khz cross at the tweeter, which isn't ideal but as said this is temporary anyway.


X2... if temp you will have a gaping hole in the upper mid/ lower tweeter region... depending on your placement in your car.. might actually sound ok.. lol...

if you plan to go active, and you LOVE the tweeter's sound.. go for it.. the tweeter will be over protected, x-over point wise... ready for active duty.

Rob
2004 WRX Wagon


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## typericey (May 13, 2006)

thanks for the replies danger and wagon.

yeah, i should've searched first. did my search after and saw a thread like this dated 2006. my bad.


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## bafukie (Nov 23, 2007)

Dangerranger said:


> The crossover frequency will effectively double. Aka instead of 2.6khz, you'll have a 5.2khz crossover point. Won't be optimal, but should still work decent as a tie-over in a car door since off axis rolloff will kill the top end response of the woofer regardless.
> 
> The tweeters are both likely 8 ohm, but if its make for 4 ohm tweeters and you put 8 ohm tweeters on there, the result is the same, x/o frequency doubles. If they're both 8 ohm, you'll have a 5.2khz woofer cross point combined with a 2.6khz cross at the tweeter, which isn't ideal but as said this is temporary anyway.


this is something new (different ohm different x-over point). I hope u dont mind me asking why is that so? I have been asking around regarding this question too but ppl tend to say apart from the different rms power from the amp, the rest doesnt change


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## Dangerranger (Apr 12, 2006)

bafukie said:


> this is something new (different ohm different x-over point). I hope u dont mind me asking why is that so? I have been asking around regarding this question too but ppl tend to say apart from the different rms power from the amp, the rest doesnt change


If you were running active, the x/o frequency is set because the DSP, head unit, whatever, doesn't see the impedance of the drivers and it's all done at signal level. 

In the case of passives, you're using an inductor and a capacitor to cross the drivers over. Inductors, as we know, act as a low pass and beyond the cutoff frequency they increase impedance as frequencies go up. capacitors act as a high pass, increasing impedance at frequencies below the cutoff point. 

For woofers, the inductor is in series and acts as a low pass, and (if 2nd order or above) a capacitor is connected in parallel because it shunts high frequencies to ground, meaning when connected in parallel the capacitor acts as a low pass as well and causes the higher rolloff rate. 

With high pass crossovers, the opposite is true, the capacitor is in series, blocking low frequencies, where the inductor shunts low frequencies to ground, acting as a high pass and causing a steeper rolloff.

The cutoff frequency is relative to the impedance of the drivers. An 8 ohm driver will require twice the inductance/capacitance to achieve the same crossover frequency, because the impedance is double to begin with. So if using a crossover designed for a 4 ohm driver, the effective crossover frequency doubles because the 8 ohm driver would require twice the inductor/capacitor to get the same crossover freq. 

Make sense?


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## 14642 (May 19, 2008)

The passive crossover modifies the impedance of the speaker to adjust the output power of the amplifier. The amp makes less power into high impedance than into low impedance. The values of the passive components are chosen so that the time constants of the capacitors and inductors used in the circuits cause a particular shape of the impedance curve. Change the terminal load (speaker impedance) and you change the shape of the impedance curve. Good passive crossovers are designed to work with the impedance curve of a particular speaker to produce a target acoustic response. 

The simplified explanation about doubling or halving the frequency would work if speakers were resistors. They're not. 

In short, this is a practice which should be avoided.

Using different speakers with that crossover is kinda like putting the front bumper from a Chevy astro van on a Dodge pickup. You might be able to make it fit, but...


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## meteoro84 (Jul 10, 2006)

hi ,reading some answers , *"The crossover frequency will effectively double. Aka instead of 2.6khz, you'll have a 5.2khz crossover point"* with a tw 8ohms in a 4ohms xover , i think if in a xover for 8ohms tw putting a tw 4 ohms the frec will be a half ? taking the example is a 8 ohms 2,6k and put 4 ohms tw the frec wiil be 1,3khz???
thk


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## 14642 (May 19, 2008)

yes, but only if the tweeter is a resistor. This is a practice that should be avoided.


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## Abaddon (Aug 28, 2007)

Andy Wehmeyer said:


> yes, but only if the tweeter is a resistor. This is a practice that should be avoided.


I find this to be one of the most misunderstood thing about speakers. We always look at the "impedance" of a speaker, but most people don't understand the concept of reactive loads. I get the feeling that a lot of people, when looking at Frequency response plots, don't understand what that second plot is trying to tell them.


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## exmaxima1 (May 31, 2007)

Dangerranger said:


> The cutoff frequency is relative to the impedance of the drivers. An 8 ohm driver will require twice the inductance/capacitance to achieve the same crossover frequency, because the impedance is double to begin with. So if using a crossover designed for a 4 ohm driver, the effective crossover frequency doubles because the 8 ohm driver would require twice the inductor/capacitor to get the same crossover freq.
> 
> Make sense?


Crossovers, as you noted, use both caps and inductors which must be sized for the task. AFAIK, for a given capacitor, doubling the load impedance will HALVE the crossover frequency. For a given inductor, doubling the load impedance will DOUBLE the frequency. For multi-pole networks, such as a 12 db/oct type (2 poles), you will get some unintended results from dramatic changes in load impedance. 

You cannot use a 4 ohm crossover with 8 ohm drivers.


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## bafukie (Nov 23, 2007)

Dangerranger said:


> If you were running active, the x/o frequency is set because the DSP, head unit, whatever, doesn't see the impedance of the drivers and it's all done at signal level.
> 
> In the case of passives, you're using an inductor and a capacitor to cross the drivers over. Inductors, as we know, act as a low pass and beyond the cutoff frequency they increase impedance as frequencies go up. capacitors act as a high pass, increasing impedance at frequencies below the cutoff point.
> 
> ...


noted. thanks


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## Dangerranger (Apr 12, 2006)

exmaxima1 said:


> Crossovers, as you noted, use both caps and inductors which must be sized for the task. AFAIK, for a given capacitor, doubling the load impedance will HALVE the crossover frequency. For a given inductor, doubling the load impedance will DOUBLE the frequency. For multi-pole networks, such as a 12 db/oct type (2 poles), you will get some unintended results from dramatic changes in load impedance.
> 
> You cannot use a 4 ohm crossover with 8 ohm drivers.


You're right, I had it backwards on capacitance. Brain fart. Doubling load impedance on a given capacitor halves the crossover frequency.

And in response to Andy and others, I fully agree that an ideallized passive crossover setup won't be had plopping a 4 ohm crossover on 8 ohm drivers due to both the impedance difference in a resistive sense and reactive sense. As always the impedance curve of the drivers at hand will affect the true frequency cutoff of the crossover as well as resultant phase and power response, but I don't think the OP was really expecting fabulous results out of the setup as much as he wanted it to tie him over and keep him from blowing the drivers up until he gets the $$ to go active with them. 

But the tweeters used are *probably* 8 ohm, which that should be checked, the midbass impedance will probably be the only difference.


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