Subwoofer Crossover question

[randallhoffmann]

Hello,

I have a passive crossover rated at 120hz 12 db octave slope at 4 ohms - 200 watts RMS.
I would like to use this crossover on a subwoofer, but the subwoofer is 8 ohms. What would be the hz cutoff and RMS rating when used with this 8 ohm subwoofer?

Thanks in adavnce.

[Paul O]

Used with an 8ohm driver this is no longer a 12db filter. It's now two 6db filters, one at half and one at double the rated 4ohm cutoff frequency.

[daryl]

Provided Link: Daryl's Speaker Stuff.


A crossover such as you are describing wont work with any subwoofer or speaker regardless of rated impedance due to the impedance frequency response functions of subwoofers and speakers near the crossover frequency.

It is simply an old school crossover based on equations for resistive loads and flat frequency responses which are non-existant with practical subwoofers and speakers.

Just trash it and use a home theater Pre-pro/Reciever or an active crossover that has the option of using a lower order for the highpass section.

[emerye]

> I have a passive crossover rated at 120hz 12
> db octave slope at 4 ohms - 200 watts RMS.
> I would like to use this crossover on a
> subwoofer, but the subwoofer is 8 ohms. What
> would be the hz cutoff and RMS rating when
> used with this 8 ohm subwoofer?

This crossover will not work like a crossover designed for an 8 Ohm subwoofer, but you might be able to use it anyhow.

Does this crossover have connections for the main speakers (above 120Hz)?

A 4 Ohm butterworth crossover at 12db/oct has about 7.5mH inductor and a 235uf capacitor per the formula. So, this is probably close to your crossover on the woofer section. If used with an 8 Ohm woofer you will get a crossover about 200Hz. It could still handle similar power ratings, but it is not optimized for your 8 Ohm woofer. So, it may give OK crossover functions, but not optimum.

You could always try it and see how it sounds.

If you use the high frequency section of a 4 ohm crossover with 8 Ohm speakers the crossover will be much lower than 120Hz. It will be closer to 70Hz, and again not optimized for your 8 Ohm speakers. The bigger problem will be the overlap between the smaller speakers making sound above 70Hz and the subwoofer making sounds up to 200Hz. This is the overlap section between the two speakers, which will sound bad and may give your amplifier problems with a 4 Ohm or lower impedence at these frequencies.

You should probably use a different high pass crossover for your main speakers. Or better yet use an active crossover with a subwoofer plate amp.