High Pass Crossover for FE87E (Protection)

[mrevie]

I recently purchased a pair of Fostex FE87E fullrange drivers to use in a pair of rear surround speakers. Having done some reading, I've discovered that it's a good idea to prevent significant amounts of bass from being passed to the driver to prevent too large an xmax damaging it. Seeing as it's not practical for me to use a digital/electronic crossover in this isntance I need to design a passive crossover to do the job.

I've decided to go for a second order linkwitz-riley as I feel it offers the best medium between too slow a rolloff and too sharp. Ideally, I'd like the cutoff frequency (-3db point) for this arrangement to be about 200Hz. If I were to use a higher order crossover then I'd want it about 150-180. The measured impedance of the driver at ~200Hz is around 12 Ohms.

When calculating the values for the components I discover them to be:

Cap - 33uF
Coil - 19mH

Now, 19mH seems a somewhat ridiculous value for an inductor in terms of cost and size. So I decided to play with the value a little bit in simulation. Lowering the inductance of the coil to 4.7mH (a much more reasonable level in my opininon) doesn't seem to have a negative effect on the simulated crossover.

Are there any drawbacks to using the smaller inductor? Is there a better way to achieve what I want without using a passive HP filter?

[ttan98]

Hi,

I bought a similar pair from Ebay as well, I suppose they from the same seller.

I mounted them on a small cabinet, for such a small speakers they throw a quite a sound stage. SOund good too will improve with time.

Good buy.

[mrevie]

Yeah, looking at the data on them they do look quite nice for surrounds. I was considering using them in a semi-omnidirectional rig, purely because they'll mostly be used for effects.

Mine will most likely be mounted in a sealed enclosure with a max volume of about 5L.

Did you use any form of crossover to protect it from too much bass?

[ttan98]

I use an active x-over.

YOu can use active x-over or even design passive x-over at line level.

[planet10]

Getting a passive XO to work on these at the frequency you want is an extremely difficult job since you are working just above the impedance peak.

dave

[woody]

I have a similar question. For a 24db biamped oct xover at 215hz could you put this driver ( fs 140, qts .92 ) in an enclosure so it's fs is 215hz and qts is 1.4 . Then feed it's amp from
a 12db xover with a Q of .5 so the sum of the 2 xover sloaps
come out to a Q of .7 and 24 db octive?

[mrevie]

Any suggestions on how to improve the passive crossover that I want? Going active really isn't an option for me.

[mrevie]

This is the response get when simulating

[mrevie]

From this circuit