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I did some internet research on elliptical crossovers. One site, that read quite well, stated that the writer would never use them because they had too high a Q and rang. Can anyone shed some light on this one way or the other?
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An elliptic filter has the steepest slope for any given filter order. I believe they are mainly used in RF applications to make very selective bandpass filters with brick wall slopes.
For loudspeakers, if the design & driver selection requires a crossover with a very narrow transition band, you can realize it with the fewest components by using elliptic filters. Two significant downsides to using elliptic filters in a crossover are ripple in the pass-band and a less linear phase response than other filter types (assuming the latter matters of course). It's also harder to create matching elliptic low pass and high pass filters that sum together without creating a peak or dip, although by tuning parameter values you can trade some steepness for flatness.
I'm not a speaker designer, but it seems logical to use elliptic filters when working with a driver that is ill-behaved out of its pass-band. Like a metal or ceramic or other super-hard midrange driver or mid-woofer that has a nasty resonance or breakup in the low treble. For example, the Seas magnesium cone driver that Jeff Joseph uses has strong resonance around 5-6 KHz which is a little over one octave above the crossover. Joseph uses the Modafferi "infinite slope" crossover, which is a type of elliptic implemented with transformers.
Regarding the comment about elliptic filters having high Q and ringing, there are a couple of things to keep in mind. First, with any filter topology including elliptic, Q is variable. Second, it's the impulse response of the combined LPF + HPF crossover network that matters. A competently designed crossover will not ring at the crossover frequency regardless of what filter type is used.
Thanks. Sounds like good info and some food for thought, what I was looking for.
Wasn't the "infinite slope" xover of certain Joseph speakers based on the elliptical (cauer) filter as designed by Modefari. As I recall, those speakers were generally well reviewed.
The "infinite" slope crossovers used by Joseph Audio are different, though I've never taken one apart I've looked at the designs online.
While the slopes are quite high, they aren't high because of very large capacitor or inductors. They are high because of some very clever use of inductive coupling. I don't remember the precise details, but the patent is online.
As I recall, it's close to using a bucking transformer as part of the crossover. By using the inverted phase output of the transformer and adding it to the output of the crossover you get much higher slopes without the ringing.
Having said that, only Joseph uses them and they haven't dominated the speaker field so I doubt it's the ultimate answer to crossovers either. Still, very clever. :)
Best,
Erik
..."infinite slope" Modaferi crossover started with JSE speakers.
Joseph bought the patent and still uses them.
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