3way XO questions

[augerpro]

I have a couple stupid questions for all of you. I'm doing my first 3way XO for my BB project.
1)Any tips to keep Z up in the bandpass? Steep filters and wide bandpass seem to help, but I can only take that so far.
2) Say I've designed a 4th order filter for the woofer's lowpass. After optimization the first shunt cap comes back at like .001 uF. I could remove it. What happens when you have two inductors in a row?
3)If I have to pad the bandpass any preference on where to put the resistor? Placement changes the FR shaping of course, but assume I can get the same shaping from any position. I believe LDC has a bit to say about this.

[augerpro]

Concerning my second question above. The first shunt cap, C1, was .0001 or so, and the second shunt cap, C2, was 70 uF. Intuitively i decided to make both 35 uF and the FR was nearly the same. So I changed C1 to 70uF and just removed C2 and now have a 3rd order. i've learned the optimizer definately has to be setup correctly. GIGO. I remove tiny value compnents and keep others from exploding, and otherwise just lock and adjust so that the values make sense. But I'm doing all this by intution for the most part. I guess this comes back to filter Q and all that that I just don't have info on. I have to ask myself what is the difference between the 4th order with both caps at 35 uF and the 3rd order with the one cap at 70uF? Ignoring the order for the moment what is the difference electrically? I guess that's a big question so I don't expect an answer. Just some ignorance of the subject that had to vent about :M

[JonMarsh]

It's called pole spreading, and it's giving you a four element network that responds like a 3rd order network. That's not uncommon- you wouldn't normally use a full four element crossover for a fourth order response UNLESS your drivers are quite flat in the initial transition band.

You're discovering a bit of basic filter theory by trial and error. A book or two might be faster...

[TacoD]

What Jon already pointed out, you have to account for both acoustical response of the unfiltered driver and the electrical response of the x-over. You can end up with a 4th order acoustical with a 2nd order electrical filter (typically the case for a HP filter on a closed box driver).

[augerpro]

I see. Thanks guys. The lowpass woofer is the HiVi M4N which I'm crossing around 450Hz. I figured the reason I had to use a 4th order network for a 4th order acoustic response was because of exactly what Jon said. The M4N obviously has a smooth response far past 450Hz! So what books would you recomend? It's been over 10 years since I had calculus so I'm pretty rusty with that sort of thing.

[JonMarsh]

Just some basic loudspeaker theory and concepts- Joseph D'Apollito "Testing Loudspeakers" might be a good one to consider (counter intuitive considering the title, but trust me). No calculus required; let LspCAD do that for you.