Need help with PLLXO High Pass filter for full range's!

I've built a couple of 7l vented boxes for TangBand W4-1052sa's for use as full range computer/small HT speakers, and I'm powering them exclusively with a Sonic Impact T-amp.

I used a SBB4 alignment, which gives me good power handling and extension down to about 55hz, but below that the t-amp makes those litte 4" cones go nuts trying to play low bass. I simulated different filters in WinISD, and settled on a 2nd order high-pass filter at 65hz as the best balance of power handling and extension.

Problem is, the components for a high-level filter at that frequency (and at 4 ohms) would be:

A. Huge
B. Almost as expensive at the rest of the project
C. Detrimental to efficiency/distortion (?)

Then I read Dave (planet10)'s primer on PLLXO's: http://www.t-linespeakers.org/tech/filters/passiveHLxo.html

Seems like this would be the perfect solution and fit with the bargain-basemet hi-fi aesthetic of the whole thing. Problem is, I've never designed a filter before, and the PLLXO stuff is particularly confusing to me. I know I need to take the input impedance of the T-amp (10k ohms, AFAIK) into consideration, but I can't see exactly how this value fits into the equations for the 2nd order filter....

Also, will there be a problem since the two channels share a common ground at input on the 1/8" stereo jack of the T-amp?

... and is the choice of R1 purely arbitrary, or is there a reason for choosing 5k vs 10k, for example...?

I think so... I'll have to check when I get home tonight. What Q value do you think would model this filter?

And just to clarify, by "low Q", do you mean that the corner will be less sharp and the transision to a true 12db/oct will take longer (happen at a lower frequency)?

Thanks for the clarification on the filter formulas, by the way!

Using 10k for R1 and 111k for R2, I get a C1 of 245 nf and a C2 of 24.5 nf. Does that sound right?

So the f3 will stay at 65hz in this case, it's just that the slope of the filter curve at this point would vary somewhat...

lower Q = Lower slope at f3 = higher start of the rolloff
higher Q = Higher slope at f3 = lower start of the rolloff

...right?

If this is the case, it should work out fine for my design... I'll model a .5 Q filter later tonight and see how it looks.

Thanks for all the help! I was able to model a .5 Q filter, and I was a bit taken aback at first! The rolloff does start quite a bit sooner, so much so that my 65hz f3 turned into a 100+ hz f3!

So I played around a bit, and eventually wound up lowering the HPF to 50hz. I also tried modeling a larger box with a more traditional alignment to see it I could get more bass response, with surprising results...

The 100hz bump that the smaller SBB4 alignment has naturally combines nicely with the low-Q filter to make what looks like a higher-Q-filter rolloff. The larger alignment has a hair more extension in the very low end, but rolls off so early that the trade-off in midbass isn't worth it, IMO.

Here's the excursion plots at 10 watts for both boxes (QB4 is blue, SBB4 is gray):

...and here's the transfer functions...

I know... I've been following his thread here in the Class D forum. I had prototyped these boxes in cardboard first, and in these leaky, resonant enclosures I had overexcursion all the time. Most likely the tuning was a lot lower than my design spec.

Now in a proper box, I find the combo is pretty indestructable due to the inherent bass roll-off of the SI.

My thanks to Dave anyways - at least I learned something!