DIY active variable sub x-over?

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How difficult would it be to make to a quality sub crossover with variable frequency cutoff, variable gain/output, and finely variable phase?

I have a Paradigm x-30 that is great, but it takes line-level inputs & only provides summed mono output.

I'd prefer to maintain true L/R stereo output and speaker-level inputs. Line-level RCA won't work for my new setup because my preamp(TVC) is a 40 ft balanced cable run from my monoblock amps next to my speakers.

Any suggestions or websites with schematics?

Thanks for reminding me about Elliot Sounds, I should thought of that myself. Rod's got a solution for just about everything there.

I probably should have given more information about my particular system. I've got 97dB Cain & Cain Double-Horn IM-Ben (1/2 way down page) speakers which run their Fostex FE-168 Sigma 6.5" fullrange w/o x-over & T900A supertweeter nearly fullrange with adjustable, second order 12 db per octave x-over @16.5khz on the super tweeter.

The speaker's overall frequency response is ~40-35kHz.

For subwoofers, I have a pair C&C's passive Baileys . Terry Cain, the designer/manufacturer, suggest crossing the Baileys in at somewhere around 80Hz, adjusting for room specifics.

So, I won't need the high pass portion of a crossover, just the LP. I'd prefer to keep the FE-168 free from any x-over.

"78 3-Way 12dB/ Octave Crossover " Buffers, HPF/LPF.Simple

How about the 24dB 24 dB/Octave 2/3-Way Linkwitz-Riley ?

I'm just guessing that the steeper 24dB slope would perform better. Is this true?

Speaker level inputs = Voltage dividers.

Hmmmm. For the short term, I could make a little voltage dropping unit to plug into the inputs of my paradigm X-30.

Do you have any idea what the ratio would be? Or, what the likely voltage output at an amplifiers binding posts would be?

I have a schematic from Electronics world which has variable phase + filtering for a servo sub.

I'd like to see the variable phase section.

That link is a good one!!

What you want is a flat frequency response.
Sealed enclosures roll off at 12db/octave while ported,24db/octave(for the most part)
so if your main speakers are ported...youl want a 24db/octave LPF on your sub with the same F3 as your mains,as a start.

A variable cutoff frequency is highly useful.
The schematic on Isaacs site is of a kit, which I've built. It's not bad, has quite a bit on it, defeatable 3rd order rumble filter, one band of parametric eq which is semi useful. I'd prefer a 4th order slope, since it would more easily more easily match vented mains, or if they are sealed, then it's better to have a 2nd order HP on them.
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