Sub xover for chip amp

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I'd like to incorporate a simple low pass filter in my chip amp to run a small subwoofer. One requirement is that it must have a variable cutoff frequency. I was surprisingly unable to find anything in my searches online that I wanted to use so I came up with this idea.
See the attached pic.

I think what I sketched is a 2nd order Sallen-Key Butterworth lowpass filter with unity gain.

Can someone tell me if I am on the right track with this design?

From my calculations with the 5k resistor and pot set to zero resistance the low pass will be at around 225hz -3dB.

With the 5k resistor and the pot set to 20k ohms for a total of 25k ohms the low pass will be around 45hz -3dB.

I used this formula: R = .707/(2*3.14*F*C)

Will this work? Is there a better way? I'm trying to keep it simple. I've never designed a crossover before.

Thanks,
Dave
 

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Yes, it has been a long time and I have gotten bogged down with work and other projects. So I have not made much progress. I plan to incorporate this design into a six channel preamp that is on my list of things to do. I will be using OPA132 or OPA134 opamps for the filter, powered by +-15 volts.
 
Variable X-over frequencies

Hey all:

I actually built a variation of the original X-over posted by DC Dave about 12 yrs ago, and it worked very well! My ckt was an adaptation of the two-taw x-over that first appeared in the July 1988 issue of Audio magazine. (At least, I think it was July 1988).

This was about the time that old-school car audio was the rage, and few brave souls were venturing into the DIY realm. This x-over was fixed at 120Hz, but I made it variable by replacing the two 100k resistors with a 2-ganged 100k linear pot, each section in series with a 11.3K 1% metal-film resistor. This yielded nearly the same 45-225Hz x-over frequency range.

Since it was a fouth-order Sallen-Key, it was in F and I could select between 12dB and 24dB per octave, simply by selecting where the output buffer was connected.

I also did its hi-pass counterpart for a complete two-way stereo x-over, slope-selectable, continuously variable independant left- and right-channel crossover.

Having the magazine article provide the formula for the cut-off frequency helped me choose the range I wanted, and the values of the tuning resistors to use.

If I find any information on this, I will post it here.

Cheers,

Steve
 
Ever thought of making both sections variable?

Actually, I did. But the problem was in pot. I couldn't find one that is 4-ganged, and didn't want to go in trouble trying to phisically connect two stereo pots so they turn together because I have no idea how to do that without destroying pots. If you know some easy solution, please let me know.
 
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