help in lowpass filter curcuit.

I am in need of a active lowpass filter for my subwoofer project. Signal input is stereo and through a mixer, given to the filter and output is fed to the amp. What changes can be done to make it as a high pass filter. Any circuit is available using low noise op amps, it will be greately appreciated.

Here's the simplest active low pass filter.
**broken link removed**

Here is a Linkwitz-Riley electronic crossover for a sub-woofer and the main speakers.
**broken link removed**
It has slopes of only 12dB/octave but another project there has slopes of 24dB/octave.
The Linkwitz-Riley alignment has each output at -6dB at the crossover (unlike the -3dB with Butterworth filters) so that the outputs do not produce a +3dB boost at the crossover frequency like a Butterworth filter when the outpouts are added.
Like all even-order filters, one of the filters must have its phase reversed, or they will cancel in a notch at the crossover frequency.

Thanks audio guru..thats what I was looking for! Looks complicated but must be a goody.