I found myself with time on my hands and enough parts to build a subwoofer and a plate amp (based on The Bog Standard) so I figured "why not?". I also needed a filter, so I drew one up. It's probably pretty naive. Just a simple preamp/buffer and a second order Sallen Key filter. I want to be able to set the cutoff frequency, so I made one of the resistors in the Sallen Key-filter variable. This works in SPICE as well as on breadboard. But is this too naive? I'm also getting about 50-60 degrees phase shift at the cutoff frequency. Should I be worried about this? Ideas for improvement are appreciated!
A second order low pass filter response should drop at 12dB per octave.
Each "order" imparts a 90 degree phase shift.
A Butterworth filter should be -3dB at the crossover frequency, Linkwitz Riley -6dB at the crossover frequency.
I don't know enough to give you any idea of how to make your filters respond like classic filters should, but using a scale of 3dB (rather than 4dB) per division makes it easier to see if what the filters are doing looks "normal".