I made a subwoofer a about a year ago old woofer no amp, so I added an amp from some old PC speakers I had. I put a filtercap in sires with it thinking that would let only the bass through. because it would charge up then release driving the woofer at bass frequency. talking to friend other day said inductor not capacitor, but not sure what to do for inducter, I have some really small tube ones and some from SMPS for filtering AC, but not sure those would work, or wether need biger like a small transformer or make one.