Why is my sub not working?

Recently i installed an amp, cap, and jl w3 sub in my truck. the cap is connected to the batterey and ground and the amp to the cap( both ground and power). when i turn on the stereo the amp turns on but the sub does not work. the cap shows 13.9V when on and 12.7 when off. when i turn off the stereo i here a poping or thud noise from the sub. what could be the problem???