Sub with standard headunit help..

[MikesDman]

Hi iv amped my standard headunit and im trying to hook up a sub to it, having a little problem as the sub plays like a big speaker with vocals etc etc..is there any think i can use to filter it to bass levels? any help would be good.
Cheers.

 

[bmh.01]

Set the crossover on the amp to about 80hz.

 

[MikesDman]

Ohhh ok cheers!