Nowadays the LCD and plasma technologies have become pretty cheap and big screen TV’s are more affordable. But your movie experience isn’t complete without good sound. DIY is a very good solution for a home theater speaker setup and subwoofers are the easiest component to build. Once you found a good speaker driver and made a cabinet according to it’s parameters all you need to do is give it a place in the whole system.
Active filters are the best solution to integrate the subwoofer in the HT. This way you add more control, flexibility and you eliminate negative influences of passive filters. The above circuit offers a very simple but effective solution. Using only a handful of electronic components cost is very low. The filter, as it is presented, has two main stages. The first stage, using half of the TL072 IC, sums the “left” and “right” channel into a single channel. By varying R3 you can adjust the gain.
The next stage forms the filter itself. It is a second order filter with 12db/octave slope. The corner frequency is set by the values of R5&7 between approximately 20Hz and 100Hz. You will need to set this frequency to match the other speakers, room and placement. If you don’t have any measuring equipment than trial and error is the way to go. Now there is one more thing you can do to make it really good. If you look at the first stage you will see that the signal will be 180 degrees shifted in phase. This may or may not affect your sound, it really depends on your room and speaker placement. However it would be good to add one simple inverter stage before the filter with gain set to 1 and with a bypass switch. This way you can select 0 or 180 degrees phase shift.
Subwoofer Active Filter: [Link]