When you decide to add some more bass to your existing PA, you have different options: 1 you add a passively filtered sub in parallel with your main speakers if the total resulting impedance doesnít drop under the minimal load your power amp must see (not very efficient, youíll be disappointed with the result); 2 you add a passively filtered sub with itís own amp parallel to your main system (better); 3 you add a sub, itís amp and an active crossover (much better); 4 you use forte and a couple of good parametric EQ plugins to economize the purchase of an active crossover (perfect!). Of course, like this:

Diagram6

(XML mods apply, donít forget). After the Main program Equalizer, the signal is split in two: one feeds the main outs through a parametric EQ, and powers the main PA , the other feeds a secondary out through another parametric EQ, and powers the Sub bass bins. Why two parametric EQís, wouldnít one on the Sub-buss be sufficient? Well, might be, might be not. In fact, no. Thatís how active crossovers work. First, overlapping low frequencies can result in phase problems. Second, it is of no use whatsoever you feed high energy low frequencies into a speaker system that does not reproduce them acoustically but only transforms this energy into heat. Filtering these frequencies out of the main (top) system will lead to greater efficiency both for the amp and the speaker: you will notice more headroom and greater dynamics. Below are two screenshots (taken from the GlissEq plugin by Voxengo) of how such a crossover could be designed. The two filters are set to an 9 dB/octave slope, crossing over at -3dB at 100Hz.

HighPass
LowPass

The LowPass section of the crossover, inserted in the main (top) signal path:

and the HighPass section, inserted in the Sub signal path.

Thatís it! Be careful though, donít start such engineering unless you fully understand what you are doing. You might want to ask: couldnít we design a full 4-way crossover system this way? The answer is: yes we can, but I wouldnít risk it. If something went wrong, the above system will not be harmed. It doesnt damage a bass speaker if it is fed with high frequency content, might something crash in the computer. On the other hand, if something goes wrong and you send accidentally 200 Watts of full range audio into a high frequency horn, damage is immediate.

So here is where this little tutorial ends. Itís not exhaustive, but I hope it has given you some ideas, up to you to invent new stuff!

Sub