You will need to connect your sub in line if you purchase a unit with onboard amplification - that is each speaker wire would go to the sub then out to the respective speaker. Both speaker wires (assuming you aren't biwired) must go to the sub. If you use a sub that requires separate amplification you will need to attach an out board amp - if your existing amp has pre-outs this is easy.
Regardless of where the amp for the sub resides (I am willing to bet you will want to buy a sub with an amp in the unit) you will need to set the frequency range you want the sub to operate in on the subs amp - the low end is usually part of the speaker spec and the amp allows a high pass at say 80+hz. This setting is going to be based on your main speakers capability. I suggest you be reasonably certain of the effective Low freq on your mains and set the sub to augment near, preferrably 10-20hz above, the point where your mains have a 3db drop in SPL.
Not knowing your existing speaker specs this is the best I can offer. As always an SPL meter goes a long way in setting up a good match. Low Freqs are generally less demanding on tone and timbre match, so a good quality sub that lives very near the manufaturer specs is generally easy to integrate. If you need low end puch for music it is also generally a very good addition to your system. And, of course, let your ears be the judge...
I have been thinking about a Paradigm PDR series sub. From pictures on the internet, it looks like they have speaker level inputs, but not outputs. Surely this means all the low bass signals are still being sent to the main speakers as well. When using the speaker level inputs is this level brought down to line level again so the onboard amp can still do the work? My speakers are Celestion f10s. From their website, the frequecy response is 70Hz to 20kHz ±2db.
Ben, Best to find one with high level outputs that have a high pass filter on the outputs. There are many fine brands that do. You are right that the sub can step down the high level signal to a low level signal for the internal amplifier to work with. Try and audition some subs with your speakers aswell so you can match the quality of sound coming from the Celestions. Something boomy or bloomy may sound contrasting alongside the clarity of the Celestions.
Thanks for the replies. I was wondering, how easy would it be for me to make my own variable frequency crossover. It could have left/right inputs/outputs and a single subwoofer output, all at line level. Does anyone have any circuit diagrams for this? Thanks
Ben, Variable frequency crossovers are somewhat complex-- because of the need to simultaneously change multiple resistance values as you adjust frequency. There are some schematics out there, though. If you post in the DIY section of this forum, you should get some helpful replies.
I've built quite a few fixed-frequency crossovers. These are quite easy. Link Removed to a 24dB/oct Linkwitz-Reilly (my favorite). You can't go wrong with Rod Elliott's projects either. check this one out. You can buy PCB's from Rod as well.