Some one please explain this to me. A head unit has 4 channels. How does the 4 channel setup work with one subwoofer? How do 2 rear channels become one subwoofer channel? Do you take the average of the two signals and send it out to subwoofer? Thanks.
Not sure what you're saying here. What exactly are you trying to do? Not trying to question your intelligence, but I have to ask- Are you trying to run the subwoofer off the head unit using the rear speaker connections? Never heard of that before.
Some decks have sub output RCA and have features in the deck to control the sub (XO point, level...ect).
If you have 4 channels out and no sub output you can use 2 of the channels (R&L) as CA is typically only stereo.
(4 channels is to allow you to fade between front and rear but are the same signal).
When you plug these into an electronic XO or amp with XO with bypass it takes the sub signal and separates it and this sub output is the signal used for the sub amp. You could use the front 2 rcs outs to run the mids/highs and the rear output into a sub amp with XO. The fader on your deck will allow for sub level changes.
If you are not talking RCA and speaker output of the deck you can use a high level adapter or find an amp that accepts high level input.
...and no, you can not power a sub off deck power....I mean you can but it would be the same as trying to swap your SUV motor with a lawnmower engine.....it might move forward but is not going to get you anywhere.
My old Pio Premeir 690 which is in the lady's ride now has the option to run one or two subs off the rear channel. You can actually bridge the radio and change a setting to have it low pass only. Not saying you'd get much but if you had a very efficient driver and weren't looking for brain-scrambling bass it's an option.