Are you using the stock head unit?


If so, I suggest using something like an Audio control LC-8 or LC-6i, located here in order to get decent signals to your amps.

If you splice into the HU feeds, you are going to reduce the signal quality. In other words, if you splice into the rear feed for both the rear signal to the JL and for the sub signal to the Alpine, you are going to hurt the signal quality, and the signal quality coming from the stock head unit is already questionable.

If you don't want to buy the Audio Control, then I would next recommend just undoing the input from the JL to the rear inputs, and using that solely for the sub input, and setting the JL to 2ch input, which will take the input plugged into the front, and use it for both the front and the rear. You will lose the ability for fade front/rear from the stock head unit, but you'll have good signal quality. You can set the gains on the amp so that your f/r fade is properly set (about 75% front, 25% rear is what I go with)

Otherwise, if you try and split an input by splicing into it, you'll cause problems.

Finally, I went with the stock head unit for a long time, thinking I could get good sound quality out of it with the right equipment. You can, it is possible to get good quality out of the stock head unit, but great quality is impossible. In other words, when I finally switched to an aftermarket head unit, I was amazed at the difference it made. I recommend a decent quality aftermarket head unit; any of the Alpines would be good.

Dave

 

My recommendation would mirror that of Caelric. The order of which I would recommend would be the following (if money were no object):

1. Upgrade HU to aftermarket (connecting JL slash amp using RCAs from front/read preamp outs on aftermarket HU). Connect Alpine sub amp to HU's sub-out preamp.

2. If not willing to replace HU, get the Audio Control. This will give you front/rear and sub preamp outs.

3. If not willing to go with audiocontrol using the bridging setup that Caelric recommends, which will cause you to lose your fading controls.

If I were doing your project, I'd ditch the rear speakers all together, but that's just me.

 

Go this way if you don't want to spend the extra money. My 3 channel Xtant 603 is connected like this in my sedan. I don't find the need to fade the front and rear speakers. And like others have said, you may want to eliminate the rear speakers altogether.

The benefit of connceting everythign in this manner is that you can use the fader on the stock HU to increase or decrease the sub level.

 

I would think that one of your inputs is out of phase. Maybe you didn't notice it being out of phase with rear speakers, but the sub would make it much more obvious.