Common Ground

[Jores]

So I'm upgrading the stereo on our 242 limited- adding an amp (kicker kxma 800/8), adding a sub (infinity), and the mid cabin speakers (polk). There are two pair of rca cables coming out of the head unit (polk) one pair labeled rear rca line out, and one pair labeled front rca line out- do I connect both pair to my amp? Next if I want to ground the hu to the new amp (common ground) do I splice into the hu harness ground (b5) and can I piggyback it to the 4ga ground cable that connects to battery ground? If not where might I connect? Thanks in advance, John

 

[tdonoughue]

John,

Yes, the RCA's all go to the amp. You are going to use the pair for the front to be the inputs corresponding to the front speaker outputs; rear for the rear.

Sounds like by 'ground' you are referring to the negative (not technically a ground on a boat). To common ground, you are going to attach ground to ground between the components. You can run the negative of the amp and the negative of the HU to the same wire to run back to the battery negative pole, but you need to make sure that the wire running back after you combine the two is of sufficient size for the run length and amperage requirement of the combined load (the amp and the HU).

We have a table around here somewhere that shows power cable sizing...

 

[Jores]

Thanks for the quick response- the kicker amp has four rca inputs but you don't have to use them all- I was planning on running 2 pair speakers at 4ohm (bow and swim deck) and 2 pair in parallel at 2 ohm (cabin), and two channels bridged for the sub- so not sure what you mean as to front or rear- not sure if the inputs on the amp correspond to speaker outputs. Negative wire is 4ga, John

 

[tdonoughue]

Hey, John. The HU will usually have a front/rear, meaning you can use the fade adjustment to decrease volume on the front or on the rear set of channels. On your setup, that means that one set of RCA's can be controlled separate from the other set of RCA's, but not fully independently (you can't adjust only the front or only the back--it is usually set up so that you are 'moving' the center of sound from the front to the back or vise-versa).

In your setup, that means that your bow and swim would be on one (call it 'front', just for fun) and the cabin would be the other (call it 'back'), so that you would be able to adjust the cabin separately from the bow/swim. But then, wired that way, you would not be able to adjust the bow separate from the swim. Some people will wire the cabin and bow together ('front') and the swim deck separately ('back'), as those are the two that are most likely to have different volume needs.

As to your negative wire, the missing two pieces of information, then, are, a) how far is your run from the farthest of your amps or your HU, and b) what are the amperage requirements of each of your amps and HU?

 

[Julian]

If you never plan to fade between the speakers (front to rear) then you could just run one set of RCAs to the amp (most amps have a 2-4 channel switch) and switch the amp to 2 channel.

 

[Jores]

Thanks, Front to rear fade might be useful as you said. So the amp has four pair of rca inputs and these correspond to the eight channels? Since the swim deck speakers would be on there own channel I could connect them to channel 7/8 and the corresponding rca on the amp (rear)- bow, cabin, and sub could be connected to 1/2, 3/4 (cabin in parallel) and 5/6 bridged for the sub and connect the rca to amp at first set of rca (front) and I think there's a fader switch for each set of channels - its all a little confusing and I have read through lots of stereo threads. The amp requires 4ga wire so I bought from KnuKoncepts and will run new wires to all speakers as well. John

 

[tdonoughue]

A couple of options. Really it works the same. I would put the sub on 7/8 bridged (just to be a little more consistent with the norm--lots of people put the sub on the last channels). Then, I agree with your 1/2, 3/4 assignment--that can come from the front set of RCA's on the HU. 5/6 can be the back for the swim deck. Not a big thing--like I said it will work the same.

What that will allow is for you to switch off the swim speakers (effectively), by fading all the way forward, or switch off the 'tub' speakers by fading all the way back (like if you have someone napping in the boat while others play out back).

The amp requirement of 4ga wire is what you want to check now that you are adding the HU to the circuit. The 4ga requirement is a function of the amperage draw an the length of the run. Try this: Google Image Result for https://images.crutchfieldonline.com/ImageHandler/scale/978/978/products/2012/47/12c/Gauge-Chart.jpg