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Old 8th June 2005, 12:12 PM   #1
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Default Powering a car amp

I'm installing a subwoofer into our boat (also happens to be my first car audio installation, so I'm kind-of unsure on some things). First, the speaker is a 12" 8-ohm Rockford Fosgate Punch Z. Not that good, but it works. The amp is even cheaper, it was a $40 Baja so-called 400W 2-ch MOSFET Amp (heatsinks look like they belong on a 100W amp though) running in their version of a bridge mode - invert one amp and not the other, which is correct, but don't send the same signal to the amps, but instead still send left to one amp and right to the other! Their logic is it sums both left and right channels, except when a sound is different on each channel, there goes half of the power!

Anyway, I have the power antenna going to the remote in, RCA cables going from the radio to amp, the crossover set to low pass at ~60hz.

All thats left is to run power from the battery to the amp. I figured I would need roughly 20ft of wire (goes from about the middle of the boat to the back then all the way across to the battery), and the manual suggests I use 10-12 guage wire with a 40A inline fuse.

But, the manual says connect the ground to the chassis, assuming it is a car. Since the boat is different, the chassis isn't even connected to the ground. What I need to know is can I just run positive to the positive on the battery, and ground to the ground on the battery?

Also, how much power do amps usually draw when they are off waiting for a signal to turn them on? The boat isn't used anywhere near as much as a car, so I'm worried that when it sits and doesn't have an alternator going to keep the battery charged the amp might drain it.

Thanks!
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Old 8th June 2005, 02:10 PM   #2
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To answer the first question yes you can hook up straight to the negative side on your battery, but you don't want to run a ground that far. The general rule in car audio is to keep the ground 12 in. or less (some say 18 in.) I'm not that familiar with boats but if it has gauges then there should be somewhere to ground the amp where the gauges and other electricals are grounded. And also for your power and ground wire I would suggest moving up to 8 AWG for a length of 20'.

As far as current draw I would get a resettable circuit breaker and use that to disconnect the power when not in use. You can find these at most car stereo shops. They are not too expensive and serves as a switch/fuse. I know Stinger and Phoenix Gold make them, but I'm sure there are several other companies.

And a side note I would cross that sub over a little higher maybe 90-100hz.
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Old 8th June 2005, 07:40 PM   #3
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Thanks for the reply.

The amp is mounted right behind the head unit, so could I just ground it directly to the metal on the head unit, or the ground connection on it?

What would happen if I had to run it to the battery? Is that just a risk of a ground loop, or am I risking damage to the amp or electrical system in the boat?

I will probably plan on getting one of those breakers then. Just out of curiosity, how much power do most amps drain when they aren't on?

Thanks again,
Mike
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Old 8th June 2005, 09:23 PM   #4
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one more question.

There is no power going through the ground, right? So I shouldn't have to use an 8-10 gauge cable to connect the ground, right?

I think I will ground the amp by running a short piece of wire from gnd on the amp to the metal chassis of the head unit. This will have them grounded at the same place and therefore have no ground loops.
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Old 9th June 2005, 04:14 AM   #5
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Yes power is going through the ground. Actually on D/C power flows - to + in a circuit. So you need to use the same gauge wire for both the ground and power. Yes the only real problem with running your ground that far is the ground loop.
As far as current draw I can't give you a hard number because there are a lot of variables involving battery, amp, etc.
BTW don't ground it to the actual ground wire on the head unit. It needs to be a metal contact obviously so check around screws, bolts, etc. and you should find something. Good luck and let me know what happens.
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Old 9th June 2005, 09:39 PM   #6
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Quote:
Originally posted by darth_audio
Yes power is going through the ground. Actually on D/C power flows - to + in a circuit. So you need to use the same gauge wire for both the ground and power. .............It needs to be a metal contact obviously so check around screws, bolts, etc. and you should find something. Good luck and let me know what happens.
The screws/bolts, if they are grounded, are probably grounded using thin wire, which will be to small to handle the current.
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Old 13th June 2005, 12:07 PM   #7
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Is this a glassfibre boat? If its steel, then normaly one don't connect the body to ground. (Has to do with corrotion.)

You should run 2 heavy gauge wire from battery to amp, one for + and one for ground. (The current in the groundwire are allways exactly the same as in the + wire, so both wires should be of the same gauge.)
Get ground (and +) for det headunit from the amp. This will depress groundloops for the soundsystem.

To check the currentconsumtion during off-state, just connet the ground on the amp to a powersource (battery), and use a multimeter (is this what you call the testinstrument? (english not being my native language)) sett to current measurement and connect it between the + of the amp and + powersource. A good idea would be to use a wire between the +'es and then removing it before you hook up the multimeter. This to prevent charging the caps inn the amp throu the multimeter which posibly could take out the fuse in the multimeter. I suspect a powerconsumtion well below 100ma. Thoug the currentconsumption are small it is, as sugested earlier in this tread, smart to disconnecting the powersource trough a autofuse. Smart, easy and safe.

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Old 13th June 2005, 12:20 PM   #8
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Thanks.

So, if I understand you correctly, as long as I run both the amp and the head unit off of the same two wires coming from the battery, I wouldn't have to worry about ground loops?

The only other thing I'm worried about is the amount of power it will use. The fuse blows at 40A, so it could almost draw that much, and I'm worried the battery won't be able to keep up with it. What do you guys think?
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Old 13th June 2005, 12:48 PM   #9
space is offline space  Norway
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Yes, if you hook up the ground of the headunit directly to the amp-ground, there should be no problems with groundloops.

At maksimum power (for audio) the peak powerconsumption will probably be close to just under 40 A, but the average will probably be well under 10 A, more like 5A, which will be LOUD.
There has been quite some hyping as to the size of the powercables to caramps. If you follow the instructions in the manual you won't be dissapionted.

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Old 13th June 2005, 12:54 PM   #10
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Thanks again.

I was more worried about it draining the battery than not being loud. Don't want to get stranded in the middle of a lake with no battery power to start it back up!!

And, my speaker is an 8-ohm (4-ohm per amp), and the amp can handle down to a 4-ohm in bridge, so it would be drawing only half of what it would had my speaker been 4-ohm.
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