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Discussion Starter · #1 ·
Hello all, (If you don't like reading a lot, stop now! :boring)

I know questions like these have been beat to death the internet-wide, but I have a peculiar problem and I'm curious if anyone has dealt with it.

My truck (a 1981 Landcruiser BJ42) is on a 24V system, in order to achieve better voltage for cranking over the diesel. (Toyota built it that way, not me) I just installed a 24V -> 12V power converter rated at 12A to run a stereo system. This is practically a bare minimum, but I'm only running 2 6.5" speakers anyways.

Given that, I'm curious if it would be possible to run a single 10" sub, at 4 ohms and 125 watts RMS, off of a monoblock sub amp, along with the two speakers and deck, while coming in under 12 amps and not destroying my power converter.

Here's some math: (from: http://www.crutchfield.com/S-oIs0eqol2z5/learn/learningcenter/car/amplifiers_faq.html)

"To calculate the current draw of an amplifier, multiply the number of channels by the RMS watts per channel (a 2 channel amp rated at 300 watts RMS per channel would be 600 watts). Double it to account for amplifier inefficiency (600 watts X 2 = 1200 watts), then divide by the average output Voltage of an alternator, 13.8 volts (1200 divided by 13.8 = 87 amps). Since the average music signal requires about 1/3rd of the average power in a test tone, divide by 3 (87 amps divided by 3 = 29 amps). The result is the amplifier's approximate average current draw."

So mine comes out to 125 doubled to 250, divided by 13.8V, divided by 3, equalling 6.03 amps.

Does that sound correct at all? If that's true than it should be no problem for my converter (as the deck must draw only 4-5 amps with 2 speakers), but I wonder if anyone has dealt with this.
 

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add an extra 12 V Alt and seperate battery....nuff said
 

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Discussion Starter · #3 ·
add an extra 12 V Alt and seperate battery....nuff said
I was thinking of that, but I have the converter now and another alternator amounts to a significant project, as I don't have much space for a 3rd battery and lack the money and skills to install an extra alternator. (I may do it in the future when I have some cash)
 

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6 amps sounds correct, but don't forget to include the amperage drawn by your CD deck as well (probably realistically close to 6 amps as well), so you'll be maxxing out your converter pretty quickly. 12 amps isn't a lot of power to suck up in a hurry.

Would a capacitor help? I've never run them but I hear they work well. My other suggestions would be either a) install another 12v battery and use the 12 amp converter to charge it and run your 12v accessories off of that (battery would act like a buffer between the stereo and converter), b) run two converters, one for your amp and another for the deck, or c) try to find a larger amperage/wattage converter and possibly keep the 12 amp unit for small 12v accessories like cell phone chargers, ect. I've never had to deal with converting over from 24v sources so I'm sorry I don't have any other suggestions. LR
 

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Discussion Starter · #5 ·
I guess it's time to get out the trusty multimeter,
 

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Discussion Starter · #6 ·
a) install another 12v battery and use the 12 amp converter to charge it and run your 12v accessories off of that (battery would act like a buffer between the stereo and converter),
Actually, I do have another battery sitting around. It's been there for a while but it still has some charge, just needs to be powered up. I may give this a try, although without an alternator I'm thinking the battery won't last very long powering a sub before it dies. (I want to avoid killing my primary batteries so it'll only charge when the truck is running)
 

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12 Amps for a complete stereo system including 10" sub isn't going to cut it. And 6amps is not the correct figure. That is for average power draw. You need to calculate based on maximum power draw.

A better way to calculate amperage is to use the following equation: Watts = Voltage x Current

AKA Current = watts / voltage.

125W / 12V = 10.4 Amps

You'll need something closer to 20A.
 

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That initial calculation sound bogus. For starters, amps are a lot better than 50% efficient. Second, most of the power goes to dynamic range. In your car or room the continuous power is probably less than one watt and rest is highly transient (which is absorbed by the amp's power supply). Of course that depends on the music you listen to and the volume you like to play. I measured my 100W home theater sub's power consumption and the most it used was 20W (using a 120V wattmeter). For your 125W I'd guess it would draw less than 2A at 13.8V. The best way is to measure it (on the 10A scale) with a multimeter.
 

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I think you need a simple to DC to DC convertor to run the stereo and amp.
 

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Discussion Starter · #10 ·
I have a DC -> DC converter. . . The problem is whether or not the 12A it supplies is enough to run what I have listed. Maybe my friend will let me test his system.
 

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DC ammeters are cheap now, just go buy one and then you can test it real time rather than doing math
 

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Discussion Starter · #13 ·
Yeah I have a Motomaster Multimeter. . . (I know I know. . . But it was cheap and I've had good look with the Canadian Tire brand) Just waiting for some nice weather to get some work done, plus I'm starting school, blah blah blah.

BTW the sub is a JL Audio 10" WX Series @ 4 ohms, 250W Max, 125 RMS. I'm unsure of what amp I will get but it'll be some variety of single channel amp for 125W RMS. Don't plan on over-juicing the sub.
 

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is it 2 12 v batts in series?
just grab the pos and neg from one battery, it will still be 12 volts, and will charge fine. then you dont have to worry about draws and such...
 

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Discussion Starter · #15 ·
I have thought about this, but I have also heard from several other owners of these trucks (on other forums) that this is a bad idea, as drawing voltage from just one of the batteries can cause an imbalance that will slowly but surely wreck both batteries. I'm not exactly sure of how this happens.
 
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