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How to Choose an Amplifier.


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danjarus1975 
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Joined: November 25, 2009
Posted: November 25, 2009 at 12:09 PM / IP Logged  
How do I find a amp that has the exact RMS of my subs, I have four 2000w subs (1000w rms @ 4 ohm). When I shop for amps, I only see amp with an lesser RMS like 3200w never 4000w exactly. I want to run them altogether to make them 2 ohm stable! I see mono amps that are 4500w max (1500w RMS x 1 @ 2) will that power each sub to it's normal RMS? Or, where can I find an amp that will work for me?
Laron "Danja" Williams
laoz3 
Member - Posts: 5
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Joined: August 12, 2009
Location: Kansas, United States
Posted: November 26, 2009 at 5:21 AM / IP Logged  

hmm that is a good one but from my past experiece is that you would  not find the numbers to match up exactly. Either the rms on the amp is too low or too high and really i would perfer the amp to be lower. The reason for that would be just is that you dont over push the sub woofer.Most subwoofers have a green zone so to speak that a speaker can handle the constant power going through it and if your pushin it right at the max that ova time that subwoofer would blow.

laoz3 
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Joined: August 12, 2009
Location: Kansas, United States
Posted: November 26, 2009 at 5:34 AM / IP Logged  

sorry didnt see your other Q's r the subs single or dual. If there single voice coil then i would go 2 subs runin parallel basically + to + and - to - then to your amp if they r dual voice coil i would buy an amp that can handle a 1ohm load and run them in parallel again and so you would need 2 amps that can handle a 1 ohm load and then just basically connect 2 subs to each amp in parallel mode.

Also im not a pro at installing a system I picked it up from alot of trial and error and BELIEVE me I did alot of errors if any of this info is incorrect im sorry and if im right then im glad i can help u

danjarus1975 
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Joined: November 25, 2009
Posted: December 01, 2009 at 6:57 PM / IP Logged  

thay all are DVC!

Laron "Danja" Williams
danjarus1975 
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Joined: November 25, 2009
Posted: December 01, 2009 at 6:58 PM / IP Logged  

Sorry, I mean they are DVC!

Laron "Danja" Williams
briankmizell 
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Joined: December 05, 2009
Location: Arkansas, United States
Posted: December 07, 2009 at 7:49 AM / IP Logged  

I would like to thank you for such and informative post you have made. I have several questions for you about my set up.

I have two JL audio amplifiers, a 300/2 and a 250/1

The 300/2 calls for a 40 amp fuse, doing the math I think that equals out to 52% efficiency.

The 250/1 calls for a 30 amp fuse, doing the math this one should be 57.8% efficient.

1st question is, do these numbers sound right?

Now, my truck came equiped with a 55 amp alternator.

I have added two 12" electric fans that have a 50 amp relay on them. 

I have a electric fuel pump that pulls about 7 amps.

The max draw of all my compents would be approximately 180 amps (thats my guess)

I will be running a 2/0 gauge power wire from the battery to a distribution block and then from there two 4 gauge wires will be run, one  to each amp with a 30 amp fuse on one, and a 40 amp fuse on the other.

I will also add a 4 gauge wire running from the battery to the positive output post on the alternator.

2nd question is, would I need a 200 amp alternator, or would 140 amps be sufficient?

3rd question is, what size of fuse would I need to put in the 2/0 gauge power wire next to the battery?

4th question is, do I need a fuse on the 4 gauge wire between the battery and the alternator, if so what size?

5th and final question, will I need to upgrade anything else on the truck since I will be running such a high out put alternator?

If you need more info such as speakers and resistence of each one etc, let me know and I will get them for you.

Thanks in advance for your reply,

Brian Mizell

briankmizell 
Member - Posts: 28
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Joined: December 05, 2009
Location: Arkansas, United States
Posted: December 12, 2009 at 10:55 PM / IP Logged  
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oldspark 
Gold - Posts: 4,913
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Joined: November 03, 2008
Location: Australia
Posted: December 13, 2009 at 12:18 AM / IP Logged  
briankmizell wrote:

The max draw of all my compents would be approximately 180 amps (thats my guess)

... would I need a 200 amp alternator, or would 140 amps be sufficient?

You have answered your own question - 140A is not enough for 180A.
That's unless you run the 2nd alternator as a separate system for your audio (you can share a common ground, but not their power outputs).
Be aware that just because fans have a 50 amp relay doesn't mean they draw 50A. 10 Amps each is more common (ie, typically 80-100W each.)
Fusing between the battery and alternator is an interesting question. Older vehicles never had them.
Alternators are generally self-limiting in terms of their output current.   
The main reason for adding a fuse was for "normal protection" against battery discharge in case of shorted cables.
Some argue the fuse is also to protect the alternator diodes in case of a reverse connected battery, but I find they rarely do.
From the alternator's point of view, except as above, there is little reason for the fuse except where a short from a high-output alternator may become a safety issue.
The fuse sizing will be to protect the cable (ie, not above the cable rating), else whatever battery or alternator maximum or short-circuit current limit is desired. (Many vehicles have fuses less than the alternator rated current, but they will usually have loads taken off the alternator - eg - headlight power.)
The above are not answers per se, but are considerations only.
If the above change your implementation, then some trivial dribble follows...
Where 2 alternators are possible, many keep the existing system intact and add a second alternator with its own battery to power the audio system (or whatever), and add sufficient ground straps (that's something that is often overlooked - hence resulting in a "electronic fry" situation! How to Choose an Amplifier. - Page 8 -- posted image. ).
If only one alternator is possible, a new alternator that covers standard plus additional "average" load is substituted.
Although a bigger battery may be used, usually a second sealed battery is added as close as possible to the heavy current system - eg, an AGM battery next to the audio amps - with the 2nd battery connected to the alternator ONLY whilst the alternator is charging.
This means localised heavy wiring only with few voltage-sag problems (ie, no caps needed).
The alternator to 2nd battery cable need not be as heavy as the normal audio amp cabling - it only carries the charge & load current to the second battery, hence should be rated to the alternator's max current output.
But the latter alternator to 2nd battery current will be limited by the size of the relay used. (Typically 60A or larger with a circuit breaker; could be fused if the cable & relay matches or exceeds the alternator output.) (This is separate to the 2nd battery's own fuse/protection.)
08canyon 
Member - Posts: 24
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Joined: December 02, 2009
Location: Michigan, United States
Posted: December 15, 2009 at 2:21 PM / IP Logged  
OK... For some reason this is confusing me. I have an RE Audio XT-800.2 amp, 100W RMS x 2 Channel @ 4Ohms, unit has a 25 amp fuse in it. Is the advertised RMS for real?
oldspark 
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Joined: November 03, 2008
Location: Australia
Posted: December 15, 2009 at 4:51 PM / IP Logged  
Why the confusion?
25A x 12V = 300W   (based on a literal "12V" system)
25A x 13.8V = 345W ("normal" rating voltage)
25A x 14.4V = 360W   (common max system voltage for lead-acid batteries)
All exceed the claimed 2x100W = 200W rating.
(All figures above are RMS - I rarely quote anything else unless denoted!)
If you are still confused - do NOT continue reading.....
FYI - a common electrical practice is to run fuses at no more than 70% capacity - ie, 200W/.7 = 285W minimum "fuse rating" (divide by whatever voltage is applicable).
But, some devices may run fuses higher than that if the device doesn't tolerate a 42% overload etc (ie 70% x 142% = 100%).
And the amplifier's inefficiency must be added to the above 200W.
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