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How to Choose an Amplifier.

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Member - Posts: 28
Member spacespace
Joined: December 05, 2009
Location: Arkansas, United States
Posted: December 16, 2009 at 11:55 PM / IP Logged  
oldspark wrote:
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 9 -- 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.)

Thank you for your reply, some of that info I had to read a couple of times for it to soak but I got it.

Member - Posts: 41
Member spacespace
Joined: February 20, 2009
Location: Washington, United States
Posted: January 27, 2010 at 10:07 AM / IP Logged  

great post haemphyst thanks!

but i had a question, when trying to find the "Efficiency" on an amp, you dont say what rms to calculate? for example im looking into buying an amp that says its 400w rms @ 4ohm's and 600w rms @ 2ohm's. i added up the calculations for 2ohm's first as i plan on running the amp at 2ohm's, but the numbers didnt add up. so i decided to retry using the 4ohm's settings and everything seems to add up nicely. what i dont understand is, will i only get efficient power if i run the amp @ 4ohm's?

so just to make sure, i was wondering if you could give me your input on if this amp is good for me or not. im planning on running a 600w rms sub in a single vented box @ 2ohm's. thanks in advance!  How to Choose an Amplifier. - Page 9 -- posted image.

800 Watts Total Output Power

600 Watts RMS x 1 Channel @ 2 Ohm

400 Watts RMS x 1 Channel @ 4 Ohm

Full MOSFET Circuitry

60dB Channel Separation

90dB Signal to Noise Ratio

22K Ohm Input Impedance

200mV - 4V Input Sensitivity

Frequency Response: 20Hz - 20kHz

THD <0.05%

Fuse Rating: 2 x 25A

Page of 9

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