this is going to sound like a total newbie question, but im running a 12 w3 d4, ive had an mtx 421d running it at 2 ohms pushing 350-400 watts to it rms, JL's site says 250 rms........is that each coil or all together??:( any info would be great.........i know stopid q....but hey gotta start some where
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1 JL 12 w3 D4
1 MTX 421 D Monoblock
1 Sony Xplod 75x4
Front: Alpine SPX-177A
Rear: JL audio VR690-CXI
The RMS is always for the entire speaker, and never broken down to individual voice coil. At least, not with the quality manufacturers, but basic tenets are being broken all the time now out of Korea and China...
Most competitors, it seems, "overpower" their subs, like what you are doing according to the info you provided. There will be no harm to anything as long as you take the time and effort to get a meter reading of the voltage output at the two terminals that the sub is connected to for the purpose of setting the amp to push 250 watts to the sub and not more. You will need info (if you don't already know it) on setting gain with a digital multimeter (DMM), and use the ohms law formula E=(P*R)^.5 to find the voltage readout for 250 watts. So, using the formula, if the resistance is 2 ohms, and the power you want is 250 watts, the voltage should be 22.36 volts AC to the sub at maximum clean volume of the head unit. You will probably be setting the gain back a little.
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Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.