Car Stereos: subwoofers, watts rms, 4 ohms

Question
QUESTION: hey i just bought 2 kicker L7 subs and i was wondering what would be the best amp to use with them?

thanks

ANSWER: Hi Matt,

L7's come in a few different sizes and impedance ratings, so this will affect the amplifier you'll want to choose for them.

Here's how to figure out how much power you need:

First, you'll have to calculate the final impedance of the two subwoofers.  You can find impedance calculators and wiring diagrams at several manufacturers' web sites; I like to use the support page at JLAudio.com.  If you have the 2-ohm version of the L7, then the final impedance at the amplifier will probably be 2 ohms; if you have the 4-ohm L7, then you can choose to make the final impedance 4 ohms or 1 ohm.

Then, you'll want to look at the RMS power rating of the sub.  The total system power handling is the sum of the RMS power rating of all the subs.  So if you have two subs rated for 500 watts RMS, the two of them can handle 1000 watts.  (You should disregard "peak" or "max" power ratings, both on the subwoofer and the amp).

As a rule of thumb, you'll want an amplifier with an RMS power rating between 50% and 100% of the subwoofers' power rating, at the subwoofers' impedance.

So, for example, let's say you have the 12", 2-ohm version of the L7.  This sub is rated at 750 watts RMS (if it's used in a sealed box).  Two subs together would be a final impedance of 2 ohms, with a power handling of 1500 watts.  You'd want to look for an amplifier rated between 750 and 1500 watts RMS at 2 ohms.  There are quite a few good amplifiers that fall in this range.  

If you had the 4-ohm version of the 12" L7, you'd still be looking for 750 to 1500 watts of power, but the impedance rating would need to be 1 ohm or 4 ohms.  Amps with these specs are available, but a little harder to find.  Most mono amps produce their maximum rated power at 2 ohms, but there are some 1-ohm stable amps available, as well as a few amps that can produce that power level into 4 ohms.

I'd recommend looking for amps with CEA-2006 compliant power ratings, so you can be sure you're getting the power that the manufacturer claims.  Also, if you're planning to use a ported box, look for an amplifier that's equipped with a subsonic filter.  

Hope this helps!

Brian



---------- FOLLOW-UP ----------

QUESTION: the subs i bought are actually two 10" L7s. I'm not sure the ohms because i bought them used from a friend of mine but i will ask him. could you suggest an amp for the 2-ohm and and 4-ohm 10" subs.
thanks

Answer
Matt,

I think an amp in the 1000-watt range would be a good match for your subs.  If you've got the dual 2-ohm version, then some good choices would be Kicker's ZX1000.1,  Alpine's MRP-M1000, or MTX's 1000XD.  Each of these amps is rated to produce 1000 watts RMS with a 2-ohm load.

If you have the dual 4-ohm version, it might be a little harder to find an amp with output power comparable to the ones above. You'll either need something that's 1-ohm stable, or one that can produce around 1000 watts into a 4-ohm load.  Some amps to consider would be Rockford Fosgate's P1000-1BD, or Alpine's PDX1.1000.

Hope this helps!

Brian