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Car Audio RMS & Ohms Calculation?

I am needing help to calculate how much RMS watts each speaker will get.

60w RMS x 4 amp @4 ohms

front speakers 45 RMS @ 4 ohms

back speakers 65 RMS @ 4 ohms

but I am going to connect them to two channels instead of 4 as stereo only have 2 pre outs (one for sub and one for speakers)

wondering how much in series or parallel the watts RMS and ohms output from the amp would be.

and any suggestions as to hook them plus a sub up with more efficient rms

2 Answers

  • 2 months ago
    Favorite Answer

    Would need to know what amp to answer the question.

    Wouldn't hurt to know what head unit either.

    HU's with just two channel pre-amps (just 2, left and right?  Right?) are not that common any more.

    With just one pair you can't use one for speakers, one for sub.  One is for left speakers, the other for right.  You'd need to use y-cables into the amp so all 4 channels have input signals.  CH 1 and 2 would be front left and front right.  Channels 3 and 4 would be rear left and rear right, or for you, it sounds like you'll be bridging them for a sub.

    Concerning running all 4 speakers on 2 amp channels -- this will work fine, but I can only guess how much power they'll get because your amp will be running at 2 ohms stereo and you didn't provide that information.  If an amp's 60x4 at 4 ohms it could be as much as 120 x 4 at 2 ohms or as little as 75 or 80, meaning you could get ~60w per speaker or 40.

    With a 4 ohm sub on the rear channels (bridged) you could get 150-240

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  • 2 months ago

    With all speakers being 4 ohm speakers, they will get the same power their channel outputs. When the amp outputs 60 rms watts into each of the 4 channels, all the speakers can get up to that much. If the speaker is rated for 45 rms watts that doesn't change what the amp can give, but it does affect how the *speaker* performs. 60 rms watts is more than a 45 watt speaker is rated to handle. Expect them to distort at higher than their rated power. The 65 rms watt speakers shouldn't break a sweat being given the full 60 rms watts the amp has to give per channel. You may be able to connect a 45 watt and 65 watt speaker together to share the front channel, then use the rear channels to drive an amp and sub woofer. 

    If your amp is bridgeable you can add up the per-channel power, connecting the front and rear channels together. You need to refer to the factory specifications of the amp to know if that is possible.

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