50x4 is 200 watts although in reality no head unit is 50x4 RMS. its really about 20 watts x 4 RMS. if the sub is connected to an amp, and the amp puts out 200 watts at the load presented by the sub, then your sub is getting....
And no you wouldnt count the extra 20 watts because first, HU's are made to power the inside speakers or components. And second of all you cant wire a HU and an amp to the same sub, it'll destroy something.
the amp is the ONLY thing driving the sub. therefore the sub is getting 200 watts. period.
if the head unit is 20 x 4, then each of the four speakers in the car's doors, dash, or rear package shelf are getting 20 watts each for a total of 80 watts divided between 4 speakers.
o ok, I got it the head unit is just giving a signal to the amp and it is giving no additional wattage. Thanks for clearing it up. Im noob but im still learning.
you got it right on the head. the head unit pre-outs are a low voltage signal. a "line level" signal, unamplified. the amplifier takes that clean, noiseless unamplified signal, and boosts it to drive the speakers at a high voltage derived from the current drawn by the amplifier from the car's electrical system.
the head unit often has a smaller 4 channel amplifier built into it to drive the door speakers, but that's separate from the pre-outs that the amplifier(s) use.
