Crna Legija wrote:If my car head unit has 3 50w rca pre-outs(front+rear+subwoofer) and i have a 150wrms amp will it add the 50w to the 150?
The head unit does not put out power out the RCA jacks. Sorry that won't add. You can put speakers on the head unit speaker outputs and jack the RCA outputs into a power amp which will only put out it's rating.
Please note that there is several standards for measuring amp power. Peak power is based on the peak output voltage into the minimum load impedance. This is an advertising gimmick to give wimpy amps a high wattage number. True power is measured in RMS average into a nominal impedance. Often a 25 watt RMS per channel amp will knock the socks off a 250 watt peak power amp. Read the specs.
One puts out almost 50 volts peak to peak. The other puts out less than 24 volts peak to peak. I used to measure amps with a scope and dummy load. Don't buy the cheap junk based on the watts printed on the box.
More info..
http://www.outrageousaudio.com/page_fil ... attage.pdf
The old school way of measuring wattage was based on the maximum undistorted (not clipped) sine wave the amplifier could drive into a stated resistance load. Often the maximum distortion is given such as 0.01% THD. For example my band amp is rated for 500 watts RMS per channel (not total, thats 500 by two) into a 4 ohm load with both channels driven. Most car stereo is not measured by this standard. Cheap amps measure power by 100% distorted clipped peak power total into a artificially low impedance.
For example that amp will drive the load at about 45 volts AC RMS at a little over 11 Amps per channel. That is not clipped at 127 volts peak to peak into a speaker. That is not in the same class as a 500 watt amp that is lucky to hit a 40 volt peak without clipping.
Info on electric power is here with an online calculator.
http://www.sengpielaudio.com/calculator-ohm.htm
I have only tested one car amp that got my dummy load resistors too hot to touch. That amp had a 60 amp fuse.
Here is some specs from a real amp. It shows what power it will put out into a given load with what maximum distortion. Cheap amps won't provide these specs.
Output Power (typical all channels driven):
4 Ω, 1 kHz, 1% THD, 324 W
4 Ω, 1 kHz, 0.5% THD, 275 W
8 Ω, 1 kHz, 1% THD, 235 W
8 Ω, 1 kHz, <.019% THD, 175 W
8 Ω bridged, 1 kHz, 1% THD, 650 W
8 Ω bridged, 1 kHz, < .08% THD, 550W
Frequency Response (-3 dB): <10 Hz to 85 kHz ±3 dB
Frequency Response (1 watt): 20 Hz to 20 kHz, -.4/-.25
Crosstalk (adj. channels): -74 dB, 1 kHz @ rated output
Distortion (THD+N, 20 Hz to 20 kHz) (80 kHz LPF):
at rated power: < .06%
at 1 watt: <05> 100
Dynamic Range: 100 dB below rated output
I should have it built in time to distract me from schoolwork this fall. 
