Ok, I guess I'm just too lazy to dig out the books to refresh my memory....
The question is: I have a an Alpine deck that I just installed my car. The amp is rated at 26 watts RMS and 60 max at 4 ohms. I'm currently running it into 10 ohm stock speakers, so what is the output power like?
Full Version: Amp power question
About 10 watts and 25 respectively. 
Boojum is right, it works like this: p = v^2/z where p is power, v is voltage and z is impedence
For RMS of a single tone (which is the way RMS power should be measured) p = (v*v)/(sqrt(2)*z).
So, for your deck, playing 26watts RMS into 4 ohms, the output voltage is 12.13V. For 60W peak, the output voltage is 15.5V. These same voltages into 10 ohms will give you 10.4W RMS and 24W peak.
Now for the rant - considering your car battery and alternator generally supply your deck with somewhere between 12 and 14V, where does it get 15.5V for 60W peak? The answer should be (as is the case in standalone car amps) from a switchmode power supply which switches the DC from the car battery into high frequency AC, transforms it to a higher voltage, and rectifies it back to DC. However, no headunits I have ever seen contain a switch mode supply. Instead, they just lie on their power ratings.
Don't believe any head unit which claims to be able to put out more than 32W RMS and 46W peak into a 4ohm load. More realistic maximum ratings would be 25W RMS and 36W peak. This problem will of course be solved if car manufacturers change on mass to the proposed 48V auto power standard, but I wouldn't hold my breath.
For RMS of a single tone (which is the way RMS power should be measured) p = (v*v)/(sqrt(2)*z).
So, for your deck, playing 26watts RMS into 4 ohms, the output voltage is 12.13V. For 60W peak, the output voltage is 15.5V. These same voltages into 10 ohms will give you 10.4W RMS and 24W peak.
Now for the rant - considering your car battery and alternator generally supply your deck with somewhere between 12 and 14V, where does it get 15.5V for 60W peak? The answer should be (as is the case in standalone car amps) from a switchmode power supply which switches the DC from the car battery into high frequency AC, transforms it to a higher voltage, and rectifies it back to DC. However, no headunits I have ever seen contain a switch mode supply. Instead, they just lie on their power ratings.
Don't believe any head unit which claims to be able to put out more than 32W RMS and 46W peak into a 4ohm load. More realistic maximum ratings would be 25W RMS and 36W peak. This problem will of course be solved if car manufacturers change on mass to the proposed 48V auto power standard, but I wouldn't hold my breath.
QUOTE
Boojum is right, it works like this: p = v^2/z where p is power, v is voltage and z is impedence
For RMS of a single tone (which is the way RMS power should be measured) p = (v*v)/(sqrt(2)*z).
So, for your deck, playing 26watts RMS into 4 ohms, the output voltage is 12.13V. For 60W peak, the output voltage is 15.5V. These same voltages into 10 ohms will give you 10.4W RMS and 24W peak.
Ratio of impedances is 10/4=2.5 As much will reduce output power: 26/2.5=10.4, 60/2.5=24. Or, if its easier, multiply 26*4/10 and 60*4/10For RMS of a single tone (which is the way RMS power should be measured) p = (v*v)/(sqrt(2)*z).
So, for your deck, playing 26watts RMS into 4 ohms, the output voltage is 12.13V. For 60W peak, the output voltage is 15.5V. These same voltages into 10 ohms will give you 10.4W RMS and 24W peak.
Okay, got it. The reason I was asking is because I'm hearing some distortion from this deck which is supposed to be highly regarded for it's sound quality. Sounds like ~160-300hz range is breaking up (distorting) a bit. I think this may be due to "pushing" the amp to hard (I have it running about 60-90% for the volume level I want.
I think it's time to look into proper 4ohm speakers.
I think it's time to look into proper 4ohm speakers.
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