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Old 19th September 2004, 05:16 PM   #1
mjarve is offline mjarve  United States
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Default Figuring out Output current.

I was playing around with a new amp this morning and I noticed on the back, near the speaker binding posts a label that said:

"4(ohms) 400Watts" then below:
"40 volts RMS"

Now this is stating the output per channel (400Watts @ 4(ohms)), of course, but I got to thinking what the output current would be (I'm funny this way)?

So if I know the Voltage, the Impedance, and the ultimate Power Output for a given load, how do I determine output Current? I seem to remember some formula that went along the lines of: P=(Ei)/R, but using this, the current turns out to be a rather incredible 40-amps, and I know that cannot be correct.

Thank you for helping me with my little brain fart.
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Old 19th September 2004, 05:40 PM   #2
lubbie is offline lubbie  Canada
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According to a little excel sheet i designed.. With the info you supplied the current would be 14 amps..

Output Voltage(RMS)/Speaker Resistance. <- down and dirty.
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Old 19th September 2004, 07:32 PM   #3
mjarve is offline mjarve  United States
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Thanks. 14 amps seems much more reasonable than 40, and much closer to the mark too (judging from the power transformer and output array).
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Old 19th September 2004, 09:25 PM   #4
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Actually..... I=sqrt(400W/4Ohms)=10A.

10A X 40V=400W This would be 10A RMS. Which peak current is 10A X sqrt of 2 or ~14.1A. 14.1A X 4Ohms is 56.6V peak.
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Old 19th September 2004, 10:29 PM   #5
lubbie is offline lubbie  Canada
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You are correct.. That is peak output current through the transistors. I used peak to determin output transistor requirements.
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