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Old 7th June 2005, 07:09 AM   #41
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
W=I^2*R = V^2/R = I*V
If V is rms then W is rms equivalent.
If V is peak then W is peak = 2*Wrms when dealing with sine waves.
You use the version of the formula that you have values for. You don't need to do a current calculation in between if you don't have a value.

If the +Vrail is 24Vdc and you can swing to within 5V of either rail when loaded then the pk to pk swing is 24-(2*5)=14 and Vpk=7v.
Close to your 12.5Vpp quoted but not near 10.5Vpk=21Vpp.

10.5Vpp =5.25Vpk Wpk=Vpk^2/R =5.25*5.25/8 = 3.4Wpk = 1.7Wrms.

If you don't agree on any of this come back.

But please define your voltages, currents and power as either rms, pk or pp if there is risk of confusion to the reader.
regards Andrew T.
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Old 8th June 2005, 12:12 AM   #42
diyAudio Member
Join Date: Jul 2002
Location: Naptown
Like I said, I should have clearified that I measure from peak to peak. And that's what i said in the second post:

And since I was measuring Peak to Peak, the amp stays within 5 volts of the positive rail and 5 volts of ground with about .66v input.
As for the swing, with the bias set at 12V at the sources of the Fets, the swing is from 6.75 v to 17.25 v. This is within 5v of positive and 5v of ground (IE, within: 5v to 19v).

I have to admit that it's post like this that make me refrain from posting at all. I was simply demonstrating a 'Simple AB amp', with my own interpretation of it.
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Old 8th June 2005, 02:55 PM   #43
AndrewT is offline AndrewT  Scotland
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders
please keep posting useful info especially to beginners asking for guidance.
I reluctantly had to correct the error otherwise that newbie and any others reading the thread would be totally confused.
If you had avoided the watts conversion the whole problem would have gone away and the relevance of your post would have been just as good.
regards Andrew T.
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