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Wrong wattage calculation on a review of an audio amp.

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BrianHG:
Wrong wattage calculation on a review of an audio amp.

Help, did this guy calculate the wattages wrong or have I've been doing audio wrong my entire life...
See his table at 10:50 where he lists the scope voltage reading in peak-to-peak and his calculated wattage...



This is what I calculate:
72v p-p into 8 ohm = 36v / 8ohm = 4.5 amp.
4.5 amp X 36v = 162 watts peak.
Or 162 watts peak * 0.707 = 115 watts RMS.
A far cry from what the presenter listed as 81 watts.

It is me that wrong with my 162/115 watts, or the presenter with his half peak power watts of 81?

The rest of his figures all all off by the same amount...

themadhippy:

--- Quote ---It is me that wrong
--- End quote ---
could be,

--- Quote ---72v p-p into 8 ohm = 36v / 8ohm = 4.5 amp.
--- End quote ---
yer calculating the rms voltage from the peak to peak voltage wrong. you forgot to divide multiple it by 0.707

DonKu:
162 watts peak * (0.707)^2 = 80.98 watts RMS.

BrianHG:

--- Quote from: DonKu on January 02, 2025, 06:47:00 pm ---162 watts peak * (0.707)^2 = 80.98 watts RMS.

--- End quote ---
Really?

Google: how to convert peak watts to true RMS watts
Response:

--- Quote ---To convert peak watts to true RMS watts, multiply the peak watts by 0.707.
--- End quote ---

Remember, the square root of 2 = 1.41421356237 and 1/1.41421356237 = 0.7071067812.
You are not supposed to * by .707 to the power of 2, you are supposed to take the peak power and divide by the square root of 2, hence, 162/(sqr-root2)  or 162/1.41421356237 = 115 watts, not 81 watts.

True RMS wattage is not 50% peak wattage like with you 81 watt calculation.  That is just plain wrong!

162 watts peak would mean a square wave at 72vp-p.
115 watts RMS would means a sine wave with 72vp-p.
81 watts would mean a square wave with 50% on duty cycle.  Sine waves deliver more effective power than that.

TimFox:
The peak power for a sine wave is twice the mean power (rms power is a vulgar error).
72 V pk-pk = 36 V peak = 25.46 V rms.
(25.46)2 / 8 = 81 W mean power.

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