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Thread: watt to volt conversion question

  1. #1
    Supporting Member Randall's Avatar
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    watt to volt conversion question

    I am working on a Lab Series L5 solid state amp that claims it is 100 watts at 8 ohms. I ran a 1K sine through it, scoped the output to find the clipping point, and put my Fluke 77IV across the dummy load. I get 33.5 vac. The conversion chart I have says that's around 140 watts! The chart is RMS voltage correct? Am I mistaken somewhere, or is this amp putting a clean 140 watts? Sure doesn't sound 140 watts loud to me.
    It's weird, because it WAS working fine.....

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    I suspect that your Fluke is not giving you accurate RMS conversion. There are a number of reasons that could be happening.

    Scope the output as you have already done, and note the peak of the biggest clean sine wave. Then divide that by 1.414, the conversion factor for the peak to RMS value for a true sine wave. That number is your RMS voltage.

    If your load was a resistor, the power would be the computed RMS squared divided by the resistance. I say resistance because speakers are NOT resistive loads. Their impedance has peaks and valleys.

    100W into 8 ohms is P = V^2 / R; solving for P = 100, R = 8, we get V^2 = P*R = 100 * 8, so Vrms = 28.28, Vpk = 39.99; call it 40 volts peak.

    140W into 8 ohms is 33.46Vrms, as you computed, and 47.32V peak.

    Meters get funny about AC waveforms and frequencies, even "true rms" meters.
    The music world seems to be a three-way race between the princess and the pea, the blind men and the elephant, and the emperor's new clothes.

    Early data suggests pedestrian deaths surged in 2015; legislation filed to require license to use cell phone while walking, and require five-day waiting period for both cell phones and shoes.

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    Quote Originally Posted by Randall View Post
    I am working on a Lab Series L5 solid state amp that claims it is 100 watts at 8 ohms. I ran a 1K sine through it, scoped the output to find the clipping point, and put my Fluke 77IV across the dummy load. I get 33.5 vac. The conversion chart I have says that's around 140 watts! The chart is RMS voltage correct? Am I mistaken somewhere, or is this amp putting a clean 140 watts? Sure doesn't sound 140 watts loud to me.
    Is your load really 8ohms?

    nosaj

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    Supporting Member Randall's Avatar
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    Thank you R.G. Could you please double check your math entries above? I think there is a confusing = symbol.

    So by your method I scope 86 Vac p-p. It took me a while to remember we only use half the wave, so that makes it 43 v peak. So, 43 / 1.414 = 30.41 Vrms.

    30.41 * 30.41 / 8 ohms = 115.6 watts, correct?

    nosaj, my load is exactly 8 ohms according to my Fluke. It is a large 8 ohm, 100 watt wirewound hollow resistor soldered to a short speaker cord to a 1/4" plug. I trust it. It did get hot pretty fast, though.
    It's weird, because it WAS working fine.....

  5. #5
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    Yep, 86V p-p is 43V pk, and that's 115.6 into an 8 ohm resistor.
    The music world seems to be a three-way race between the princess and the pea, the blind men and the elephant, and the emperor's new clothes.

    Early data suggests pedestrian deaths surged in 2015; legislation filed to require license to use cell phone while walking, and require five-day waiting period for both cell phones and shoes.

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