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Old 2nd June 2012, 02:21 AM   #1
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Default How to measure output power

I have just finished my second amp, a custom designed stereo ipod amp. The design was for 3.5 watts / channel and I'd like to see what I'm actually getting. I have a 1KHz sine wave mp3 file which gives me 0.71Vrms at the input. I'm thinking I can put a 0.1 ohm resistor in series with the speaker and measure the Vrms across the resistor and across the speaker. Doing the math to convert the voltage across the resistor to amps, is it simply V x A ?
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Old 2nd June 2012, 02:35 AM   #2
TheGimp is offline TheGimp  United States
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While it is not a speaker, I use a resistive load for power measurements as speaker impedance can pretty much be considered off specified value at almost any frequency.

Measure the resistor with a precision ohm meter, or measure by feeding an accurately measured voltage to get your cal value.

From there, I adjust the output power while looking at the waveform on a scope and increase input till I get the onset of clipping. This lets me measure max continuous power.

I also like to look at distortion with a spectrum analyzer (Audio Tester) at 1W out.

Then I ignore my measurements and go listen to it.
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Old 2nd June 2012, 03:08 AM   #3
Pano is offline Pano  United States
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Ditto. You have to know that you are clipping or not, or have reached a certain level of distortion. Do you have a way to determine that?
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Old 2nd June 2012, 03:25 AM   #4
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Take this oppotunity I wonder how I calculate output power on paper??

Bias x B+ x Output Transformer=output Watts

Bias and B+ is easy to find, but which value is about the output transformer??
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Old 2nd June 2012, 03:42 AM   #5
tomchr is offline tomchr  Canada
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I rig the amp to a resistive load and crank up the input signal until I get 2~3 % THD on the output (measured with a distortion analyzer). Then I measure the output RMS voltage and use P = E^2/R to find the output power. I use the same method to measure output power at 1 % THD and measure the THD at 1 W out. I perform this measurement at mid band (1 kHz) with spot checks at lower and higher frequencies. I usually also run a distortion vs power vs frequency sweep.

Now, why 2~3 % THD? Well... In my 300B SET with a source follower grid drive on the 300B, I have lots and lots of grid current available. But the 300B isn't designed for A2 operation as far as I know. I have noticed that once significant grid current starts to flow, the THD basically falls off a cliff. That tends to happen around 2~3 % THD, so I stop my measurements there. At that point the output waveform is also rather ugly on the o'scope.

If you don't have a dedicated distortion analyzer, you can use a computer sound card and some software. TrueRTA comes to mind.

As far as calculating output power on paper, I'd just look it up in the datasheet for the output tube. Or perhaps dust off RDH4 and look in Chapter 13 (AFAIR).

Modulus-86: 40W/8Ω @ 0.000061% THD. HP-1: 3W/20Ω, 450mW/300Ω @ 0.000032% THD. DG300B and Other Tube Circuits.
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Last edited by tomchr; 2nd June 2012 at 03:45 AM.
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Old 2nd June 2012, 04:21 AM   #6
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Thank you for the answers. Unfortunately, I don't have distortion analyzers, or even a scope. I was hoping it could be done with a multimeter. I'm not looking for exact, precise numbers. I just to want to see if I'm in the ballpark.
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Old 2nd June 2012, 05:58 AM   #7
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You'll have to see what the AC frequency response of your multimeter is.
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Old 2nd June 2012, 12:49 PM   #8
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How do you figure this? Bias x B+ x Output Transformer=output Watts
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Old 2nd June 2012, 01:10 PM   #9
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maybe build this?

Power Amp Clipping Indicator

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Old 2nd June 2012, 03:13 PM   #10
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Meter is a VC99, spec for AC volts frequency response is 40-400 Hz.
I'm assuming I can't use this multimeter to measure frequencies in the KHz ?

Let's forget about clipping and % distortion for a minute. I just want to know
how I can get a rough measure (+/- 10%) of the power being delivered to the speaker.
As TheGimp mentioned, I'll actually be using an 8 ohm 5 Watt resistor.

Last edited by EddieRich; 2nd June 2012 at 03:18 PM.
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