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Old 7th January 2011, 02:01 PM   #1
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Default How is an amp's output measured?

I understand V=sqrt(W*R). Using 10W and 8ohm you get 8.9vrms.

So how do you measure the output? (I presume DIMM or O'Scope)

At what Hz?

What do you use for the 8ohm load?

If there are other questions I should have asked please feel free to fill in the blanks.


Thanks

T
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Old 7th January 2011, 02:11 PM   #2
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I'm not sure how it's done 'correctly'; perhaps there isn't a real standard. What I do myself is input 1kHz at line level and connect a an appropriate ohmic load to the output. Then measure Vpp on the output, convert that to RMS and use the number to calculate output power. When reporting figures, I will mention input signal, frequency and the value of the ohmic load.

I guess that's the best you can do, really: describe the conditions you used to measure power. All too often, power numbers are mentioned, but all the vital information is left out.
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Old 7th January 2011, 02:17 PM   #3
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Hello Troncones

You need to first know what form the measurement instument's numbers represent. In the case of a DMM it will present the output AC voltage level in rms volts. In this case you simply measure the output voltage at the test frequency across a 8 ohm dummy load and then do the math (W = E^2/8).

On the other hand, if you measure the ouput using an oscilloscope the AC sinewave measured from the top of the positive peak to the bottom of the negative peak will be a measurement of peak to peak voltage. In this case you need to do the math by converting the voltage to rms in the eaquation. The math then is W = ((Vpp/2.828)^2)/8.

The loads are usually made up of multiple non inductive resistors paralleled up to equal 8 ohms at a high enough wattage to serve as an effective dummy load.

Amplifiers are tested for output power while being driven with sine waves and done so at multiple frequencies to understand the amplifiers ability to deliver power say at 20hz, 40hz, 80 hz, 120 hz 1,000hz and so forth up to say 20khz. This allows one to fully characterize the amplifier power delivery ability.

Mickeystan
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Old 7th January 2011, 02:23 PM   #4
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At what distortion level?
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Old 7th January 2011, 02:41 PM   #5
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"At what distortion level? " If it's just a quick check of maximum power from an amp then it is common to measure the output at the point where clipping is just visible on an oscilloscope screen. Usually at 1KHz.
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Old 7th January 2011, 02:58 PM   #6
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Old 7th January 2011, 03:26 PM   #7
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A dummy load can also be built by using wire wound resistors.
I for example have done one by connecting parallel 27 pcs. of 220 ohms 5 W wire wound resistors. The parallel connection makes the total inductance of such dummy load very low. Actually the small inductance it still contains makes this load to resemble a real loudspeaker better than pure resistive load.
Anyhow, the difference is minimal and this sort of load can be used for all testing purposes.
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Old 7th January 2011, 04:27 PM   #8
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Thanks everybody.

I don't have a Distortion Meter. I might play around with the RoMac sound card meter. I'm open for suggestions about an economical entry level Distortion Meter.

For now, I was just going to use a 25watt 8 ohm and watch the scope for signs of clipping. This should at least give me circuit A to circuit B comparison.
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Old 7th January 2011, 04:33 PM   #9
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Quote:
Originally Posted by Troncones View Post
Thanks everybody.

I don't have a Distortion Meter. I might play around with the RoMac sound card meter. I'm open for suggestions about an economical entry level Distortion Meter.
Try looking in the 2ndhand ads or drop by at a hamfest.
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Old 7th January 2011, 05:44 PM   #10
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If using a soundcard and software to do the job watch out for input overload and destroying the soundcard!
You will need some sort of calibrated attenuator and some protection for the card. There will probably be something suitable on this very site
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