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Old 6th February 2015, 05:23 PM   #1
mitsof is offline mitsof  Burkina Faso
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Default How to measure Vrms of laptop headphones jack output?

I need to measure the Vrms of what the headphones jack of my laptop outputs.How can this be done without an oscilloscope?
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Old 6th February 2015, 05:31 PM   #2
Mooly is offline Mooly  United Kingdom
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Use an accurate DVM and a test tone at 0db/400Hz. If your meter isn't up to measuring low AC voltages then make a simple opamp amplifier (a 741 is fine) of known gain and use the DVM to measure the output of that. Then just divide what you measure by the gain of your opamp circuit to get the level at the input.
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Old 6th February 2015, 07:38 PM   #3
mitsof is offline mitsof  Burkina Faso
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Quote:
Originally Posted by Mooly View Post
Use an accurate DVM and a test tone at 0db/400Hz. If your meter isn't up to measuring low AC voltages then make a simple opamp amplifier (a 741 is fine) of known gain and use the DVM to measure the output of that. Then just divide what you measure by the gain of your opamp circuit to get the level at the input.
What is 0db?Does this mean at full volume?
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Old 7th February 2015, 07:02 AM   #4
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by mitsof View Post
What is 0db?Does this mean at full volume?
Not necessarily.

You need a test tone that will fully drive the output to max volume which means that at some point it will clip as you turn the volume up.

You can make a test CD or MP3 file using Audacity,
Installing and using Audacity. A get you started guide.

To determine when it clips you could use headphones and a series resistor to limit the volume. Clipping is very obvious audibly.
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Old 7th February 2015, 10:54 AM   #5
mitsof is offline mitsof  Burkina Faso
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There is also the frequency range problem of cheap multimeters.I think they can measure only up to 60hz.
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Old 7th February 2015, 11:46 AM   #6
Mooly is offline Mooly  United Kingdom
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You can use a lower frequency if you have doubts. Any meter should be good for at least 400Hz because that is a recognised power line frequency in specific applications (ships, aircraft etc)
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