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Old 17th May 2005, 09:16 PM   #1
Gluca is offline Gluca  Italy
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Default hum measurement

Ciao,

sorry but I couldn't find anywhere how to measure hum?! The (my first actually) amp I just soldered (one monoblock almost finished) is really quiet as I need to place the ear very close to the speaker to detect some hum but ... how to measure it?



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Old 17th May 2005, 11:22 PM   #2
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I'm not sure how these sorts of things are done 'professionally', but it would make sense to measure the magnitude of the AC voltage coming out of the amp without any music playing as a measure of the hum/noise.
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Old 17th May 2005, 11:52 PM   #3
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I am not intimedated by exprts taking measurements with expensive testing
gear my measurements with lesser humbler gear have proved more reliable
over and over again

Expert-Mastertech
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Old 18th May 2005, 03:10 AM   #4
kmtang is offline kmtang  Canada
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The easiest way is to use an oscilloscope to measure the residue hum/noises from the amplifier. Just clip the test probe at the output terminals 0-8 ohm. You will then see the hum/noise in the scope. You might need to adjust the time scal to 10mS or less to see the clear picture of the hum.

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Old 18th May 2005, 09:25 AM   #5
Gluca is offline Gluca  Italy
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Thanks I will try later on tonight... I will look for 50hz and I suppose I don't need to apply any signal on the input

but

on certain occasions I saw hum stated as mv @ 1W output power??? This puzzles me...

Gianluca

@ Mastertech: I got a nice digital HP oscope from ebay for 250 bucks and it works fine and helps quite a lot!
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Old 18th May 2005, 09:29 AM   #6
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"on certain occasions I saw hum stated as mv @ 1W output power??? This puzzles me..."

It puzzles me too. How can you measure residual hum in the presence of an audio signal? You can't.
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Old 18th May 2005, 09:36 AM   #7
EC8010 is offline EC8010  United Kingdom
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If you have a digital oscilloscope, it probably has things like "average" which will allow the hum to be seen without the noise. It's conventional to short-circuit the input of the amplifier for a hum measurement. The reason hum measurement methods tend not to be published is that if there's enough to measure, then it shouldn't be there! Think about it - 80dB down on 10W into 4 Ohms is 1.8mVpk-pk - and 80dB down isn't particularly stringent.
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Old 18th May 2005, 09:53 AM   #8
dhaen is offline dhaen  Europe
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Quote:
Originally posted by Frank Berry
"on certain occasions I saw hum stated as mv @ 1W output power??? This puzzles me..."

It puzzles me too. How can you measure residual hum in the presence of an audio signal? You can't.
It's a valid measurement, as the power supply perormance on a class B amp may well be different "on load".

Measurement isn't trivial, but the hum can be found as part of the distortion component.
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Old 18th May 2005, 10:05 AM   #9
EC8010 is offline EC8010  United Kingdom
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Quote:
Originally posted by dhaen
Measurement isn't trivial, but the hum can be found as part of the distortion component.
Yes, hum tends to show up as sidebands around the distortion spectra. Of course, these days, everyone does have a spectrum analyser (soundcard plus software) so it's no longer as difficult as it used to be...
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Old 18th May 2005, 10:12 AM   #10
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"Yes, hum tends to show up as sidebands around the distortion spectra. Of course, these days, everyone does have a spectrum analyser (soundcard plus software) so it's no longer as difficult as it used to be..."

Hum is measured with no audio present.

When the amplifier is passing audio, you are measuring Noise+Distortion ... not hum.
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