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.
Johnny
Johnny
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!
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!
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 The reason hum measurement methods tend not to be published is that if there's enough to measure, then it shouldn't be there!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.
Frank Berry said:"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.
dhaen said: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...
"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.
Hum is measured with no audio present.
When the amplifier is passing audio, you are measuring Noise+Distortion ... not hum.
What we're measuring....
This is why "hum" can be far more insidious in P-P amps, epecially class B.
To me, this is still hum - call it what you will.
part of the distortion component
This is why "hum" can be far more insidious in P-P amps, epecially class B.
To me, this is still hum - call it what you will.
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