• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

hum measurement

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.

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


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


@ Mastertech: I got a nice digital HP oscope from ebay for 250 bucks and it works fine and helps quite a lot! :)


2003-01-18 7:57 am
Near London. UK
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 The reason hum measurement methods tend not to be published is that if there's enough to measure, then it shouldn't be there!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.