How to measure/trigger/view low level 60/120hz hum? - diyAudio
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Old 18th April 2013, 10:49 PM   #1
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Default How to measure/trigger/view low level 60/120hz hum?

Do you use an oscilloscope or a measuring mic to measure the inherent 60/120hz hum from your amp? Without anything hooked up at the input, the output of the amp has a very small hum that is only audible if you put your ear up to the speaker. I'm trying to quantize the hum level and maybe do something about it (wire placement/isolation/etc), but my oscilloscope can't even trigger it.

Is a better way to measure this with a measuring mic? What is your technique?
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Old 18th April 2013, 11:02 PM   #2
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You can measure it with the amp input shorted and a scope on the output.
You will need to put the scope on a low voltage range to pick it up enough to trigger the scope.

Hum problems are usually down to bad ground routing and/or poor decoupling.

The best way is a star ground at the smoothing capacitors.
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Old 18th April 2013, 11:35 PM   #3
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Originally Posted by nigelwright7557 View Post
You can measure it with the amp input shorted and a scope on the output.
You will need to put the scope on a low voltage range to pick it up enough to trigger the scope.

Hum problems are usually down to bad ground routing and/or poor decoupling.

The best way is a star ground at the smoothing capacitors.
Yes that what I'm doing right now, my oscope can't seem to trigger noise that low. If i apply a signal (even a small one, like 50mV), it can trigger fine.
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Old 18th April 2013, 11:51 PM   #4
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If you have to put your head right up to the speaker to hear the hum then I don't think you have much of a problem.

You can try earthing the zero volt line, that might help.
Or even lifting it, sometimes earth loops can cause problems.

I did have slight problem wit ha class AB amplifier with hum with no input signal and that turned out to be poor decoupling between input and output parts of the amplifier.
I put a 33R resistor in series with the B+ and B- lines from the output power supply to the input supply.
It would need a decoupling capacitor after the 33r.
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Old 19th April 2013, 06:01 AM   #5
RJM1 is offline RJM1  United States
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The trigger input selector on my scope has 3 positions, internal, external and line. If you switch the trigger input to line it should trigger on the 60Hz. You could always trigger on a second channel of your scope or the external trigger input connected to a low voltage AC transformer.
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Old 19th April 2013, 06:16 AM   #6
Salas is online now Salas  Greece
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He can pull the Mag. X5 also. Best is to FFT on a 24bit audio card so to see the spread of harmonic noise in spikes across the bands.
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Old 19th April 2013, 10:13 PM   #7
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Best is to FFT on a 24bit audio card so to see the spread of harmonic noise in spikes across the bands.
+1 I invested $200 odd in an emu sound card and fft software and have not looked back. I've reduced my system noise floor and distortion considerably by being able to measure changes in earthing, psu and bias setup.
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Old 20th April 2013, 07:41 AM   #8
AndrewT is offline AndrewT  Scotland
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Originally Posted by triplej View Post
+1 I invested $200 odd in an emu sound card and fft software and have not looked back. I've reduced my system noise floor and distortion considerably by being able to measure changes in earthing, psu and bias setup.
Have you the time to give us a tutorial on how to connect the emu?

I downloaded a set up from this Forum. I have had no sound ever since. The software file changed some setting and I have never been able to undo that change. even reinstalling has not worked.
__________________
regards Andrew T.
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Old 20th April 2013, 12:14 PM   #9
ctrlx is offline ctrlx  United Kingdom
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Set timebase to around 2mS, set amplitude to minimum say 5mV/div, turn the trigger variable knob to come off "auto" to allow manual control of triggering.
Depending on the earthing of scope/amp weather or not to 0V scope probe to allow clean image.
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Old 20th April 2013, 01:48 PM   #10
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Even my ancient Eico 435 scope has a "Line" trigger.

With a dual trace scope you can pull off the signal on the filter cap to set the trigger, then probe with the second channel.

DON'T try to pull the signal off the mains -- you'll blow out your probes.
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