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Audio detector that ignores 50Hz mains hum using arduino/attiny

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schmitt trigger said:
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the hum you can listen to, is actually at TWICE the powerline frequency, 100 or 120 Hz. Check it out with a scope if you don't believe me.
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Click to expand...
Hum from the ripple in a full-wave power supply will be twice the line frequency, but hum pickup up from a ground loop will be at the line frequency.
Why would it be double if there's nothing to rectify it? :???:
 

Yes indeed...

The hum generated WITHIN equipment is usually from the rectifier, so naturally it will be at twice line.

But hum from any external wiring running BETWEEN different equipment, including classic grounding problems will mostly be at mains fundamental.
 

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