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I have a normal 2.1 Stereo speakers with an aux 3.5 cable coming out as input source, i plug this aux cable to a source when needed else it remains hanging, whole speaker this is plugged in to an inverter output for power,

now when there is mains power everything is fine but when power fails and system comes on inverter power suddenly a noticeable Hummm sound comes out of speaker, loudness of hummm sound depends on the volume knob of the speaker, aux cable when touched the sound drastically increases, when mains power comes back again things come back to normal.

Note : On inverter power when speaker is creating hmm noise if i plug aux cable into a phone, computer or headphone with female 3.5mm input then the HMMM sound is completely nil and things work fine.

What may be the reason behind this sound ?

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    \$\begingroup\$ Sounds like the inverter isn't properly earthed back to the AC earth i.e. it's probably a floating output. \$\endgroup\$ – Andy aka Aug 12 '18 at 16:16
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    \$\begingroup\$ How is your inverter grounded in the first place? \$\endgroup\$ – winny Aug 12 '18 at 16:38
  • \$\begingroup\$ the plug where inverter's input wire is connected is temporarily grounded by electrician because inverter and everything connected to it like computer etc were giving shock, he just took the grounding wire and attached it to a iron rod coming out of one of the pillars of the house outside and that solved the shock problem, this had to be done because i don't have grounding in this rented place. is this the problwm > \$\endgroup\$ – Gaurav Kumar Aug 12 '18 at 17:43
  • \$\begingroup\$ If you have access to an oscilloscope, check if the inverter has a raw pwm or inadequately filtered output. \$\endgroup\$ – K H Aug 13 '18 at 18:55
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Your inverter probably has an unsymmetric but strong capacitive connection from the output to the input. That can generate some unwanted AC electric field which your open audio input catches.

One solution: make a shorted (=all wires together) dummy jack where you put your unconnected audio input plug. You can use a tightly wrapped tin foil piece as a first aid.

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Inverter don't give a pure sine output. There are some kind of inverter which gives pure sine output but those inverters are much more costlier than normal inverters. Normal inverters will output kind of modified sine wave which doesn't be same as sine wave that you get from the mains power supply.This might be a cause for your kind of humming sound from your speaker system. You can also notice this kind of humming sound from your ceiling fan when it's working in inveters.

My Suggestion is that you can try to get a pure sine wave inverter and can fix these humming issue.

You mentioned that humming sound goes off when you insert it to phone or PC. This because they still give some kind of output even if you don't play any music with it. They always give much more higher SNR sound output even though you don't play any music

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