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Every small 5VDC power supply (wall wart style) I see has the standard 2-prong Europlug, i.e. no ground. Even those with a British 3-prong plug just use a plastic prong for ground.

Background - I am powering a small ARM board with such a power supply (micro-USB plug) and I am getting a mains hum on a microphone. As soon as I connect ground from my oscilloscope though, the hum disappears. I have also tried using a slightly bigger power supply, in a small metal cabinet, with a mains ground - same result, no hum.

Why does the wall wart not have a ground and is there a safe way around it?

Per Jessen
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  • How about powering your ARM board from a USB powerbank and using it outside, far from any building that has **mains power**. My prediction is that there will be no hum! Yet there is also no ground connection. My point: in your particular scenario the hum is solved by making a connection to mains earth. That doesn't mean the ARMboard can only work properly with an earth connection. A smartphone also has microphones and it doesn't need an earth connection to pick up sound without hum. – Bimpelrekkie Jan 23 '21 at 13:54
  • It doesn't need it (for safety and most applications) because it's double insulated. (Look at its label - you should see the box-in-a-box symbol). The way round it is to provide a proper ground, e.g. use the other PSU. –  Jan 23 '21 at 14:05
  • Yes, it has the class II box-in-a-box symbol, it never even occurred to me to check. I have tried a couple, they all have it and there are no metal parts exposed, of course. Omitting the ground prong on those wall warts seems an unnecessary saving, and the next PSU size up is the clunky laptop "brick" with an IEC320 socket. – Per Jessen Jan 23 '21 at 14:30
  • Horses for courses - they are not intended to do what you want. – Andy aka Jan 23 '21 at 15:13
  • You mean they're intended as battery or phone chargers, not power supplies? yeah, I suppose that is true, although it is a fairly typical use case too. They are also often being sold specifically for e.g. ARM and Arduino boards. – Per Jessen Jan 23 '21 at 17:04

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Wall warts although isolated from earth gnd, tend to have poor RF isolation and the switching frequency fields tend to be induced in the high impedance mic cables due to poor balance or CMRR, common mode rejection ratio.

There are several methods found effective to reduce this. Some are done in the SMPS with crossover RF caps to cancel the electromagnetic coupling of mains current. Others are commonly done with high mu CM toroidal chokes molded on the mic cable. Others benefit from RF caps from 0V thru grounded interface cables to peripherals like analog monitors.

You can test yours with a Various caps to earth ground to determine where attenuation starts and thus compute the equal impedance half power point -3dB which is near the perception threshold for changes in power.(crude approx)

There are many more methods such as cable types , shielded balanced pair and differential amplifiers. The CMRR loss is usually in the cable type and shield or the use of multiple SMPS in a connected system so ground current paths are a concern. (Ground here just means 0V)

You can also try changing power to linear regulated supplies and keep away from strong SMPS stray fields.

I hope this gives you some ideas.

Tony Stewart EE75
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  • Thanks - I have seen the crossover caps in the SMPS I mentioned, and that approach seems to work. I have also tried the shielded balanced pair for the microphone, didn't help though. I would much prefer sticking to the wallwart style power supply, I just can't find any with a ground connector. (Swiss T12 plug) – Per Jessen Jan 23 '21 at 14:04
  • How to measure the variation in hum by hand holding to 0V and mic? Is any other method to raise CM impedance used? Is it line modulated carrier crosstalk or baseband? – Tony Stewart EE75 Jan 23 '21 at 14:12
  • I believe it is line modulated, low frequency. The microphone (from an old analog phone) is directed connected to the ARM board/codec. (Nanopi Neo Air), no other circuitry involved. – Per Jessen Jan 23 '21 at 14:21
  • Can you inject hum to cancel it or use a notch filter? or use a large CM choke? – Tony Stewart EE75 Jan 23 '21 at 14:23
  • It's a standard thin USB cable, but I could possibly open it, yes. Might be worth sacrificing a USB cable to try out a choke like that. – Per Jessen Jan 23 '21 at 14:36
  • clamshell chokes exist but depends on f and Z – Tony Stewart EE75 Jan 23 '21 at 14:38
  • if touching the mic body helps it is a CM problem, if worse, its a ground loop problem with multiple supplies – Tony Stewart EE75 Jan 23 '21 at 14:53
  • Touching the mike with my finger roughly doubles the hum volume. So as suspected, a ground loop issue. If only I could find a 5VDC wall wart with an earth prong. – Per Jessen Jan 23 '21 at 17:11
  • Got any PC nearby or PSU? or run a magnet wire out from the panel screw with a ring lug – Tony Stewart EE75 Jan 23 '21 at 17:15
  • Yeah, got a laptop right next to it - but I have tried shifting it, maybe 1 meter away. Supplying the ARM board from a USB port on the laptop also gives me zero hum, by the way. – Per Jessen Jan 23 '21 at 17:18
  • untill you plug in the charger.... – Tony Stewart EE75 Jan 23 '21 at 17:20
  • The charger has remained plugged in when I have tested the USB supply from the laptop. I have also used four different chargers, same problem with all of them. – Per Jessen Jan 23 '21 at 18:50