Hello,

I have a problem with a power supply 50Hz hum. When i power my telephone with a switching adapter (it doesn't have protective earth, it's a cheap china product) i hear a 50Hz hum/noise.

But when i power it with a lab power supply or power supply adapter which has a transformer and a diode bridge in it and etc (it's not switching type adapter), then the 50Hz hum disappears.

Also, when i touch my telephone PCB's ground with my finger or oscilloscope probe, the 50Hz hum disappears too.

What is happening here? is it something related with ground loops?
How could i reduce it? (when using the cheap switching power adapter)

 

Use earth connection dummy!

 

But the lab power supply doesn't have earth connection too. Why using it everything is ok?

 

what makes u think tht?

 

Because i don't hear any when using non switching type power supply.
And my device isn't intended to be earthed.

what makes u think tht?

 

OK...at frequency do u think switching type operates?

 

The switching supply could be unregulated, it's not unheard of in cheap chinese stuff.

 

tht was not what I asked.

but u can try replacing the capacitors