NOTE: Please be indulgent to my English I live in a big city (Moscow). We don't have a very clean power grid. There's a lot of interference in the power grid. This is probably a common trouble of big cities. Sometimes that noise gets right into your ears when you listen to music. It made me struggle with this unpleasant phenomenon. For a start, we decided to determine the spectrum of the voltage in the power supply. To do this, I downloaded the program SpectraPLUS on an old computer. Through the step-down transformer supplied voltage from the mains to the sound card. And saw a terrible picture. A lot of noise in the sound range. With low frequency noise rectifier power probably cope. High-frequency noise penetrates the ears probably due to the parasitic capacitance. I decided to check the industrial EMI filter. Here is the spectrum at the output of the industrial EMI filter. The result is close to useless. The use of such a filter for audio devices is equivalent to the use of a navel warmer. My next step was to use a self-made simple filter of inductance and capacitance. I used a choke from an old computer power supply and a capacitor from fluorescent lamps. Calculations have shown that these two elements form a low-pass filter with a boundary frequency of 367 Hertz. That's enough. After a quick build, I checked the spectrum after a homemade filter. Result. A self-made filter of two simple elements works more efficiently for audio devices. Research will continue. Now I have to put the filter in the case and listen for a while.