Speakers Sven BF-21R, quiet 50 Hz, independent of volume. What to do?

Question

Do it yourself

Speakers Sven BF-21R. After the move, a quiet 50-Hz background was discovered, which does not depend on anything.
• Not loud.
• Not from an audio cable connected or disconnected.
• Neither from the selected audio input.
• And in the passive column too.
Perhaps he had been before, but because of the noise he could not realize.
What to do?

Answer 1

There are not many options.
1. Problems with the power supply of the speakers - you need to see if there is a background on the supply voltage of the amplifiers (if nothing is connected to the speakers, but there is a background - look inside).
2. Problems with the ground - 50 Hz "leak" through the "ground" wire (common signal). You can try (silly as it may sound) by flipping the speaker plug in the outlet. It might get better. If it does not help (and most likely does not help), connect the ground of the speakers (there should be a ground terminal) and the ground of the sound source (case). The background should disappear, or significantly weaken.

Answer 2

difficult to guess,
diodes, electrolytes, possibly build up electrolytes, shielding, wire cross section