I have some old speakers in the house that I wanted to test. So instead of messing with the real stereo unit from the 80s, I bought a $30 stereo from Amazon, and new speaker wire.
The speakers sound okay, but there’s a hum coming from them all the time. Even with the stereo volume knob turned all the way down. Is this normal for stereo systems and I just haven’t noticed?
A low hum is almost certainly going to be 60Hz (or 50Hz in some parts of the world) line noise.
As many have said, it’s often a ground loop - but since you have a single device plugging in to unpowered (presumably!) speakers, that’s not the problem. In your case, the stereo itself is producing the hum.
So if you’re in North America, there’s one thing to check before returning. I’m guessing that a cheap Amazon amp has either a wall-wart or two-prong plug. If possible, try rotating the plug 180 degrees and see if that helps.
However, there’s a good chance that the plug is polarized (i.e. one prong is larger than the other) in which case you won’t be able to flip it.
If I bonk my PC speakers, the hum goes away for a while. Wiggling the wires can make it come and go. Probably a short in a wire or connecter, right? They’re 20yo.
NOT a polarized plug. The hum seems the same both ways. The hum is also unaffected by twiddling the volume knob
Actually, it is 100/120Hz because of weak capacitors/bad filters in the power supply.