Is your speaker powered? If so it's probably a bad power supply in it.
13 responses Add your response
Given your 50hz comment I'm assuming you're outside the US. If the equivalent of 3-prong adapters exists for your local plug configuration, get one and see if the noise goes away. If it does, you have a grounding problem, which you'll have to work on component by component. If the noise stays, it's more likely your amp or an upstream component creating the noise.
Since you have said the hum only occurs with nothing connected to the amp I'm guessing the amp. I'm also assuming the sources are both unplugged from the wall and the interconnects to the amp. Is this a problem that just occurred or is the amp new to you? Have you tried moving the amp and speakers to another room or even a different location like a friends home or audio dealer?
If there is nothing connected to the speaker except power cord, and we are assuming it's a powered speaker, (no RCA or XLR signal cables) -- if you are still getting 50hz buzz, then I agree with Eric that the power supply is failing (probably a bad capacitor). The amp/ps board in the speaker likely needs to be replaced.
It actually sounds like a minor ground loop hum. If you have to get within 5cm of the speaker to actually hear it, then don't worry about it. My system has the same thing. Sometimes this will be very noticeable in systems and people can have a very hard time getting rid of ground loop humm. Humbuster may or may not fix this. Sometimes, you'll need to get the Granite Ground Zero to resolve this ($500+)