I hope this is the right section for this problem, but powers that be please feel free to move it if it isn't. Please also note that I'm a Brit, so I've 'translated' the bits that may be esoteric, but please let me know if anything is still opaque! I have a cheap JBL speaker system with an active sub powering the other speakers. It emits a hum, but only when it is receiving an input. I have the sub connected to a Sky+ box [basically, the equivalent of Tivo] via optical cable and to a cheap and cheerful Toshiba DVD player using a digital coaxial link. The hum sounds to my innocent ears llike mains hum. It remains at the same volume no matter what is put through it, and is loud enough to be annoying at very low volumes and periods of silence. Okay, so all this sounds like what many of you've heard before in similar 'my system is maing funny noises - help!' type pleas, but there are some oddities in this one: (1) the sub is hum-free if there is no signal being fed into it. Putting the connecting cables in, taking them out, etc, has no effect. (2) the hum is only apparent when the volume is on, even microscopically. If the sub is set at mute or I turn the volume to zero, the hum goes. But if the volume is set to 0.00000001%, the hum is there. (3) obviously on Sky+ the hum is there from switching it on. In the case of playing the DVD, the situation is rather more unusual. If the DVD is playing a menu screen, there is no hum. But as soon as the player starts playing a disc, the hum starts. (4) I don't think the hum is there when playing the sound through the TV set (though equally, it may lack the bass to pick it up). (5) This is where is gets really weird. Neither my wife nor I noticed the humming problem at all until a few weeks ago, round about the time we bought some new electrical equipment. This is roughly the sequence of our purchases: (a) computer in the study downstairs (b) new Hitachi LCD in living room (directly above the study - we live in a 3-storey townhouse) and a smaller LCD in my daughter's playroom above that. The Hitachi hums horribly whenever the screen goes dark, and gets returned. At the same time I began noticing that our cheap Panasonic all in one home theatre system (which also had a powered sub) is humming. (c) new Panasonic Viera LCD replaces the Hitachi. This doesn't hum, but humming continues in the home theatre system. (d) The DVD player etc are plugged into a bank of sockets in the wall. I change this arrangement and plug them all into a Belkin anti-surge multisocket extension lead. Hum continues. (e) I then buy a separate cheap Toshiba and a JBL xite speaker kit with active sub as described above. Humming if anything is worse than under the old system. I should perhaps add that in addition to the Sky+ box, Toshiba and JBL already mentioned, there is a Sony 710 DVD recorder, but this is only ever switched on when I want to record something (and switching it on or off has zilch effect on the hum). The Toshiba is usually attached via HDMI to the TV, but switching to component doesn't seem to have an effect. It should also be mentioned that my 12 yr-old son claims he can hear humming on his LCD screen attached to his Playstation/Xbox in his bedroom, but this might be psychological suggestion. Likewise, I may be hearing a slight hum on the cheap amp and speakers attached to my computer, but again, this might be imagination. I've racked my brains to think of any feasible explanation for this problem. I don't know if this is relevant, but we are living next to a new housing development, which might be draining the juice, but I find this implausible. Okay, folks, any suggestions?