I have a Torus 15 amp power conditioner, about 5 years old. It has until now always been silent and problem-free. Typically 9 of its 10 outlets are in use.\n\nAbout two weeks ago I bought a new Rotel 200 x 5 watt\/channel power amp which draws max 1100 watts (=about 9.2 amps) but at idle draws < 1 amp. The power amp is connected to the power conditioner. Both units have substantial manufacturer power cords. \n\nWhen I turn on the power amp, I hear a soft low frequency hum directly from the Torus unit. The amp itself remains silent. When I turn off the amp, the hum from the Torus stops.\n\nI only hear the hum up to a few feet away. Beyond that distance it isn’t audible.\n\nTo make sure no other component is involved, I disconnected everything but the power amp from the Torus and disconnected everything from the amp including the speakers. When the amp is switched on, the Torus hums. When off, the Torus is silent.\n\nI tried the same test replacing the new amp with an older Bryston stereo amp. There is no hum from either unit.\n\nI tried connecting the new power amp directly to the power outlet. The amp is silent. The amp produces no hum when in use, either.\n\nAs the hum isn’t loud enough to audibly affect music or other sound, I don’t consider it a serious problem. But I am very curious about it. My uneducated guess is that it’s caused by a slight incompatibility between the two units, not a specific defect. Can anyone suggest a specific cause for the hum?