Hum from an electronic device is often due to a bad ground connection. In fact, it happened to me with a Dot when first connecting the Dot's audio out cable the first time. The Dot's jack is very tight at first, and although it felt completely inserted, it was not; hence a hum, so I removed and plugged in cable a few times to Dot and made sure it was fully inserted, and hum was, and has been, gone.