Hi! I have a problem that I could not resolve during a long period of time. I have 2 Shure KSM 109 mics that are connected to M-audio DMP3 preamp. Preamp is connected to PC sound card. 1) When I turn on a phantom power I hear a low frequency 50 Hz hum in both chanels. 2) When I touch lightly a 0dB\-15dB attenuator with my finger (when mic lies on the table) the 50 Hz hum swells (increases). We have a 50Hz power line in our country. I have tried: 1) Checked XLR cables (1-1, 2-2, 3-3 pins) with tester. Cables are OK. 2) Installed Ebthech hum eliminator between PC and mic preamp. It did not help. 3) Checked 48 V from M-Audio preamp - OK. 4) Checked a voltage (with mics connected to preamp) between 1 and 2 pins, and between 1 and 3 pin = 30 V. 5) Checked with another preamp: (Almost the same result - hum when I touch an attenuator, no hum from power line, may be because preamp powered only via USB?). I have found then that there is NO HUM when I run my PC and preamp from wall outlet WITH GROUNDING. Also I have found that (when I run from wall outlet WITHOUT GROUNDING) when I touch with my one hand a microphone and with another hand I touch my Yamaha hi-fi amplifier chassis (the metal jack of head-phones connected to amplifier chassis), then the hum almost disappears. I have found also - my problem is not related to balanced-unbalanced issue. Now, I have a hum in this scheme: microphone->preamp. I have no any devices connected to preamp. How I have checked a hum without connecting to PC souncard? My M-audio dmp3 preamp has a VU meters that show an output level, calibrated to correspond to digital peak meter. I can visually see (without sound) if I have any hum from mics without any devices (such as PC) connected directly to mic preamp. Please, see my very short videos related to this issue: 1.YouTube - 113 (without PC connected) 2. YouTube - 50 Hz HUM when I touch a Shure KSM 109 microphone attenuator (with PC connected) 3. YouTube - 50 Hz Hum when I touch a Shure KSM 109 microphone attenuator (without PC connected) On this videos I only touch an attenuator with my finger and VU meters show immediatelly that my hand produces a noise pickup (hum) in mics. I have checked mic-preamp combination running from UPS battery (without wall outlet, the result is the same like running from USB powered preamp, see above). And I can conclude - this is not a power line issue. Should mic and preamp work good without earth grounding? I guess they must do, because there is portable recorders that do not have any grounding. Thank you very much for any help!