It sounds like a design issue with the decks. Does the phono output still work if you switch off the power to the turntables? If not then the Line output or USB output circuitry is actively cut into the signal path.
As a precaution, make sure your phono cables from the turntables are seperated from any power cables. If you have to cross the cables, try to cross the power and phono cables at 90 degree angles to eachother.
It sounds like your signal is being corrupted by noise from a 50 / 60Hz supply. As the same thing happens with the Line Output I think it's unlikely to be something after the deck's output. |