Testing with open plugs doesn't actually prove much. You need o choose an input source, and short the corresponding RCA plugs. Really you should use a resistor, but shorting is better than an open input. Also, buzzing is different than hum. If the buzzing occurs with power, it is likely (again from the schematic) a problem with the power supply section. Did I mention that I don't like this pre's design? Buzzing with no power... never heard of that. Only heard of 60Hz ground loops, and noisy open inputs. Maybe you should try to contact the manufacturer, and get their take on the problem. Martin.