Connecting a couple of amplified nearfield monitor to an iMac (in the specific case, a 27 inches, i7 2,93MhZ) through the 3,5 mm audio socket will likely cause a disturbing noise one one of the stereo channels (in our experience, the left one.) This is probably due to the fact that the iMac audio socket is pre-amplified thus affecting the “work” of the monitors.
We first thought that there was some problem with the monitor so we tried all the tricks that came to our mind: inverting the monitors’ connection, changing the cables, using another computer but the damned noise was still there. Then, we tried to connect the monitors to the iMac through the balanced inputs linked to an old Line6 TonePort UX2 and everything worked fine. But this is not the end of the story. Since we wanted a firewire-equipped audio card, we tried with a Behringer FCA-202 and… the hum came back.