weird problem with new imac

[djw]

dear all, hope you have some input on this.

i bought a late 2009 model 21.5" imac two days ago, from my local apple resale agent. all well, i knew what i was getting and it suited my needs just fine, currently still on version 6 of ableton but upgrading soon.

here's the problem. i run a couple of krk rokits rp6 through my m-audio firewire 410 soundcard, and bought a 800 to 400 pin to connect the speakers to my new imac. when the studio monitors are on, i get an awful noise - its like the monitors are amplifying the (rather non-existent) computer noise; for instance, when i scroll the mouse, i get static out of the speakers. when i open a program, i get static out of the speakers. when i'm writing this, i get static out of the speakers. moreover, it does not go away when i play music.

i thought that it might have something to do with the 800 to 400 pin. therefore, i connected my macbook pro to the soundcard, perfect sound and no static. i then connected my macbook to the external power supply, thinking this might be the problem, but no. still perfect. moreover, when plugging my headphones into the headphone socket on the 410, i do not hear this static...

i returned the imac to the apple resale agent, who did several tests and found nothing wrong with the computer. i spoke to the staff in a local music store who had experienced a similar problem, and their solution was to power the soundcard externally, as the problem was in the "loop" between the firewire soundcard, the computer and the speakers. m-audio warns against using external power on their firewire soundcards in connection wtih macintosh computers, so i am a bit sceptical.

as it turns out, i have also lost my original power supply so i have to go out and get a new one today, but i just wanted to voice this to you guys and ask whether some of you have experienced similar issues? does the external power-supply idea sound reasonable?

what can possibly be wrong here and what can i do to solve it? as they find nothing wrong, i'm stuck with an imac i obviously cannot use production-wise....

thanks in advance.

[skorzy]

I have the same problem as you and after some research i found this:
http://www.dv247.com/studio-equipment/m ... ler--51928 costs you 25€ but solves your problem. it has something to do with the power supply of the mac creating ground loops which causes the hum in the speakers.