very strange hum from int. amp ....

Hi,

I recently got a new int. amp, and like it very much.
But has a very strange hum problem.....

Whenever I turned my floor lamp on(in the same room), my amp hums; once I turned it off, the amp becomes silent.

I tried to just plug my amp to the wall and only connect to my speakers, still the same.

Any ideas what cause this problem and how to fix it ?

My another question is: why my old amp does not have this problem, under exatly the same setup ? is it a problem of amp itself ?

Any input will be appreciated.

Thanks,