on pedalboards, power and ground

[Renegadrian]

I made myself a small pedalboard for bass, just a tuner a digital pre from korg and a little control switch.
I got this power transformer (should be a LED transformer) who puts out 3A at 12V, so plenty of juice.
Also added a LED strip at the bottom for the show.
now, the inputs of this transformer are blue and brown, no green/yellow (ground).
at the other hand, red and ground, are the outputs for the led, the tuner, an heatsinked 7805 for the pre. (it works on 4.5V)
It works so good at home, but I noticed at reharsals a distinct noise on my guitar player's amp. not on mine, my system is silent as before (this transformer is in place of two different wall warts I used to power the pedals) but that noise on his amp...may it be the room ground!? may it be other equipment?! Is there anything I should do to my gear!?

[Jdansti]

Does the noise on the guitar amp stop when you unplug your power supply?

[Renegadrian]

yes, so I am sure it's from that power supply.

[Fender3D]

Try keeping your PS' ground separate from other devices' ground(s).
In Italy safe "ground resistance value" not always coincides with safest audio ground...

[intripped]

what about an EMI filter?
I read somewhere that these filters prevent noise not only from entering, but also going out to the power line.