@Badboy
It could be that some EMI is getting through to the amplifier. I suggest that:-
1) you make a couple of 1uf film capacitors between + and ground and - and ground on the output of the smps. This will filter HF noises on the supply rails
2) when the smps is fitted in the amp box make a metal screen between smps and amplifier this will screen a lot of interference generated by the smps. I think you still got a lot of wires going round and these all serve as antennas. I also suggest you add an input filter at the mains. Try also to decouple the center ground with a small ceramic cap to chassis.
Is that fuzzy signal audible? Does it increase with more power? Did you try a radio near the smps ? (about a foot away) you will hear some noise for sure and the more load the more the noise. This will tell you what you are dealing with. Try to screen the cables to the input of the amp and also the supply wires from smps.
EMI is not so easy to get rid of and good filtering is needed.
Silvio
Thank you for your reply
since I'm using this app for driving a subwoofer, i can't hear any noise on my subwoofer (like it's not even connected to amp) and also this happens as the output power on amp increases (when smps goes under load)
I also used the input main filtering from an Pc psu and that did't effect anything
I also made a new pcb for my amp and now there is much less wires involved
I just connect the amp to 50HZ transformer and put the amp next to the smps as it was under load (about 600W)
there was no distortions and very clean signal in amp output
I made this test with and without input EMI filters on smps and there was definetely no differences
So I guess this problem has nothing to do with magnetic interference or SMPS input EMI filtering
I test the radio thing and yes, I definetely could hear some noise in 1 meter radius while smps was under 600W load
I think this problem comes from SMPS output rails
I will try that 1uf idea and let you know the result