Audio mixer: how to reduce audio buzz from psu?

Hiya!

I recently got a very nice and small Hama stereo audio mixer, 2x Stereo IN and 1xOUT , perfect for use with a retroPC that has an external midi module... plus it has a retro-look that is great next to 90s midi hardware .



I know it's a basic/simple one but it's small and tbh offers the basic function that I need. But it came without a PSU. I tried using a generic 9v one that I use for consoles and other stuff but it produces a constant triangle-wavish sound (?) (like the example here: http://en.wikipedia.org/wiki/Triangle_wave) that is audible from ~50% of master volume. Clearly too much noise from the PSU , so I tried an original Roland 9v psu from a SC-55 midi module. The parasite is still there but more quiet and more like a deep buzz now.
Ofcourse using a 9v battery solves this problem but I don't really want to mess with batteries...
So I just opened the unit , the 9v input is on the upper left side:

(click for larger pic)

From what I know I don't see any input filtering (or it's a very basic one? I assume the diode is to prevent reverse current?) and I was wondering if anyone has any suggestions of how to fix this... maybe add a capacitor or two? Or some filtering component?
Any suggestion will be highly appreciated, thanks in advance!

 

Perhaps add a 1000-2200 uF cap on the power input?

 

that seems to be the easiest thing to try, will try that tomorrow and see how things are , thanks!

 

That is how i shut up my atari 2600 jr with switchmode style power supply. 2200uF 25v cap across the reg, nice n quiet now.

 

thanks for the help, the problem was solved by adding a 1500uf+100nf caps on the power line and using another psu... There was no room for the 2200uf ones I have available atm but the current result is great for this little cheap mixer



funny thing is that the new psu is a no-name one... apparently the "good" one is very noisy :\