You'll probably need a regulated adapter. You can easily make one yourself using any 12V DC adapter, a few caps, and a 7809 IC. An example circuit can be found in the 78xx datasheet.

If you are using a regulated adapter already, the mixer might be pulling too much current from it.

Adding a large buffer cap to an unregulated 9V DC adapter might be sufficient, too.

hmm
The PSU I tried is a "switching" one, variable voltages 1.2A ... it's one of the new light ones, the roland one is 10 times heavier.
I got some more questions now, from the 78xx datasheet here to which example are you referring too? the "Figure 9. Fixed-Output Regulator" one?
And about that buffer cap (that seems the easiest to test) how can I test it? does it go between vcc/gnd?

The circuit in figure 9 will do. If you need more current than 1A then go with the ciruit in figure 13 or use a different voltage regulator.

For a buffer cap you can just take any large electrolytic and connect it between +9V and ground, yes. Make sure not to connect it the wrong way around, of course 😉
A switched adapter has a lot of high-frequency harmonics though, which are difficult to filter out with an electrolytic cap. You could try adding a ceramic cap of about 10-100nf in parallel.

thanks jwt27! Will test tomorrow and report 😀 both are easy.
I assume that CO 0.1μF on the datasheet is a ceramic one right? I can use a regulated 12V psu and make an internal mod with the 7809 , it seems like a good plan 😎

update:
I tried today with a 2200uf + 100nf caps in the input line, sadly this made no difference using the same psu I tested before. BUT I found another switching psu (that I was considering to be of inferior quality) that does not make the parasite... So I will use that 😀
I thought of leaving the filtering inside with only one change because of room problems: the 2200uf cap became a 1500uf one

So yeah, it works fine now (for a cheap mixer that is) . Thanks again jwt27 for the help!

I remember having a ton of issues with this.

Some of my old videos have this buzzing in the audio 😀

I tried many things, recording on a notebook on batteries, USB sound cards. In the end I got a decent Sound Card for recording (X-Fi Titanium HD), a decent mixer (it's a Phonic AM-240) and new cables. I still had issues with the video signal. Ranging from tiny jitters to constant wobbly picture. Using splitters, KVM and the VGA to S-Video adapter in various combinations minimised the issue. Best signal I got was directly of the S-Video out of a video card.

I'm curious to see how I go with my new VGA capture card. But I might soon go all digital (DVI > HDMI) on my time-machine for the purpose of capturing.

yeah, it's a common problem with cheap mixers and cheap PSUs... I have another similar stereo mixer , a cheap XFADE VL-303A one, it came with it's own "double grounded" psu and is absolutely quiet. No parasites at all. But I can't really complain on the HAMA one, it was a 5eur amibay purchase and well worth it's money 🤣
Be sure to get good shielded cables Mau1wurf1977 for audio/video, cheap ones tend to pick up noise from everywhere, I had a stereo cable that picked up noise from a ps2 mouse if the cables were close 🤣