Thanks for the recommendations.
I have been doing some experiments with the setup and the Zoom R24 only adds just a little bit of noise, there were other sources for hum and noise:
I had a Bass guitar connected to the Hi-Z input and the volumes of all inputs were set at 100 which added some noise too.
1.) Setup: Noting connected to the R24 except power supply and headphones.
Each input obviously adds noise depensing on the volume setting. Starting at about volume 90 you can hear it in the headphones.
This basic noise adds up which every input I add. If I set all gains to 0 there is silence.
This basic noise probably cannot be avoided. What I don't know is wheter the R24 is worse here than other product - I have nothin that I could compare with.
2.) Especially input 1 but also input 2 (because it’s close to input 1 ?) are sensitive to crosstalk/stray pickup (I’m not sure if this is the right term, I’m german).
When I touch the housing in the upper left corner I hear hum in the headphones. This gets very strong when I touch the screws of the XLR connector.
Strehngth of the hum depends on the setting (Hi-Z or normal) and on the gain setting. Similar effect on input 2 bbut not as strong. The other inputs are not sensitive in that respect.
I guess this could be normal for Hi-Z input ? Are these just more sensitive than other inputs ?
3.) I had a bass guitar attached tot he Hi-Z input and this thing is humming (except you turn Gain down to zero).
I guess this is pretty much normal to abass guitar ?
4.) My Midi-Keyboard (Kawai ES6) has a slight humming on the line outputs.
It sounds like some crosstalk of the digital electronics part of the keyboard. I don’t think there is much I can do here
5.) When I connect both, the R24 and the Midi Keyboard to USB of my notebook at the same time, this adds some more noise to the keyboard line outs. Seems that this is creating som ground loop.
The power supply of the R24 does not seam tob e aproblem, I don’t here a difference between power supply and battery operation. The notebook was running on battery.
I think that the R24 is not bad or defect, I just have effects that you get when you plug things together with analog signals and usb.
Need to experience some more to find the best setup I guess.
Thanks for your help