I have 3 devices (see image):
ZOOM H1 recorder
The recorder is connected to laptop through USB and works as a digital audio interface. I use it to record sound from the synthesizer (through audio cable).
Everything is OK, sound is clean, until I connect USB cable to the synthesizer too (used as USB-MIDI interface). Then there is a high pitch noise in the sound (approx 680 Hz and 1 kHz).
There is a lot of questions like this on the internet and there are many answers too. I got confused by everything and I don't know what solution to choose.
Suggestions that I have found:
Use an audio isolator: e.g. this Might degrade sound quality.
Use USB isolator: e.g. this
Use a modified USB cable with disconnected ground Just dismantle a spare USB cable and cut off the ground. Not sure how safe this is.
Connect grounds of USB cables together Not sure how to do this...
Use powered USB hub Tried and did not help.
Connect everything in one AC outlet I have everything in one AC, so this does not help.
Can somebody help please?
Further details: I have read several posts about this problem, and I am not even sure that my problem is ground-loop related. I have tried to disconnect the laptop from AC (running on battery only) and connected only headphones to the recorder, so the only thing that was connected to AC was the synth, so there should be just one ground, right? But this did not help and the same noise was still present.
Update: The same noise is present even if I unplug the synth from AC. So, nothing is connected to AC, not even laptop, and still noise is present.