It is a random thing so no way to predict it. But again, vast majority of time this is not a problem. So don't be too anxious about it.
If you get SMSL SU-8 and pair it with a balanced amp input, then you have more of an insurance here.
My current system: (Note: ODAC Rev B is usb-powered DAC)
Computer back IO panel ----> Cheap USB Cable ----> ODAC Rev B ----> Cheap RCA Cable ----> Schiit Lyr 3
My DX7s shipping is delayed due to labor strike in Canada, it will arrive next Monday or Tuesday (3-4 more days), but since I am so afraid that I will have the same ground loop issue as my original DAC. I went out and bought idefender and iPower just in case. (I am going to return them if they don't work.)
===== Here is the testing Results: =====
Original:
Computer back IO panel ----> Cheap USB Cable ----> ODAC Rev B ----> Cheap RCA Cable ----> Schiit Lyr 3
Result:
Max volume on Lyr 3 using low Gain: observed static and hum noise (very clearly audible), when I move my mouse, a high pitch beep sound occurs
Max volume on Lyr 3 using high Gain: observed heavy static and heavy hum noise (very loud), when I move my mouse, a loud high pitch beep sound occurs
Conclusion:
Observed static and hum noise (very clearly audible).
With iDefender Only:
Computer back IO panel ----> iDefender----> Cheap USB Cable ----> ODAC Rev B ----> Cheap RCA Cable ----> Schiit Lyr 3
Result:
Max volume on Lyr 3 using low Gain: observed static and hum noise (very clearly audible), when I move my mouse, a high pitch beep sound occurs
Max volume on Lyr 3 using high Gain: observed heavy static and heavy hum noise (very loud), when I move my mouse, a loud high pitch beep sound occurs
Conclusion:
Absolutely no change whatsoever compared to Original setup.
With iDefender + iPower:
Computer back IO panel ----> iDefender + iPower ----> Cheap USB Cable ----> ODAC Rev B ----> Cheap RCA Cable ----> Schiit Lyr 3
Result:
Max volume on Lyr 3 using low Gain: No audible noise, no noise when moving mouse
Max volume on Lyr 3 using high Gain: No audible noise, no noise when moving mouse
Conclusion:
Audible noise is completely killed. There is no magic here. iDefender is useless by itself. iDefender + iPower combo just replace the usb power from computer power supply to a cleaner 5V power.
Note:
My DX7s will arrive in 3-4 days, I will test it with and without the iDefender + iPower combo and see if there is any difference. (For science!!!!!!!!)
@amirm thanks for trying to help me, but I am still very very very anxious about it, I ABSOLUTELY don't want to use iPower and iDefender at all, but if the noise is still there even with DX7s, then.... I will.....................................be sooo mad!