Thank you rstofer! It is very helpful. I already ordered Boss B25N RCA to RCA Ground Loop Isolator. I wanted to clarify here. It seems like that using B25N will be 100% safe to send tones from a signal generator (computer or any), not need to follow options 1 and 2 above. Right?
However if I understood correctly, it is not safe for signal source for a circuit testing. Alternative for that is to have OP AMP solution. Is that correct? Please clarify.
I was looking for OP AMP solutions. I found mostly DIYs to maker it myself. I may do it but most likely later. Would you recommended something that available for purchase or appropriate kit to make it myself. I was looking for the options and did not find something within 20Hz -20kHz yet. Thank you again.
I didn't find a suitable 'kit' that covered the entire spectrum. I didn't search eBay and I didn't spend a lot of time looking. I kind of like the multi-range unit I linked to above. But it is a DIY project - fun but time consuming. I did find some fixed frequency signal generators but I don't see the utility in those.
For running a sound card, how about an old PC that is due for retirement. I have at least 3 Dell OptiPlex systems that are expendable. But you can absolutely bet on the fact I will never use my Surface Book as a signal source.
Technically, the isolation transformer should be entirely adequate for eliminating ground loop (hum) but what happens if you cross up 120V with the secondary of the transformer. Well, it's still a transformer so the 120V appears on the sound card and the motherboard. I'm just not going there.
No matter what it took, if I were doing debugging mains powered stuff where there was any possibility of getting into voltages above 5V, I want an expendable source.
Other people will scoff at my hyper-vigilance but that's the way I feel about troubleshooting stuff that could potentially destroy my equipment. I started with one problem, now I have two!You could take every precaution and check every test point with a multimeter but still slip up and hit something that might not mix well with your source.