Telamon wrote:
...
Either one will not be an effective block unless they are designed to
prevent passing common mode RF noise. It takes very little in stray
capacitance or mutual inductance to go around the opto-isolator.


If you are an electronics engineer, I'd venture you are a damn poor one.
The opto-isolation I proposed would be about 10,000x more isolation
than an audio xfmr.

Coupling of rf noise though the audio xfmr is just a question of "how
much?" Indeed, I'd be surprised if they even provided proper shielding
(grounded metal sheet over inner winding with ends of sheet insulated
from each other) of one winding from another!

If the core material is ferrite, like in the cheap china junk, I'd throw
it away for such uses. Silicate-steel laminations might be usable when
rf is a problem ...

Regards,
JS