Originally Posted by JimmyM
The SWR SM-500 I used to have said in the manual not to use the XLR with phantom, but I did many times and it never blew up the amp.
IIRC the ST220 & SM400 had the OP amp DI module without the DC block, the SM500, SM900 and the other newer ones had the "improved" design.
Sure the OP amp DI is a step upwards from the resistor circuit, but a transformer is really the only way to go.
Galvanic isolation isn't possible otherwise.
Actually, the DC block is more important with some audio transformers than it is with OP amp design. Even measuring the resistance incorrectly may saturate the core. Then it's $100+ out of Your pocket instead of ~2$.
Fortunately the manufacturers who use such transformers have the sense to protect 'em as well.