[lebuffon]

Back in the good old days of analog audio we made differential audio inputs with Iron and copper. :-)

Transformers are amazing at reducing common mode noise like induced hum, but rather expensive compared to the price of these boards.

Fun fact: The entire landline telephone system used to rely on balanced lines with transformers for in/out. The hum nulling is so good that trunk lines had no shielding inside buildings.

[olelele]

there are transformers and transformers.

A lundahl or murata, the kind used in Studer, Neve & Yamaha mixer preamps can cost a whole lot.

There are pretty decent cheap ones from china these days too.

[lebuffon]

And don't forget AMI Tabfunkenwerk from USA.