On Wednesday, January 9, 2019 at 9:32:20 AM UTC-8, wrote:
I see really expensive audio output and power transformers for sale
as well as cheap ones. And lots of folks really want older ones
because they are better than the new crop. So what is the difference?

The core of a transformer has laminations, because a solid metal lump
would have electrical conduction losses (eddy currents) to cause large
heat losses (and reduce output efficiency). A power transformer
has thick laminations (60 Hz losses are small anyhow, but at 60 kHz,
that would be inefficient), and a switchmode transformer is made of
high-electrical-resistivity ferrite (at 60 kHz, the size can be small, though
such a low-magnetization material wouldn't be great for lower
frequency power).

Audio transformers are most efficient when made from very thin laminations of
soft iron, which is a more expensive construction than is used for either
60 Hz or 60 kHz power transformers. It just has more small parts.