Truly high fidelity audio
transformer design is something of an art -- the audio output
transformer was often a major limiting factor in tube-type audio
amplifier performance.

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And then somebody invented negative feedback.

Which, of course, continues to be used with transistor audio amplifiers
which don't require transformers.

We may have advanced beyond moving iron loudpeakers but the trouble remains
with loudspeaker fidelity - as it always did.