Q: What is burn-in?

A: Burn-in is the process by which the parts of a device change physically, “settle down”. Headphones are usually a bit stiff-sounding “out of the box”; headphone diaphragms are manufactured “stiff”, designed with certain durability as a target. They need time to “soften up” and settle into the performance envelope they’re designed for.

Q: How can headphones burn in?

A: By playing sound. A music collection in a player left on “shuffle/repeat” overnight may do it, though some people use pink noise combined with pauses (e. g. 20 minutes of pink noise and 5 minutes of silence) for burn-in, to prevent overdriving headphones. The Audiotest CD has 30-second pink noise and digital silence tracks - those can be combined in a playlist to last as long as necessary (e. g. ten minutes of noise and two of silence).

Q: How do amplifiers burn-in?

A: Lynn Olson, of Nutshell Hi-fi, has replied to the question of why amplifiers require burn-in.

Question:

How exactly do amps burn-in?
Headphones and speakers have their rigid parts soften, but what about
amp components? Which parts exactly benefit from burning-in, and in
what way?

Lynn’s answer:

Capacitors, especially electrolytics, can exhibit subtle changes with
time and temperature - enough so that electrolytics need to be
routinely replaced after 20 years, whether they’ve been used or not.
I suspect nearly all “burn-in” changes are due to electrochemical
effects - that would be consistent with the time constants of 5~100
hours that people report.