What Exactly Does "Burn In" do for Electronics?

I know this won't be good enough, but I'll try one last time.

This is directly out of the manual for my Ayre V-5.

"Break-In
100 to 500 hours of music
played through the system will
ensure full break-in.
Due to the manufacturing processes used for the
printed circuit boards, wires, and capacitors, a
break-in period is necessary for the amplifier to
reach its full performance potential."

Here's one from Pass Labs.

"Amplifiers
Do the amps need to break in. Yes. Most improvement comes in the first 24-36 hours, after which the amps will continue to improve as long as they are left on.

Preamplifiers

How long do the preamps need to break in?

That depends. The XP-10, XP-20 and XP-15 need about 24 hours to fully warm up. The XP-25 and XP-30 take closer to 36 hours. After initial warm up period the preamps will continue to improve as long as they are left on."

If you want more examples like this, there are thousands. All you have to do is look for them. If you read reviews from magazines like TAS and Stereophile, break in details are included in almost every equipment review they publish. (I haven't read reviews for about 10 years now. I'm assuming they still talk about break in.). You can also call a manufacturer directly. They are usually happy to answer any break in questions pertaining to their equipment.

Other that that, I don't know what more I can offer on this topic.

06-23-14: Georgelofi
So it seems like we are in agreement, that to us "break in" for better or worse can occur with the mechanical devices such as speakers drivers, cartridges, and tubes, as you also said.
And you and I will not commit without skirting around the edges and state to "break in" periods of semiconductors or active components can get "better sounding" with said "break in" times of 100's of hours, unless some technical proof is brought forward, forgetting hearsay of friends and customers.

George, that is not quite what I said. Note particularly item no. 2 in my previous post, which is entirely consistent with what is stated in the references ZD cited just above.

Peter (Csontos), the potentiometer-related effect you described certainly strikes me as being plausible, and a clever observation, but my suspicion is that it is just one of a great many effects that can occur.

Best regards,
-- Al

"ZD cited just above." Is again not a technical explaination. Could conscrewed as getting used to the sound so you don't take it back for refund.

As for trimpots Peter (Csontos), yes if they are left in one position for a while they do tend to loose contact between the wiper contact point (metalic), and track (carbon or conductive plastic) because of current flow between dissimilar materials.

Part of the same reason for the Lightspeed Attenuator, no contacts of dissimilar materials (diode effect). When ever I see this, I take out the single turn trimpots and replace with Bourns multiturn, they last a bit longer but still have the same problem after a while. Maybe we should use the Lightspeed system as bias pots as well, now you've given me another idea.

Cheers George

I'd love to try that George.