Physical explanation of amp's break in?

"Just curious, is there ANY component in the amp's circuitry that known to cause such a behaviour?"

Nope.

If I agree with Bomarc, would anyone be surprised?

My understanding of break in (of components and cables) is that current flowing in new circuitry is finding the path of least resistance through the lattice of molecules that make up conductors.
For components the highest concentration of conducting material is usually in the transformer(s) of the power supply.
Complete break in takes much longer than most manufacturers will admit. Typically, there is a noticeable improvement in the initial hours of operation with gradual further improvement. But in my experience full break in can take six months or longer.

Resistors shift value, diodes will alter their point of conduction, transistors will alter their point of conduction and their gain curves, the dielectric in capacitors are forming, etc... All of these things do this on an individual basis on their own schedule and the differences between new and "broken in" might seem quite small on their own. When all of these "shifts" are added up and combined though, both the measured and sonic results can be quite different for the entire circuit.

Just bare in mind that stressing any given component, whether it be due to amplitude of signal, thermal changes, etc.. will cause further shifting of value on some components. This can have a domino effect as the newer shifts in value can cause other circuitry to respond differently. As such, it is possible for a device to change even further ( measurably and sonically ) after you think that it is "fully broken in". This is especially true if you start running it harder or in a very different manner than what you used to.

In extreme cases, these "value shifts" for individual parts can be significant enough to cause the unit to operate erratically or not at all. The fact that components do shift in value is what causes them to "wear out" and need repair. I have even seen where a 6" piece of wire inside of a unit developed a resistance of appr 50 ohms. While i could not explain how this happened, it did. To be quite honest, this threw me for a loop for quite a bit of time too. The last thing that one would ever expect would be resistive wire in a low voltage / low current circuit, yet stuff like this happens.

Other than that, anybody that tells you that components don't break in or "settle" is either uneducated about the subject at hand and throwing out a guess at your expense ( IF you believe them ) or knows better and is blatantly lying to you. When it comes to subjects like this, you either know what you are talking about based on book smarts and education and / or first hand experience or you are talking out of the top of your hat. Given the fact that i've been making money by troubleshooting / repairing / modifying / designing electronic circuitry for 25 years now should give you some idea of where i'm coming from in terms of experience. Sean
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PS... I'm sorry if this comes across as "confrontational" to some, but when you try to pass off bullshit or a personal opinion that can't be verified in any way, shape or form as fact, i have little room for compassion.

I believe in "break in."

But, I also believe there is an emotional component to audio.

Your sense of hearing can change based on your emotions.

When you are stressed, certain frequencies can become subdued
while others come to the fore -- not a good thing if one is listening
to music and trying to hear the full range.

After purchasing a component, we often go through an emotional period. If one has splurged a little bit, bought an expensive piece of equipment, and any piece can feel expensive as one upgrades,
one can have unrealistic expectations, feeling that the new piece should be perfect, should feed the dog, make coffee, and make you feel like the musicians are giving you deep tissue massages
on their breaks.

Instead, because one is nervous and certain frequencies are
jumping out while others are subdued, the system can be extremely dissapointing. After awhile, you get used to the fact
that you spent the money, that the piece is not going to be healing wounds or raising the dead, and you go through a period of
acceptance. That's when your stress goes away and -- voila --
your new equipement sounds a LOT better.

On the other hand, you may have read that a number of "audiophiles" have used this particular cable and it made cardboard boxes sound like Wilson Watt-Puppies! In this case,
peer pressure over-rules any other stress and -- damn if you don't
plug those cables in and -- wow -- you can hear EVERYTHING!
Not only can you tell what kind of underwear the singer is wearing,
but you can even discern the brand of detergent in which they were washed -- you are ONE OF THE GANG!

What happens if one buys these cables and fails the underwear
and detergent test? Well, that's because the cales need to..... "BREAK IN!"

I believe some stuff does sound better after a break in period, but
I also believe there is an emotional element to all of this as well
as Maslow's hierarchy of needs. The need to belong can be intense, buyer's remorse is very real, the feeling that you were
duped into buying epensive gear can grab you like a pitbull once
you bring it home and listen with your stress altered auditory nerves.