Physical explanation of amp's break in?

there is no break in...its you getting used to the sound

Sean is right, as a piece ages, the components inside it change value. Resitors changes value, electrolytic caps start to dry out and ALL of them will eventually fail if used long enough, substances such as plastics used as dielectrics change especially when they get hot, and it is indisputable that tubes change as they are used.

I have rebuilt numerous tube guitar amps over the years and the change in sound is quite amazing when the carbon resistors that have risen in value and the old, dried out electrolytics are replaced.

Electronic components have a rated lifetime but they don't have an instantaneous change in value at the end of their life. They change gradually over a period time until finally they are stressed past the point they can withstand or the circuit they are in no longer functions properly.

Solder joints are a very good example. There have been numerous studies about this and volumes of data that support the fact that these joints change over time.

Whether you want to call this "breaking in" or not is up to you. Whatever you call it it is very real.

Sean is absolutely correct--all of the phenomena he mentioned do in fact occur. While electronics changes are perhaps more subtle and gradual than changes in speakers, they happen and are audible for one who has the ears to hear it. His reference to changes in use patterns was particularly interesting. I used a pair of $300 bookshelf speakers for 2 years while looking for new ones. I began pushing them harder and harder as I got closer to replacing them, and listening one day, I had to admit that even after 2 years, they had undergone a major change in sound, and up till then were not truly broken in. I've had my Magnepans for well over a year now, and even with extensive hard use, I have no trouble believing that they are still undergoing changes. In short, believe it!
Chuck

A few points that were mentioned in the above post and responses:

- manual mention 6-week break in period for the unit (SimAudio)
- component values shift over time (known fact,though this aging process is much longer than six weeks)
- break in or "settle" (Sean) VS breaking-in until it breaks down (Pbb).

The last one is definitely worth my 2 cents. Personally, being an EE, I prefer the term "settling time". And our ears settle to the unit's sonic signature long before any radical change in any passive component value (active components, being bias dependent, are excluded). Of course, no break-down is ever desirable.

"Recently purchased Moon i-5, manual mention 6-week break in period, when bass will first get weaker, and after 2-3 weeks start to normalize. Just curious, is there ANY component in the amp's circuitry that known to cause such a behaviour?"

None of the effects Sean noted are known to cause the behavior Dmitrydr asked about. Hence my original answer.