I've actually looked into this quite a bit. Originally I was very skeptical of burn in, but after having experienced it I set about trying find out whats going on.
What I found was that there are two classes of componants that can exhibit significant burn in effects, capacitors and magnetic components (chokes, transformers). ALL magnetic devices in the audio path that have magnetic "cores" exhibit burn in, its a property of the magnetic materials. These work by collections of atoms called magnetic domains chainging orientations due to applied magnetic fields. For a freshly manufactured piece of the metal the properties of this "orientation flipping" change as they are flipped. As they flip back and forth the amount of energy required to make the flip and various other properties change slightly. After a while things stabilize and it stops changing. Some of these materials will tend to gradually revert to their original state if you don't use them for a long time, so you have to burn them in again after a long period of unuse. Capacitors are more interesting. A capacitor consists of two metal "plates" separated by an insullator (the dialectric). When a voltage is developed between the plates it creates an electric field between them which produces a force between when. When that force is varying (say from a music signal) that force also varies. This creates vibrations between the plates, causing the distance between the plates to vary, changing the capacitance along with the signal. This gives rise to distortion of the signal. Its not very much but it IS there and is measurable. To make this worse almost all capacitor construction techniques produce mechanical resonances in the audio band. These resonances particularly affect transients, again its not a large affect, but it is measurable. Thus changes to the mechanical resonances of a capacitor can affect what it does to a signal. Take for example a film and foil polypropelene cap, as its used the "micro vibrations" can slightly change the relationships between the film and the foil, changing the resonant properties. Sort of like "the contents of this box may have settled due to handling". There is a type of capacitor that exhibits this effect to a very large degree: the paper in oil capacitor. The dialectric is a sheet of paper soaked in oil. As the cap is used the vibrations cause the oil to further penetrate the paper matrix, significantly changing its resonant properties. After about 50 hours this process reaches a stable point. In this condition the capacitor is very well damped and has very low resonance effects, which makes it one of the best sounding capacitors. BUT during the "burn in" period those resonances vary significantly, causing the cap to sound horrible for part of that time. Its really startling how bad these caps sound during the burn in, its like everything is getting sent through a guitar fuzz box! The first time I built something with these caps I was very perplexed by this, it sounded very good at first, but after about 5 hours it sounded terrible, I thought something had burned up, come un soldered etc. But everything seemed fine, all the voltages were right where they were supposed to be. I let it play for another day, and it started sounding much better, after two days it sounded way better than it had originally. I have tried several different brands of paper in oil caps and they ALL do this, but the time frame can vary from one to another. One primary feature of paper in oil caps is a high voltage rating so they tend to only get used in tube circuits. They work VERY well in SS circuits as well, but are rarely used commercially because they are fairly expensive and are large. Because metalic cored magnetic components and paper in oil caps are almost exclusively used in tube circuits, people with SS gear may notice very little burn in. (anything with caps in the signal path will have SOME burn in, but its probably pretty small) There is a lot more to say on the subject, but thats all the time I have right now. John S. -- JohnSwenson ------------------------------------------------------------------------ JohnSwenson's Profile: http://forums.slimdevices.com/member.php?userid=5974 View this thread: http://forums.slimdevices.com/showthread.php?t=86359 _______________________________________________ audiophiles mailing list [email protected] http://lists.slimdevices.com/mailman/listinfo/audiophiles
