Phil Leigh;539396 Wrote: > No - I specifically mentioned valve amps as the valves, HT caps (because > of the 450-600V) and o/p tranny laminations all do "settle down" from > new... > All of my guitar amps have done this. > > Low voltage transistor and IC gear does not have this issue. On the > other hand, Class-A tranny amps and valve amps need a period of warm-up > to let the quiescent current/temperature stabilise.
I HAVE heard caps get worse during "burn in". Specifically VitaminQ paper in oil high voltage caps will sound pretty good when first installed, but after 10 hours of running music they sound abysmal, harsh, grungy, very yucky sound. After another 15 hours or so they start getting better and by about 30 hours are about where they started. Between 30 and 50 hours they improve some more and sound better than they did originally. Every VitaminQ I have ever used goes through this process. Other brands of paper in oil caps go through something similar but with a different time table. Some take only 5 hours to sound awful. Plastic film caps (polypropelyne, polystyrene, polyester) don't have this wild behavior. I have heard some improve somewhat over time, but not nearly like the paper in oil. Transformers also definitely burn in. Something about the magnetic properties of the core changing while the domains realign themselves constantly for the first several hours of use. How much and how long it takes is very dependent on the core material used. Nickel behaves very differently than steel in this regard, and cobalt yet again quite different. There is another completely different effect which is warm up. This happens every time the device is turned on. Most electronic components DO change there electrical properties with temperature. Even digital audio can be affected by this. I have looked into this extensively and found at least two methods by how temperature changes can effect digital audio. First is the resistance of the transistors inside the digital circuits. Cold FETS have a lower resistance than hot FETS. This causes the circuits to switch faster which means the switching currents induce greater voltage drops/bounce on the power traces inside chips and even in the ground planes on boards. This is not fiction, its actually quite easy so see. Unless extraordinary measures are taken this noise WILL affect clock circuits and even analog circuits. Its especially noticeable in some DAC chips. They sound much better as they warm up. I did an experiment where I took a cheap DVD player and glued a resistor to the top of the DAC chip and fed it DC current to warm up the chip. I also added a thermocouple to the chip to take its temperature. I could radically change the sound of that DVD player by warming up the DAC chip. It sounded much better as it got hot. Of course at some point it stopped working altogether, so it CAN be over done! This might be one of the reasons I like DAC chips that get fairly warm in operation. BTW some DAC chips are very sensitive to temperature and others not nearly as much. The other method is in oscillators themselves. Many crystal oscillators actually have lower jitter when they are warm. Crystal ovens are not JUST for stabilizing the frequency. In my own designs I like to use oscillators that get fairly warm, the jitter is lower than cool running ones. These effects are actually exhibited in the Touch, but since the temperature doesn't rise by a lot between cold and on for a long time the difference is not very great. I've seen DVD players that have powerful DSP chips that get VERY hot exhibit these effects to a large degree. I did not notice any burn in effects in the Touch, but I CAN hear warm up effects, but its not very much. I suppose I should try the "resistor on the DAC chip" test on the Touch and see what happens. John S. -- JohnSwenson ------------------------------------------------------------------------ JohnSwenson's Profile: http://forums.slimdevices.com/member.php?userid=5974 View this thread: http://forums.slimdevices.com/showthread.php?t=77727 _______________________________________________ Touch mailing list Touch@lists.slimdevices.com http://lists.slimdevices.com/mailman/listinfo/touch
