Can temperature fluctuations affect audio gear?

That strikes me as a good datasheet to use for purposes of focusing the discussion, Magfan.

It's interesting to note, extrapolating from the data that is provided, that its maximum rated power dissipation of 115 watts at a case temperature of 25 degC/77 degF (commonly referred to as "room ambient") does not drop off by a factor of 2 (to 57.5 watts) until the case temperature has risen to 112.5 degC/234.5 degF! And it can go considerably higher than that, as well, if the power that it is called upon to dissipate is reduced such that the junction temperature is kept below the rated maximum of 200 degC/392 degF!

Some related things that should be kept in mind, though:

1)The numbers provided are "maximum" ratings, commonly referred to in other datasheets as "absolute maximum" ratings. Those are the ratings which if exceeded stand a good chance of causing immediate failure. A good design will provide a very large margin between those ratings and the actual operating conditions. As noted in the reference you provided earlier, a rough rule of thumb is that each 10 degC reduction in junction temperature doubles mtbf.

2)"Derating" can refer to two different things. It is used in the datasheet to refer to the falloff in MAXIMUM power handling capability that occurs as case temperature increases. "Derating" is also used to refer to the amount of margin that the design provides between the rated maximums and the actual operating conditions.

3)The amounts by which both case temperature and junction temperature rise above ambient temperature will depend on the adequacy of the heat sinking that is provided, and on how much power the circuit application requires the device to dissipate.

Best regards,
-- Al

Lesson Learned:
In Italy their is a very nice Roman Aqueduct bridging a river. It is 3 layers high and is in perfect shape after who knows how long? If needed, it'd probably be easy to put it back in service.

Recent analysis shows it to be built to 'modern' standards of about 2:1 over the maximum anticipated stress.\
Article was in Scientific American, so I could probably look it up if anyone was curious.

Running stuff at redline is a sure-fire start of problems.

whats actually funny is that if you really look at how cooling in electronic systems are designed you would be amazed at how much is "approximations" and "best practices" over spiffy math. Lots of software now to help but still lots of design with margin in case you screwed up. I suggest just following the MFG recommendations. Reason is someplace some poor engineer is figuring out the board layout from a thermal standpoint and he or she is using the recommendations from the component vendors anyway so when you get right to it everything is coming from the manufacturer of the descrete components. Only variable is how paranoid the integrator is feeling. cool discussion all.

Paulsax, Yes there is a lot of approximation. Junction to ambient temp. coefficient often assumes certain size of heatsink on vertical PC board center located in 1 cubic foot of enclosed space. It never happens so designer has to approximate a lot using large design margins. According to Texas Instr. study probability of semiconductor failure rises fast above 100 deg C junction temp.

I would not worry for semiconductors in properly designed electronics as much as for the life of electrolytic caps that is shortened by half for every 10 deg C increase (starting at about 50k hours at 20deg C).

Free air convection inside of audio cabinet is poor because of shelves and often lack of vent holes. Making such holes or even inserting tiny silent microprocessor fan to force air thru the cabinet would help a lot.

If I felt the need to design something needing heat management, I'd try to over engineer by at least a factor of 2x. This would be using all rule of thumb estimates and data sheet numbers. If a doubt existed, buy the next larger heat sink. More space between caps. Vent the transformer. Any wacky thing I could think of to shed heat.
If I could work the math, I could probably cut it closer and save money, time and bulk.

Read up on some heat management issues. Just an example:: If the heat sink is fins are up / down the natural convection will funnel air up. You should provide venting above and below to facilitate this flow.

Put the SAME heatsink horizontal, with the same load, and suddenly you have way too little heat sink.

Add some forced air to EITHER and you are ahead. Forced air fans may require or tolerate different fin spacing.

I look at the ratty heatsink from an old CPU. What an awful design. Fan blew down into it and then out the sides. Problem? Well, I can't imagine much airflow in the center, at the base of the fins. I use this otherwise worthless extrusion as a letter holder. Works GREAT for that!

Point? BigBucks is right. There IS a substantial science behind this stuff. Wouldn't surprise me to hear of very close heat budgets in aerospace applications where every ounce shot into space requires gallons of fuel and every cubic inch counts. Knowing EXACTLY how much heat at what junction temp and how fast it migrates is within the realm of 'knowable'.....