Can temperature fluctuations affect audio gear?


Don't know about this...some owner's manuals say that you should allow equipment and tubes to warm to room temperature before using them, but this is different. My audio room is upstairs, isolated from the thermostat. Have to keep the door closed so the dogs don't venture in there and create havoc. Hence, in summer, the temperature in the room regularly goes to 85 degrees or so. In winter (like now), it will easily drop below 60 degrees. No need to worry about equilibration, since the gear is always in there, but should I worry about the temp fluctuations? Could get a baby gate to keep the dogs out, then it would stay 70-72, but otherwise, in winter a space heater is the only option.
afc
The mention of temperature in literature is really for extreme cold. If a very cold piece of electronics is brought into a warm room, it will condense moisture out of the air on ALL the electronis inside. You want to wait until this condensation goes away before turning it on.
That is the primary reason for the warning.
So if the stuff is the same temperature as the environment, that warning doess not apply.
Enjoy.
I agree with Elizabeth. I wouldn't worry about the possibility of damage, if that was the point to your question.

I would add, though, that from a technical standpoint it does seem conceivable to me that a 25 or 30 degreeF difference in ambient temperature could result in sonic differences, at least to a subtle degree. But I would expect those differences, if any, to be equipment-dependent and unpredictable.

Also, it wouldn't surprise me if a temperature difference of that magnitude caused differences in your own hearing mechanisms as well. :-)

Regards,
-- Al
If you plan on using the system,I would open the door,and blow a fan on it(system),while it warms up.Maybe a hour,or so first.The warm humid air from downstairs,plus your humidity might cause condensation in them,if it gets fairly cool.
The amount of heat your gear will 'give up' to the environment is related to the difference in temp. If your amp runs at 100f in a 100f room, very little heat will be transferred. In a 65f room, you've got no problems.

During the hot part of the year, at least make sure you are well ventilated or perhaps even install a fan near any hot gear.....But be careful not to just blow at hot stuff, if you restrict the natural convection, you could possibly even make something run warmer...while trying to cool it. Any fan I place is above the hot piece and blowing AWAY to suck air thru the piece and allow 'nature' to do its work.

And from our FWIW department, those nutty radio astronomers run their amplifiers and much electronics in a liquid nitrogen bath at colder than -320f.
This is to reduce the 'thermal' noise of atoms bumping into one another. In the very highest resolution systems this may make a difference. Or not. LOL.

Other temp effects may be on the speakers where characteristics of some synthetics....perhaps woof cones, may change from the coldest to warmest temps encountered. I'd also suspect greater power handling capacity in the coldest weather. The voice coils will cool much better.
12-03-10: Magfan
The amount of heat your gear will 'give up' to the environment is related to the difference in temp. If your amp runs at 100f in a 100f room, very little heat will be transferred. In a 65f room, you've got no problems.
--------------------

Maybe you were trying to say something else, but it didn't come out right. The electronics will essentially be at a constant delta above room ambient (more or less). If your amp runs at 100F when the room is 100F, it means that at 65F room, the amp would run at 65F. Of course, that isn't going to happen.

If the amp is at 100F when the room is at 65F, then when the room is at 100F, the amp will be at 135F. The heat transfer is constant, that's why the amp runs at a constant delta T above ambient.
The ambient temp can effect speaker performance. I experienced this first hand with a pair of Monitor Audio Studio 20s. Below about 70° the sound would lose its openess. It could be due to the metal cones in the Monitor Audio, but I haven't noticed the effect with other metal cone speakers I've had.

I could easily see how the cold could effect a listeners perceptive abilities both positively or negatively.
I see how I miswrote. But I'll stand by.....that as the temperatures of the room and amp get closer together, heat transfer slows.
Point is, cooler is better and you can cook it in a hot environment.
Heat and temperature are 2 different things.
And NO, the amp won't always be the same temp delta from ambient. In SS, for example, you have a max temp possible....say the junction temp of the devices. In a hot room wont' the difference drop as the room temp approaches junction temp? Or will the junction keep getting hotter until failure? Isn't there an upper limit to the temp of an amp?
2 amps of identical efficiency and power rating being run identically will be at different temps depending on the mass of the amp. And we all know how much heatsinks cost.

It's kind of an aside, but look at a few Stereophile amp tests where they 'precondition' an amp at 1/3 power for an hour before bench measurements. Some amps fail.

I'll call a friend of mine who is a PHd in physics. he'll straighten me out. His area of expertise is semiconductors, so it'll be good info.
My system sounds noticeably better in the winter and colder months than it does in the summer. I am not including the sound of the central air or heating system as I have done by listening comparisons with the AC and heat turned off. It seems that the warmer air effects the sound in the room. A close audiophile friend of mine has found the same.
12-04-10: Magfan
Heat and temperature are 2 different things.
And NO, the amp won't always be the same temp delta from ambient. In SS, for example, you have a max temp possible....say the junction temp of the devices. In a hot room wont' the difference drop as the room temp approaches junction temp? Or will the junction keep getting hotter until failure? Isn't there an upper limit to the temp of an amp?
-------------------------------------------------

That's the point. The junction temperature of SS electronics is a delta above the board temperature. As the ambient increases, the board temp and the junction temp increase proportionally until evntually you exceed the max allowable device temp and then failrue will result some time thereafter.
Magfan & Bigbucks, I'll say first that thermodynamics was definitely not one of the shining successes among the courses I took in college, but pending further info from Magfan's PhD friend I believe that Bigbucks is correct.
Magfan: In SS, for example, you have a max temp possible....say the junction temp of the devices. In a hot room wont' the difference drop as the room temp approaches junction temp? Or will the junction keep getting hotter until failure? Isn't there an upper limit to the temp of an amp?
The maximum rated junction temperature of a semiconductor device, less some derating (margin), is the maximum temperature that is safely allowable. It is by no means the maximum temperature that is "possible." And yes, it can keep getting hotter until its mtbf (mean time between failure) is severely degraded, or until immediate failure occurs.

Think of it this way: If everything has been turned off for a while, everything (including internal device junction temperatures) will be at the room ambient temperature. The energy that is fed into each device, less whatever amount of energy the device outputs to other devices, and less whatever amount of heat is conducted or radiated away from it, can only have the effect of heating the device up from that starting temperature.

Best regards,
-- Al
Let's straighten out the question? OK?

The OP wanted to know if heat was OK, and if the huge seasonal temp fluctuation was OK....
Well, I think we all agree that heat in excess is bad for electronics. Cold, especially condensing cold is perhaps worse....ZAP.

Heat cycling can also damage gear. Can Expansion and contraction of solder connections work them loose? Some of the new solders are less malleable than in years past. Wasn't that one of the problems with the X-Box?

Do we agree that cold air is better at sinking heat from electronics? It would seem that as the ambient temperature and temperature of the electronics got closer and closer, the amount of HEAT transferred would get less and less. It maybe that BigBucks is right, but I don't see it. The constant delta above ambient may work but I just see stuff getting hotter faster than the room it's in....especially if the room is externally heated...sunlight, hot day...etc. At some point, the junction temp of an output device would be nearing limits and be unable to dump enough heat.....thru all forms of shedding...radiation, conduction, convection....(others?) But would that be at a constant delta from ambient?

The electronics would raise temperature as the amount of heat soaked away got less but that would catch up to you at some catastrophically high temp....which would be a much higher temp that you'd like your room!

No matter the physics equities here, I still think that a room of 85f is WAY too hot for good electronics. Maybe just sitting there....OK, but I'd never run my TV in that hot a space. Or even my 'd' amp.

After living in the same house for 20+ years, I installed AC before last summer. Glad I did, too.

And Al, I agree with you, too. Starting from 'cold' stuff starts shedding heat as stuff warms. Convection. Conduction. Radiation. All play a part in shedding heat. However, that heat goes somewhere. A bad / Extreme example is my RPTV. It kicks out a jumbo amount of heat. That lamp COOKS. Well, it sort of keeps the house thermostat artificially WARM. The TV is about 6' from the thermostat. The rest of the house cools and gets downright cold.... But that TV warmed thermostat says that all is well.

I don't mean to play the 'expert' card, but I will call my physics buddy. He is a hi-end semiconductor engineer and should be conversant with these issues. I'll ask and post back..Give me a couple days. If I have to buy him lunch, I'm billing you guys for 1/3 of the bill....each! just kidding.

some owner's manuals say that you should allow equipment and tubes to warm to room temperature before using them

i use solid state devices only so i am not familiar with such advice. the thought that comes to my mind is that the reason why the makers of tube equipment might offer this advice is so that you don't shatter the glass tubes. if you took a glass out of the freezer and immediately filled it with boiling water, it would likely shatter. when you power up a tube amplifier, i would imagine that the temperature would rise pretty quickly. so, if you left a tube amplifier out in the cold overnight, hauled it inside, immediately powered it up and started playing music at high volume, then i would imagine that the tubes would heat up pretty quickly; maybe while the glass tube housing is still cold. if the glass shatters, then your tube would be shot.

that would be my hypothesis with respect to tube electronics...
A lot of amps HAVE a thermal shutdown relay so they cannot 'cook'. I would think that would protect in the immediate case, as for the long term problem of 'being hotter shortens electronics lifespan'.. that is debatable IMO, as it is true in theory, but I have never seen any sign of it in reality (and I am ALWAYS at 78F to 82F in my place Summer or Winter (with outside variation from 102F tops Summer to -10F coldest in Winter..)
So I never have worried about stuff being a little hotter.
(that sort of worry is for the 'AudioPhile Nervosa' types,
Elizabeth,
You could make a decent case for me being about 1/2 gene away from washing my hands 30 times a day.
Heat DOES kill electronics, no question about it. The devices most prone to heat effects are power devices, which obviously use plenty of heat sinks. ICs can cook, too, some of which have extremely high circuit density. The proof can be found in any semiconductors 'reliability' testing program where devices are tested to failure.
For example, modern multi-core CPUs will dissipate maybe 70 watts? Maybe more...maybe less, I'm not current. I know voltage requirements for some devices has dropped to keep power down. And look at the obsessive lengths some computer modders go to ensure proper cooling.

I'm waiting for a passively cooled class 'a' amp with heatpipes or maybe liquid cooling, chiller and pump.

That being said, such shorter lifespan for hotter stuff has a statistical base. Silicon based semiconductors simply don't like temps much above....say 150c which depending on how much power you're talking about may actually kick out quite a bit of heat. Example:: A Penny at 150c has a lot less heat energy than say......an anvil at room temp. A power transistor running hot in a properly designed situation....proper thermal contact and enough heatsink area and mass, will get the heatsink pretty warm.

The observation you may want to make is how hot is the EQUIPMENT in your 80f room? If the gear is in an enclosed space with poor or marginal ventilation, your 'goose' is cooked and you may just be lucky. OTOH, if your stuff is in a well ventilated space and is the good gear I know you like, than you'll be fine. Even Bryston can be cooked. They design stuff with the 'noise' of actual use in mind. If EVERYONE used the amp in a cool, well ventilated space, they wouldn't use as much heatsink. But, they were thinking ahead. You are in the normal, expected range of users.

12-05-10: Magfan
Elizabeth,
You could make a decent case for me being about 1/2 gene away from washing my hands 30 times a day.
Heat DOES kill electronics, no question about it. The devices most prone to heat effects are power devices, which obviously use plenty of heat sinks. ICs can cook, too, some of which have extremely high circuit density. The proof can be found in any semiconductors 'reliability' testing program where devices are tested to failure.

i tend to agree somewhat with elizabeth's caution: your "no question about it" assertion is a deterministic statement based on statistical testing. reliability testing tests devices under accelerated conditions and then attempts to extrapolate those results for less extreme conditions. but it is statistical analysis. the reason why they do accelerated testing is because nobody can actually wait 20 years to see what would actually happen under real world conditions.
I think the OP of this thread can get away with running his gear in a 85 degree room,as long as the gear has good ventilation.At 85 degrees,he'll probably have a fan making a nice breeze up there.His owners manual would be the best guide.I don't think it will go into a thermal runaway,type of condition.Humidity can cause slower heat transfer too.The worst conditions I've seen for gear is,component stacking,or enclosed case gear.Gear done this way in a 70 degree room,is worse off than the OP of this threads condition,IMO.This can cause bad heat build up.I don't have the thermodynamics education,just some common sense,I hope.
Heat does indeed kill electronics, or at least reduce mtbf. However, if the equipment is well designed from a thermal management/heat sinking standpoint, and if parts such as electrolytic capacitors whose mtbf may be particularly heat sensitive are well chosen, the effects of heat will not become significant until temperatures are reached that are much higher than might be expected.

Those are very big "if's," of course. And I would not expect that small manufacturers of high end equipment will always or even usually have thermal design specialists on staff, not to mention that providing conservative design margins will tend to increase the cost of the product.

But to provide some perspective, commercial grade integrated circuits are most commonly rated for ambient operating temperatures of 0 to 70 degC, that being conditional in the case of higher powered devices on heat sinking provisions that maintain reasonable junction temperatures. 70 degC = 158 degF! Specifications for integrated circuits used in military avionics usually require an ambient operating temperature range of -55 to +125 degC. 125 degC = 257 degF! (Although keep in mind that "ambient" for each device means the nearby temperature inside the case of the equipment, not the external air temperature).

Devices that consume large amounts of power, such as computer cpu's, usually have considerably lower ambient temperature ratings than those numbers, but they are still higher than one might expect. Current Intel quad-core desktop cpu's have a TDP (thermal design power) of 130 watts. It boggles my mind that so much power can be dissipated in such a small package, even with the special heat sink and fan that is required. Consider how hot a 100W light bulb gets, the bulb being considerably larger than a cpu chip!

Addressing Magfan's point about the fact that computer enthusiasts (I am one one of them) pursue exotic cooling solutions, the reason for that is not to extend life but to optimize overclocking ability (running the cpu at faster than its rated speed, which enthusiast-oriented motherboards make possible). Faster speed = higher power consumption and higher internal temperatures, and higher internal temperatures will limit the maximum speed at which the cpu can be operated without crashes.

Overclocked cpu's utilizing good aftermarket cooling devices typically run reliably for many years with internal junction temperatures in the area of 40 degC (104 degF) when idle, and 75 degC (167 degF) to 90 degC (194 degF) when performing intensive processing.

Best regards,
-- Al
12-05-10: Magfan
It would seem that as the ambient temperature and temperature of the electronics got closer and closer, the amount of HEAT transferred would get less and less. It maybe that BigBucks is right, but I don't see it. The constant delta above ambient may work but I just see stuff getting hotter faster than the room it's in....especially if the room is externally heated...sunlight, hot day...etc. At some point, the junction temp of an output device would be nearing limits and be unable to dump enough heat.....thru all forms of shedding...radiation, conduction, convection....(others?) But would that be at a constant delta from ambient?
-----------------------
From above:
"It would seem that as the ambient temperature and temperature of the electronics got closer and closer"

How does that happen? If the room (equipment) is getting hotter, the components are getting hotter at the same rate (given that everything was in equilibrium in the first place.

The equipment dissipates a given amount of power (at a specific operating load). The power is dissipated as heat through the componet body, thru the board, then out thru the heat sinks. That thermal 'resistance' to heat flow is constant. So, a given heat flow (power dissipation) to ambient thru a fixed resistance must yield a constant delta T at the component end. That's why the component junction temp is a constant above ambient.

And I 'know' this because I work on aerospace electronics where we do thermal analyses all the time. I guess all these years we must've been wrong about increasing component temps by the ambient temp delta ;)
Al, computer cooling, certainly a 'side issue' here is related to hifi. The point being that 'heat kills'. Enough statistical data exists to support this.
I had a Motherboard which had an automatic overclock feature when the CPU was pushed. I kept it on the most conservative setting and ran one of the Zalman Cu/Al 'mushroom' shaped heat sinks with the fan on 'full'. I chose a conservative CPU and never pushed it. I never saw my 2.4gig CPU go above about 2.6 I kept the dust bunnies cleaned and the all the cooler fins de-linted.
Point is valid that 'heat kills'.

Processing temperatures for Silicon devices run from about 1150c for some of the Junction Drives down to very low temps like 200c or less, used for 'sinter' or 'anneal' processes...usually right at, or near the 'end of the line'. Nothing goes above about 425c after 'metalization' which is usually an aluminum alloy and deposited somewhere mid-line. I am not current on what is used in State of the Art CPUs. They may have gone to copper or some other metal. Very thing / narrow aluminum has some problems with reliability and electromigration.

The 'statistics' to which Paperw8 also refers to are valid. They are well understood and proven. That they are statistics means you are dealing with populations.....large numbers of a given object some of which will crap out immediately and others which will last.....seemingly forever. The 'take away' is that Al is also correct: MTBF is the most visible metric applied to this stuff. The object of manufacturing quality is to produce product in the middle of the spec. All 'excursions' are suspect.
Semiconductor plants, called 'fabs' spend a bundle on 'rel labs'....Reliability. Here they torture what they make in an 'accelerated' lifetime. Indeed, before a new product or process is released to production, parts must go thru what is called a '1000 hour burn-in'. Too many failures or parametric shifts during the test are grounds to deny the 'go ahead' for volume production. What went wrong? The innocent are usually than punished.

I worked at a company that had a 'Hi/Lo' group. Bad lots were investigated....what went wrong? Especially GOOD lots got the same treatment....what went right? Go Figger.

Now, if you go for the 'weakest link' line of thought, buying mil-spec stuff for part of your design and cheap-o commodity chips for other parts doesn't make sense. This is why good equipment is not only well designed...but well executed, too. One could probably build a Bryston Copy for a fraction the cost of the real thing but out of cheaper parts and NO warranty! It wouldn't surprise me to learn that companies like Bryston had 'thermal budget' folks on hand. Engineers to calculate and verify type and amount of heat sinking. Even those IR imagers to look for 'hot spots'. All sorts of cool, hi-tech lab gear.

Mil Spec parts are not necessarily different from those you can buy. I say necessarily because electrically they may be the same and even made in the same fab. The difference is that NO rework is allowed, so if a production 'lot' has a problem, it is immediately either scrapped or degraded to 'consumer'. Other rules apply and the factory audits are BRUTAL. A factory must EARN the right to build Mil-Spec parts and be so certified. Than periodically audited. The taxpayer picks up the bill. You'd use parts like that, too, if a repair call took you 200 miles into space.
Having no formal education on the subject,let me give my two cents anyway.Use F with these examples.

Semiconductor@90f mounted to heatsink @ 30f.High transfer

Semiconductor@90f mounted to heatsink @ 50f Good transfer

Semiconductor@90f mounted to haetsink @ 90f Any transfer?

Semiconductor@90f mounted to heatsink @ 91f Any transfer?
Or the reverse, absorption?

12-06-10: Bigbucks5
12-05-10: Magfan
It would seem that as the ambient temperature and temperature of the electronics got closer and closer, the amount of HEAT transferred would get less and less. It maybe that BigBucks is right, but I don't see it. The constant delta above ambient may work but I just see stuff getting hotter faster than the room it's in....especially if the room is externally heated...sunlight, hot day...etc. At some point, the junction temp of an output device would be nearing limits and be unable to dump enough heat.....thru all forms of shedding...radiation, conduction, convection....(others?) But would that be at a constant delta from ambient?
-----------------------
From above:
"It would seem that as the ambient temperature and temperature of the electronics got closer and closer"
I forgot to put my two cents in response to this statement.
This is baffling. But I DO see what you mean. Since a device and its enclosure/ sinking can move only so much heat.....so fast.... the junction will be above ambient as its heat migrates away. And I see they should stay a set difference apart. As long as you can sink the whole system.

But what happens in an enclosed space? I've had gear in confined spaces where the heat evolved simply had no place to go. Would the temp difference continue until the device failed? I'm talking Very Hot.....like over 100c, air temp. perhaps.
What happens if you put a cold / off device into a warm environment? Does the device warm....than as its temp rises to the 'delta' temp, heat begins moving the 'right' way?

My 'd' amp is on the shelf below my small dish receiver. If I close the door overnight, even with everything off/ standby, the next morning it is pretty warm inside. Even the amp is warm to the touch.

I walked into a very small demo room at a video store. They had 4 plasma TVs in about a 10x10 foot room......and it was almost too hot to breath. I'm sure the electronics was way too hot for comfort.

I spent 25 years building semiconductors from wafers to die. Apparently I didn't spend enough time in probe or reliability.


If the internal temp rises faster than the case could dissipate it,I would think it would keep rising,until failure.Don't forget,semiconductors operate at Celsius ratings.Look at the temperatures Almarg gave earlier.
HiFi,
Let me give this a try.....Big, check and see if we're getting this.

The semiconductor will, under use simply go to some temp above ambient and stay there, depending on load. The device HAS to be warmer than the heatsink.
I don't know...If you chill the sink to 30f while the device is at 90f? The device will first cool, than begin pumping heat back into the system raising its temp.

HERE:: READ THIS.....
I'm going to go thru it. I already see it makes sense of this and the math part isn't too bad.
http://homepages.which.net/~paul.hills/Heatsinks/HeatsinksBody.html

BigBucks. Please read above link and see if it meets your approval. It seems straightforward explained this way.

One further question, however.......and that deals with maximum power dissipation of semiconductor devices as well as the maximum junction temp allowed.....all apparently related back to ambient temp and the thermal resistance of the system.....

cheers........
Hifi,
You can always run something out of spec and blow it up.
Read the link I provided.

I have no idea of the speed of propagation from the junction, thru all the various interfaces and to the air.

Maybe you COULD 'shock' it.

C or F? just a conversion. The US is probably the ONLY country left in the world who uses the English metrology system. Darn French.
Everything in engineering is most likely C.
Another example would be heat and A/C.

If room temp is 70F and blows through a/c coil @ 40F great transfer.

If room temp is 70F and blows through a/c coil @ 60F little
transfer.

If room temp is 70F and blows through a/c coil @ 70F no transfer.

If room temp is 70F and blows through heat coil @ 120F high transfer.

If room temp is 70F and blows through heat coil @ 70F no transfer.

When transferring from more extremes,the transfer rate increase.This must be true,or our Heat A/C systems wouldn't work right,or at all.
HiFi.
Big is correct. Devices are apparently designed with a max junction temp at a given air temp. Lower air temp is gravey.

Read at least the written description of the math. It'll make sense. The heat-engine model doesn't apply where greater differences in temp produce greater results. transistor temps are sort of self-limiting and there are bunches of design criteria.

That being said, cooler is still better.
Sitting by a window with outdoor temp of 50F,little transfer
Sitting By a window with outdoor temp of 0 F,great transfer,
and it takes place both ways.At 0 F high rate of heat out,at same time,high rate of cold transferring in at 0 F.The more extreme, the faster the transfer rate.
As near as I can figure, that's the whole point. You'll have a zero difference when you fire it up with everything at room temp....BUT as soon as the device warms, it'll start moving heat away at such a rate as to maintain the calculated (or nearly) temp diff above ambient.
If you stick your room temp amp into the freezer, same deal.....just that now your working against the freezers ability to pull heat out (pump it) 'uphill' into the room....creating the cold box. Amp doesn't know this and will still end up warm VS the internal cold box temp.....and the same # of degrees.
If you put a power transistor in an insulated space and power it up,
you'll have a meltdown / failure in no time. No place for the heat to go. somewhere above maybe 150 to 200c, it'd just cease to function. These limits are all straight physics and chemistry.


But, based on the math the device will shed heat at a certain rate. Once it's been on and is stable, it'll run pretty much a certain amount above ambient for a long time. Even in the freezer......

This is pretty clear from the link I posted.
Also, find a datasheet. Somewhere is the 'derate' for power devices...and maybe others. The derate deals with power and temp. And how much less the device will take and at what rate, as it warms.

The OP? Long gone, but still and all, I'd not run my gear in an 85f space. Any weak link will be ruthlessly exposed. Any dry heatsink compound which isn't doing its job....a nut/screw securing a power transistor has come loose. A dust bunny clogging heatsink fins or some venting,.... All can hurt your stuff.
http://www.ee.latrobe.edu.au/internal/workshop/store/pdf/MJ2955.pdf

First datasheet I came across.
Look at figure #1 for temp derate and the bottom of page #1, just above the fig for the heat/ thermal #s.
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'.....