John Curl's Blowtorch preamplifier part II - Page 49 - diyAudio
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Old 25th July 2009, 05:14 AM   #481
anatech is offline anatech  Canada
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Hi John,
All I did was to support something that Bob mentioned, an easy concept. I have no idea why you felt it was necessary to take issue, then insult me.

Quote:
Chris, if you are not an engineer or a physicist, you are not in a position to judge.
That, is insulting.

So, what letters do you have behind your name? PEng, CET?
Does it matter? No, it really doesn't. I have spent my life doing work in this field, just as you have (I guess). I am not about to insult you, and haven't.

Quote:
Please adjust, I am in too much pain.
I'm really very sorry to hear you are uncomfortable. It's not pleasant and makes everything that much more difficult.

But, guess what John? I know several other members who are just as, if not more uncomfortable than you are. Personally, on a good day, I have a couple good hours to function somewhat. On bad days, I can't even think straight. Those are the days I don't post here at all. As it is, I had to take a break from this post. I'm on 80 mg of Oxycontin per day, plus other stuff. Increasing doses because they can't get my situation under control. Plus A.R.T., Chiro, and nerve blocks every week. There are people worse off than I am who are members here as well.

Stop whining and sniping at people. You aren't as bad off as you may think, although I'll accept that you may be in the worst pain you have felt so far. It can get worse. It's still not going to cut it as an excuse.

I do wish you well along with a speedy recovery. At least you will recover, I will not.

Try to treat others around you with some respect. now, if you have a problem with me, get over it. Instead, try letting me know in realistic terms what the problem is.

-Chris
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Old 25th July 2009, 10:02 AM   #482
PMA is offline PMA  Europe
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3 different idle currents, 2 x 50V supply, push-pull output stage, class AB.
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Old 25th July 2009, 10:09 AM   #483
PMA is offline PMA  Europe
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Same for smaller output voltage. One can see that at higher idle the power dissipation approach to a constant, or at least fluctuates much less than in a class B.
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Old 25th July 2009, 10:19 AM   #484
AndrewT is offline AndrewT  Scotland
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Hi,
it's even better than PMA is posting.
All music has transients. We all listen at much lower levels than maximum output.

Transients by their definition do not last long.

The power devices and the heatsink they are attached to have thermal inertia.

The junction temperature will fluctuate very little for low amplitude power changes.
Due to the thermal inertia, the case temperature will not fluctuate much for high amplitude transient power changes.

How do we estimate/measure/calculate the changes in junction temperature when we can feel/measure that the device case temperatures vary little in normal music reproduction?
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Old 25th July 2009, 12:28 PM   #485
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Quote:
Originally posted by john curl
Easy is better than complex. Chris, you are making it complex. This happens when techs try to do what engineers are trained to do. It is easier, in this case, to use the engineering equations.
Someone: Dimitri, Cordell, Wurcer, please bail out this situation. It is on a page of a favorite book owned by most of you.

Hi John,

I tend to agree with Chris here. As an engineer, I know equations are very important and use them every day. However, equations are not always the best way to express concepts. When one has a good grasp of the concepts and trends, then the equations become even more valuable and less subject to error or mis-interpretation. I've seen far too many people blindly plug into equations and draw wrong conclusions.

On the other hand, we must always guard against error in intuitive arguments and generalizations. This is where equations are especially important. Linked to this is simulation, which is not unlike a more convenient and detailed application of equations. There have been many times when I have discovered that my intuition has been less than perfectly accurate when subjected to the scrutiny of analysis by equations or simulation.

Cheers,
Bob
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Old 25th July 2009, 12:40 PM   #486
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Quote:
Originally posted by PMA
Same for smaller output voltage. One can see that at higher idle the power dissipation approach to a constant, or at least fluctuates much less than in a class B.

Hi PMA,

Thanks for showing this. This is the point I was making in Post 452.

BTW, an interesting side note is that this tends to be synergistic with MOSFETs, where gm doubling basically cannot happen. Bias them nice and toasty and they will have very little transconductance droop, a gairly large class A range, and rather small deviation of power dissipation as a function of output power. The price, of course, is higher quiescent dissipation.

Cheers,
Bob
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Old 25th July 2009, 12:51 PM   #487
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Quote:
Originally posted by AndrewT
Hi,
it's even better than PMA is posting.
All music has transients. We all listen at much lower levels than maximum output.

Transients by their definition do not last long.

The power devices and the heatsink they are attached to have thermal inertia.

The junction temperature will fluctuate very little for low amplitude power changes.
Due to the thermal inertia, the case temperature will not fluctuate much for high amplitude transient power changes.

How do we estimate/measure/calculate the changes in junction temperature when we can feel/measure that the device case temperatures vary little in normal music reproduction?
Hi Andrew,

There was a lot of good discussion of this over on the ThermalTrak thread. The thermal time constant of the heat sink is quite long, on the order of tens of minutes. The thermal time constant of the power transistor case (heat spreader header) is on the order of seconds (maybe 5). This is important because the case is separated from the heat sink by the thermal resistance of the insulator, perhaps on the order of 0.5 C/W.

The thermal time constant of the die/junction is much shorter, sometimes on the order of 20 ms. It is separated from the case by a thermal resistance typically between 0.5 and 0.9 C/W. Short-term transistor power dissipation changes of, say, 20W can cause a transient temperature difference that would like to asymptote to on the order of 10-18C, enough to upset short-term bias, given a junction TC on the order of 2 mV/C. That theoretical 26 mV number for voltage across RE can quickly be seriously compromized on a short term basis as a result of program-dependent power dissipation.

Cheers,
Bob
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Old 25th July 2009, 02:16 PM   #488
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It is an important distinction when talking about "time constants" that heat obeys the diffusion equations and can never be modeled exactly in SPICE by R's and C's.

BTW there is a beautiful paper by (I think) Alberto Billoti (sp?) that derives the behavior of temperature vs. arbitrary device geometry from first principles, rectangular coordinates only IIRC. Such maths would make a Russian jealous .
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Old 25th July 2009, 02:35 PM   #489
dimitri is offline dimitri  United States
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Alberto A. Bilotti, “Static Temperature Distribution in IC Chips with Isothermal Heat Sources,” IEEE Trans.
Electron. Devices, vol. ED-21, No. 3, pp. 217-226, March 1974.
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Old 25th July 2009, 02:44 PM   #490
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Thanks Dimitri, That's one journal that I don't have on my DVD archive.
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