John Curl's Blowtorch preamplifier part II - Page 51 - diyAudio
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Old 26th July 2009, 09:27 AM   #501
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by Bob Cordell
I'm not sure we're really disagreeing here. My main point was that we should not underestimate the amount of variation that can occur in junction temperature as a result of program power variations.
we do not disagree.

It depends very much on the program material and how we use our amplifiers.
Whether it's a 12W amplifier driving 104dB/W/m speakers or JC's example 800W into 4ohms driving 83dB speakers.
If we listen at levels ~ 20 to 30dB below the maximum capability of the amplifier then PMAs plots are very much a worst case scenario.

There will be periods when we stress the transistors more, but these should be rare maybe very rare, what about 20ms during an evening's listening? Or would be happier if I suggested we tolerate 300ms per listening session?
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Old 26th July 2009, 09:42 AM   #502
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Andrew, Bob is completely right. I do not know why you are in a need of protecting yourself.
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Old 26th July 2009, 04:59 PM   #503
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Please, let us not quibble with each other. It is all pretty well brought out now. Equations and all, for the most part. With added graphs, one can get a good understanding as well.
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Old 26th July 2009, 05:23 PM   #504
1audio is offline 1audio  United States
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How does the transient thermal change affect the signal passing through the amp? This is clearly not easy to measure with steady state tests and it may be significant. I have been reading about delay vs. temperature in very precision systems and the numbers are in the 10 pS/degree C in a carefully compensated system. I seriously doubt anyone would hear something that small as a steady state change but an abrupt change could modulate a signal enough to be audible (if we knew what was audible).
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Old 26th July 2009, 05:56 PM   #505
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This is why we like amps to run with high Iq with big, thick heat sinks. Thermal capacity becomes important on transients.
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Old 26th July 2009, 08:41 PM   #506
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Quote:
Originally posted by scott wurcer
can never be modeled exactly in SPICE by R's and C's.
I agree Scott and if someone likes to simulate it with Spice its important to use a model that is as good as possible and not a very simple model like I have seen someone used on this forum.

Attached is an example of a slightly better model.

BTW: the thing I showed was not made by SPICE, but by COMSOL

OT: I really hate the limitations in file size that is allowed to attach.
Attached Files
File Type: pdf temp model.pdf (94.8 KB, 82 views)
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Old 26th July 2009, 09:04 PM   #507
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Quote:
Originally posted by 1audio
How does the transient thermal change affect the signal passing through the amp? This is clearly not easy to measure with steady state tests and it may be significant. I have been reading about delay vs. temperature in very precision systems and the numbers are in the 10 pS/degree C in a carefully compensated system. I seriously doubt anyone would hear something that small as a steady state change but an abrupt change could modulate a signal enough to be audible (if we knew what was audible).
Name please one effect significant in semiconductors that don't depend on temperature, Demian! Thermal voltage is one constant, for example, that present in almost all equations.
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Old 26th July 2009, 09:17 PM   #508
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Quote:
Originally posted by john curl
This is why we like amps to run with high Iq with big, thick heat sinks.
And Big output stages.

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Old 26th July 2009, 09:27 PM   #509
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Quote:
Originally posted by Nelson Pass


And Big output stages.

Size matters?


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Old 26th July 2009, 10:30 PM   #510
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I really like your heat sinks, Nelson. I wish that I had the same. I have close, but not the same. Of course, it is the thermal capacitance of the sink that will smooth out much. Then, a real engineer, looks at the thermal capacitance of the individual devices, which is usually available from a good data sheet.
So, there are 3 approaches to smooth thermal performance, just from a thermal point of view.
First, there is the use of rich class AB-1 to smooth the transition in average power, from low levels to higher levels. Two, spreading out the power dissipation, lowers the overall thermal resistance of the output devices, by using a number of output devices. The electrical insulator matters too.
Third, with a huge thermal capacitance and a good thermal resistance, it is difficult to rock the temperature of the output devices too much.
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