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Old 11th September 2002, 10:43 AM   #1
hifiZen is offline hifiZen  Canada
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Default comments on thermal distortion?

I'm interested in hearing what others have to say about this site: http://peufeu.free.fr/audio/, and the thermal "memory" distortion ideas Pierre has presented there.

Personally, I think he's done an excellent job with the simulations, although I don't know if I like the solutions he presents as much. In particular, the cascoded CFP diff pair seems rather complex. At some point I might try it, perhaps with the cascode FET repositioned so the gate is connected to the main transistor's emitter instead of the base, and so that the FET lies between the two BJTs that make up the CFP... stability could be a serious issue here though.

When you consider the tiny thermal mass of a transistor's guts, and the time it will take any transient heat buildup to dissipate out through the case, I don't find it the least bit surprising that transistor parameters vary by large amounts. I'm sure the dynamic effects are rather more complex than Peirre's simulations indicate. Perhaps this is a good reason to use transistor pairs like the MAT02, where the thermal coupling is tight, and the pair remains closely matched...

Comments? Flames? Has anyone tried these circuit modifications?
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Old 11th September 2002, 06:41 PM   #2
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Certainly there is such a thing as thermal distortion, as
the character of any gain device will change with temperature,
and the dissipation will vary with the signal. Usually this
distortion is quite a bit smaller than the other sources, and
its time constant is sufficiently slow that it is not one of the
more noticeable forms. Think of it as subsonic modulation.

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Old 11th September 2002, 11:35 PM   #3
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Thermal considerations are very important with semiconductors. When very high precision, or very high audio quality is needed, variations of working parameters with thermal effects must be taken in account. Carefully examination of high performance oscilloscope vertical amplifiers is very instructive for the high quality audio amp designer.

I don't agree with M. Pass about long time constants. Input transistors such BC550 have very small physical dimensions, and the collector junction is as close as 1/1000e mm of the emitter junction, temperature variations caused by audio signals in collector junction can easily modulate the gain and the idle collector current in few milliseconds. This phenomenon, amplified by the whole chain of transistors, can cause severe perturbation in the final stage idle current, and therefore transient crossover distortion or other troubles can occur, this is clearly audible.

Furthermore, problems caused by thermal effects in the emitter jonction of the input transistor cannot be canceled by feedback, because this junction is not in the feedback loop... This is the reason why the input stage of an amplifier must be designed with extreme care.

Regards, P.Lacombe
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Old 12th September 2002, 01:07 AM   #4
eLarson is offline eLarson  United States
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Question maybe slightly off-topic, but still thermally related.

One of the things I had wondered about is the use of an insulating pad under the electrically live tab of a MOSFET when the signal is taken from the drain.

I envision a capacitor to chassis ground the plates of whcih are the MOSFET tab and the heatsink, and the insulator is... well whatever the insulator is (mica, plastic, whatever).

Am I out of my mind here? Or should I let the heatsink float when taking the signal from the drain. (And keep a cage around it to keep curious fingers and beagle noses away!)

Erik
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Old 12th September 2002, 03:33 AM   #5
hifiZen is offline hifiZen  Canada
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Yeah, I''m not so sure the time constants are very long either. A SS bipolar is a miniscule little flake of silicon surrounded by plastic. Plastic happens to be an excellent thermal insulator too... It's not a stretch for me to imagine some high-speed thermal modulation in these devices.

Perhaps one of the reasons the Pass amps sound so good is the larger die size of the devices used, combined with the fact that they are generally run at high bias currents. Thus, the power variances imposed during normal operation are a lesser percentage of the total dissipation. I'm certainly no expert on MOSFET tempcos, but with their larger Vgs compared to BJTs, small temperature induced variations might also be further reduced in their effect...

P.Lacombe:
thanks for the tip. I'll see if I can find some scope schematics to study...
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Old 12th September 2002, 03:49 AM   #6
BrianL is offline BrianL  United States
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Mr. Pass is generally correct. Of course he is, since he's
the "one and only"... ;-)

Note that in virtually every audio amplifier, most stages
are run in class A. Thus there is relatively little modulation
of the power dissipation with signal swing. Output stages
are one exception to this generalization.

Thermal effects can be apparent in monolithic op-amps
but careful design and layout symmetry can minimize and/or
eliminate these.

I believe that Doug Self has an example of thermal distortion
on his web site with actual measurements.

As Nelson said, in a well-designed amp, thermal distortion
effects should be more like third-order or lesser effects,
not primary distortion drivers.
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Old 12th September 2002, 06:33 AM   #7
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Default thermal distortion

With all respect for Mr Pass, I do not agree with his points.

I agree with Pierre that the thermal time constants can be relatively small in small signal stages and can lead to modulations up into the kHz region.
Also, the Pass amps are in my opinion not representative of the mainstream amps, since their stages are generally run much more in class A than the average amp, also Pass amps are not really in the forefront regarding low distortion, so thermal distortion may be 3rd or 4th order effects in them, but in very low distortion/high precision amps thermal dist may have a much more pronounced effect.

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Old 12th September 2002, 06:57 AM   #8
hifiZen is offline hifiZen  Canada
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Yes, I'm well aware of D. Self's writings on thermal distortion, I've been reading his work in EW&WW for many years. But, just because a well respected author says it is so, doesn't mean I'm going to take it on blind faith. Mr. Self seems to have the tendancy to occasionally dismiss ideas with a conviction I find a little unsettling. Otherwise, the man's work is tremendously rational, which leaves me puzzled that he isn't a little more open-minded.

Perhaps I neglected to mention that it is not the percentage contribution of thermal distortion to overall THD that I'm concerned with. I am quite certain that thermal modulation is indeed very low on the list of distortion sources. However, in my recent design efforts, I have been looking for a little more challenge, and to branch out and focus on areas which typically receive little attention. I am attempting to address the differences between conventional objective measurements and subjective results. Although I am a degreed BASc (EE), with a very methodical and objective nature, I can't deny what my ears still tell me: transistor amps, especially class-B and class-AB tend not to sound as good as tube or class-A designs. After considerable study, I am now relatively certain that the first-order distortion effects effects have been well studied and pose little threat once treated. In fact, I believe that human hearing is quite insensitive to steady-state distortions, but I have no doubt about the ability of the human ear to discriminate sounds and distortions at extremely low levels, possibly even too low to measure with most test equipment. It seems to me that it is more important to provide stable, uniform distortion behaviour across time and frequency. With this in mind, it is at least worth investigating the possible sonic benefits of reducing short-time thermal distortions.
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Old 12th September 2002, 07:18 AM   #9
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Hi Pierre

I can agree with you that for most amplifier topologies your statement about the first emitter junction not beeing within the NFB loop is true.
But that is of course not the case for all amplifier topologies and can be overcome quite easily.

Regards

Charles
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Old 12th September 2002, 07:21 AM   #10
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Quote:
Originally posted by hifiZen

[snip]
I am attempting to address the differences between conventional objective measurements and subjective results. Although I am a degreed BASc (EE), with a very methodical and objective nature, I can't deny what my ears still tell me: transistor amps, especially class-B and class-AB tend not to sound as good as tube or class-A designs. After considerable study, I am now relatively certain that the first-order distortion effects effects have been well studied and pose little threat once treated. In fact, I believe that human hearing is quite insensitive to steady-state distortions, but I have no doubt about the ability of the human ear to discriminate sounds and distortions at extremely low levels, possibly even too low to measure with most test equipment. It seems to me that it is more important to provide stable, uniform distortion behaviour across time and frequency. With this in mind, it is at least worth investigating the possible sonic benefits of reducing short-time thermal distortions.
Chad, you are tackling a formidable task; I wish you succes!
As you say, absolute objectivity and methodological approaches are essential.
But, it is already an unproven leap if from the observation that tube amps sound better, you conclude that the ear is sensitive to extremely low level distortions. Actually, you are assuming that the reason for the sound difference is such-and-such and now set out to prove it. No offense meant, but it is exttremely difficult to be unbiased; I speak from experience!

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