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Old 24th May 2012, 07:42 PM   #171
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by simon7000 View Post
Gee, and I look at what was originally presented and I see a nice polished circuit following what I perceive as the current state of the art design trend. That is to properly balance the components to get a circuit that performs very well. Some are going to more difficult to obtain parts others selection, to reach these goals.

I guess that is due to my inexperience. I have been at audio full time for only 40 years.

Once Jan gets around to sending me my copy I may have more insights.
Not to anyone in particular: There seems to be a lot of assumptions - based upon what, I dont know. This is an optimization of existing circuits put together and selected for best results. I hope to get to the not so obvoius parts when everyone reads the article. Sorry but I just can not rewrite the artcle here. Lets take an easy one but I think more intersting question for you ---- anyone care to guess why thier favorite topology or design isnt thermally stable and this one is? The transistors were not mounted to a common heat sink nor the bias circuit attached to the transistor cases. On your mark! ready. set. Go!

Last edited by RNMarsh; 24th May 2012 at 07:51 PM.
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Old 24th May 2012, 08:01 PM   #172
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What does 'thermally stable' mean? That the output offset remains relatively constant irregardless of temperature?
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Old 24th May 2012, 08:09 PM   #173
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Originally Posted by rlim View Post
What does 'thermally stable' mean? That the output offset remains relatively constant irregardless of temperature?
Also that the bias doesnt drift much with temperature.
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Old 24th May 2012, 08:36 PM   #174
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Quote:
Originally Posted by RNMarsh View Post
Not to anyone in particular: ....Sorry but I just can not rewrite the artcle here. Lets take an easy one but I think more intersting question for you ---- anyone care to guess why thier favorite topology or design isnt thermally stable and this one is? The transistors were not mounted to a common heat sink nor the bias circuit attached to the transistor cases. On your mark! ready. set. Go!
Actually I have been doing a bit showing that thermal mistracking can result in distortions that may be missed by some measurements.

Attached are things I have shown before. A distortion comparison of a carbon composition resistor compared to a metal film type of the same value, tolerance and wattage. With a THD measurement they are very close. With an IM measurement at 50Hz and 2 KHz they show differences. That is because the 50Hz is doing more cyclical heating of the resistors.
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File Type: jpg CC v MF 100 2K.JPG (384.2 KB, 632 views)
File Type: jpg THD CC V MF.JPG (362.5 KB, 609 views)
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Old 24th May 2012, 08:52 PM   #175
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Quote:
Originally Posted by simon7000 View Post
Actually I have been doing a bit showing that thermal mistracking can result in distortions that may be missed by some measurements.

Attached are things I have shown before. A distortion comparison of a carbon composition resistor compared to a metal film type of the same value, tolerance and wattage. With a THD measurement they are very close. With an IM measurement at 50Hz and 2 KHz they show differences. That is because the 50Hz is doing more cyclical heating of the resistors.
With a THD measurement at 50Hz they are not close. You don't even need to perform the IM test the thirds on the 50Hz in the left plot can be used to compute the IM sidebands on the second frequency. It is not an accurate blanket statement that "there are things that show up in an IM test that don't in a THD test". This relates to what you asked yesterday.

You need to be more careful in stating all the conditions.

As I mentioned long ago the phase of the sidebands could probably be used to separate the thermal from voltage effects.


EDIT - On second thought the thirds in the second plot don't look very close at all.
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Last edited by scott wurcer; 24th May 2012 at 09:01 PM.
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Old 24th May 2012, 09:11 PM   #176
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Scott,

Yes, I forgot to mention what we are looking at is the distortion of the same amplifier with the only difference being the feedback resistors.

The sidebands are not exactly what I expected from just the THD plots. In addition to the distortion difference in the two plots the amplifier also behaves a small bit differently. (As you can see the "noise" level has shifted!) So this is not a true resistor vs resistor comparison.
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Old 24th May 2012, 10:39 PM   #177
RNMarsh is offline RNMarsh  United States
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I usually use a much higher wattage resistor in the feedback than would seem necessary to prevent thermal issues but also to reduce the effects associated with its voltage coeffecient. Of course the metal caps at the end add distortion if they are ferrous metals. To make matters a wee bit more complicated, the materials used to dope the resistive material itself (eg metal film) can add distortion... doping to get tight tempco often uses material that is nonlinear. I wouldnt go so far as to make a blanket statment that all low tc resistors have higher thd. Just the materials used to alter the tc often are nonlinear.

Last edited by RNMarsh; 24th May 2012 at 10:41 PM.
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Old 24th May 2012, 10:43 PM   #178
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Quote:
Originally Posted by RNMarsh View Post
I usually use a much higher wattage resistor in the feedback than would seem necessary to prevent thermal issues but also to reduce the effets associated with its voltage coeffecient. Of course the metal caps at the end add distortion if they are ferrous metals. To make matters a wee bit more complicated, the materials used to dope the resistive material itself (eg metal film) can add distortion... doping to get tight tempco often uses material that is nonlinear. I wouldnt go so far as to make a blanket statment that all low tc resistors have higher thd. Just the materials used to alter the tc often are nonlinear.
In LA #1 I show the distortion of various resistors and how I measured them. I have not shown there yet how to determine the thermo electric effects. I am not finding much of a voltage coefficient that is not explained by measurable processes.
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Old 24th May 2012, 11:00 PM   #179
RNMarsh is offline RNMarsh  United States
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Originally Posted by simon7000 View Post
In LA #1 I show the distortion of various resistors and how I measured them. I have not shown there yet how to determine the thermo electric effects. I am not finding much of a voltage coefficient that is not explained by measurable processes.
That was a great article btw!! Thanks. Especially the sm types. Many of the best DAC have thd specs below the sm resistors! Anyway, I am thinking of power amps with voltage swings of up to +/- 80 volts... often across a high value 1/2w feedback resistor. Might need a high voltage or peak level test?

Last edited by RNMarsh; 24th May 2012 at 11:24 PM.
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Old 24th May 2012, 11:14 PM   #180
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Thanks!
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