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Old 9th January 2013, 05:46 PM   #32401
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Originally Posted by simon7000 View Post
Dimitri,

Third harmonic distortion is these days mostly from impurities in the resistance material or mechanical strain. Mechanical strain indicates a possible failure mechanism.

ES
Please read 4.1.3 of the reference, without DC bias TC creates third harmonic distortion. Thermodynamics, reversing the voltage does not make negative heat.
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Old 9th January 2013, 05:48 PM   #32402
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George,

The point was that the IC had less distortion than the feedback resistor. So the amplifier was was good enough or in other words not the limiting factor.

Scott,

Given 50 mW as the data sheet value (I don't know the production number) it would be a much higher number than the level at which I tested resistors.

As shown in the attached measurement of a higher wattage resistor at 50 mW, one could expect 12 db more distortion from the smaller resistor body, and another 9 to 12 from the low frequency content. (Most musical energy is found at the LF end say 150 Hz.) Then this is a sample of an Ohmite resistor, others of the same type of construction mostly measured worse and none better. So your distortion just from the feedback resistor could be -60 db. re full power. As there are thermal relaxation issues, the tail should be enough to be heard.

I suspect that is the actual limiting factor, not the rest of the circuitry. So to me this would be how to make a "Bright" or "Brings out ..." amplifier.

ES
Sigh..it's skin effect, not thermal..

jn
ps..just havin fun..Happy New Year Ed..

Last edited by jneutron; 9th January 2013 at 05:50 PM.
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Old 9th January 2013, 05:59 PM   #32403
jcx is offline jcx  United States
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a separate Voltage coefficent of resistance is recognized by R manufacturers - thermal coefficient doesn't explain everything


skin effect shouldn't produce harmonics, IMD with linear conductive materials - it can be modeled as linear mutual inductances steering current to linear parallel R paths

Last edited by jcx; 9th January 2013 at 06:02 PM.
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Old 9th January 2013, 06:03 PM   #32404
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Originally Posted by scott wurcer View Post
Please read 4.1.3 of the reference, without DC bias TC creates third harmonic distortion. Thermodynamics, reversing the voltage does not make negative heat.
Wow Scott, you are right. Third is the dominant TC distortion for a dry resistor.

JN Should we talk about PC layouts that minimize inductance but allow for uniform heating? I like eight resistors, two on the top right, two on the bottom right, the other four same on the left. Resistors parallel to each other. Signal comes in splits to 4 goes through and then the other end is connected to the paired one and returns back the way it came.

Attached is a top view, same on the bottom but staggered so the pads don't touch.

ES
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Old 9th January 2013, 06:08 PM   #32405
jcx is offline jcx  United States
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you should add guard rings fed from low Z source of the same V that is at each PCB pad/hole to bootstrap PCB dielectric effects - does a good job on surface leakage currents too
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Old 9th January 2013, 06:10 PM   #32406
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Originally Posted by simon7000 View Post
Wow Scott, you are right. Third is the dominant TC distortion for a dry resistor.

ES
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Old 9th January 2013, 06:11 PM   #32407
RNMarsh is offline RNMarsh  United States
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Just some useless knowledge to add to the R subject --> The lowest TC resistors are made as such by additives. In knowing which additives are mixed in to the resistive element finds some are none linear (thd) materials.
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Old 9th January 2013, 06:35 PM   #32408
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http://www.vishaypg.com/docs/65103/td_tc.pdf
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Old 9th January 2013, 06:37 PM   #32409
dimitri is offline dimitri  United States
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Old 9th January 2013, 07:18 PM   #32410
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Originally Posted by jcx View Post
skin effect shouldn't produce harmonics, IMD with linear conductive materials - it can be modeled as linear mutual inductances steering current to linear parallel R paths
Why not??

Skin effect alters the volume of resistive material that carries current. At higher current slew rates, less material sees current so the resistance will climb. When the resistance is a function of the current slew, man, does that raise issues or what??

An important test to validate the current slew dependence of resistance would be to return the current next to the body of the resistor and compare it to a test where the current is far away. The return current will cause a proximity effect on the resistor current density profile.

jn
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