Distortion in low resistance wirewound resistors - Page 3 - diyAudio
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Old 17th January 2014, 10:25 PM   #21
PLB is offline PLB  United Kingdom
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Hi Sbrads,

Thanks for the vote of confidence.

Regards

Peter
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Old 18th January 2014, 12:04 AM   #22
esgigt is offline esgigt  Netherlands
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Originally Posted by PLB View Post
Many years ago....... Has anybody else come across this problem or have found a technical reference to it?

Peter
Since it is a wirewound resistor that caused the rise in distortion, one aspect I missed in the discussion was the other property of the resistor: wirewound... an inductor so to speak.

The introduced fault is in the order of 10's of uV, which could easily be generated in a couple of windings...
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Old 18th January 2014, 02:51 PM   #23
PLB is offline PLB  United Kingdom
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Originally Posted by esgigt View Post
Since it is a wirewound resistor that caused the rise in distortion, one aspect I missed in the discussion was the other property of the resistor: wirewound... an inductor so to speak.

The introduced fault is in the order of 10's of uV, which could easily be generated in a couple of windings...
Hi esgigt,

I don't see how pure inductance and/or resistance can create distortion unless their equations have non-linear terms. However, I would agree that inductance can skew the spectra of distortion components. Attached is a picture (nonlinear resistor 010.jpg) of the distortion residual showing a dominant 3rd order harmonic. To minimize the inductance of the resistor, I placed it in a 6mm ID copper tube, (which was a snug fit as shown in the attached picture nonlinear resistor 003.jpg) but there was no significant change to the inductance or the magnitude of the distortion.

Regards

Peter
Attached Images
File Type: jpg nonlinear resistor 003.jpg (553.7 KB, 159 views)
File Type: jpg nonlinear resistor 010.jpg (415.5 KB, 158 views)
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Old 18th January 2014, 02:54 PM   #24
FoMoCo is offline FoMoCo  United States
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Originally Posted by DF96 View Post
In the absence of a better explanation I too would blame poor contacts. Thermal effects could only affect very low frequencies.
That's what I was thinking. I'd also expect the frequency threshold for the effect to vary with power level.
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Old 19th January 2014, 02:03 AM   #25
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Originally Posted by AndrewT View Post
I bought some ceramic encased wirewound resistors.....I smashed one up to see inside.....It was an oxide film resistor....
Hmm....I've no desire to witchhunt manufacturers but that seems like a misrepresented product. Do you have a brand/type marking you could quote there so one could take a look at datasheets? - if the source made them available that is.
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Old 19th January 2014, 05:53 AM   #26
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Hi Ian Finch,

It is not uncommon to see a metalfilm/oxide resistor in an ceramic casing.
Here is one example:
http://www.vishay.com/docs/30218/cpcx.pdf
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Old 19th January 2014, 10:27 PM   #27
PLB is offline PLB  United Kingdom
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Since I havenít seen any references posted other than that from RMJ1, I decided to do a little practical investigating myself by first removing the outer coating of insulation that covered the resistor, of which a photograph is attached (nonlinear resistor 003.jpg).
As can be seen, the connection between the tape type of resistance wire to the end caps seems to be spot welded and when I lightly stressed these connections, they appeared to be quite solid as was the lead-out wires as well.
I connected this resistor (0R05) in series with an 8R0 resistor and connected the combination (8R0+0R05) across the output of my amplifier as previously described and noted that for this particular 0R05 resistor, the measured THD was 0.14%. Previous measurements used two 0R05 resistors in series, but with this new setup, I used just one. To obtain the same input voltage to the analyzer as previously, I increased the power output, hence the higher THD level. The THD at the output of the amplifier measured 0.003%.
I then scraped the surface of the tape over the points where it connected to the end cap and with some flux, I soldered the tape to the end caps as shown in the attached photograph (nonlinear resistor 006.jpg).
After reconnecting the 0R05 resistor back into the circuit, I was surprised to see that the THD had dropped dramatically to 0.075%.
I then soldered two 1.3mm dia. solid copper wires right on top of the tape connection points to the end caps (nonlinear resistor 009.jpg) and re-ran the test again and found an even more dramatic drop in THD to 0.0045%. The distortion output from the analyzer showed a small amount of wideband noise riding on the distortion waveform. Without this noise, Iím sure the THD would be even closer to the 0.003% of the amplifier. So at this point, I feel confident that the root cause of the problem is due to the connections between the tape resistance wire and the lead-out wires with the end caps.

Regards

Peter
Attached Images
File Type: jpg nonlinear resistor 003.jpg (661.8 KB, 107 views)
File Type: jpg nonlinear resistor 006.jpg (648.3 KB, 102 views)
File Type: jpg nonlinear resistor 009.jpg (612.9 KB, 101 views)
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Old 20th January 2014, 06:21 AM   #28
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Originally Posted by tommy1000 View Post
...It is not uncommon to see a metalfilm/oxide resistor in an ceramic casing.....
True, but they probably aren't claimed to be wire-wound as is Andrew's example.
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Old 20th January 2014, 06:45 AM   #29
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Originally Posted by PLB View Post
....I feel confident that the root cause of the problem is due to the connections between the tape resistance wire and the lead-out wires with the end caps.....
Does this agree with Conrad Hoffman's suggestion (#3) that the distortion is a thermocouple effect?
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Old 20th January 2014, 02:21 PM   #30
PLB is offline PLB  United Kingdom
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Originally Posted by Ian Finch View Post
Does this agree with Conrad Hoffman's suggestion (#3) that the distortion is a thermocouple effect?
Hi Ian,

I donít have much experience with thermocouples other than having used them in temperature measurements, so I canít offer any comments as to whether it is relevant to the fundamental nature of the distortion source. Iím not sure if the following sheds any further light on the subject or not, but just for the hell of it, I stripped apart the 0R05 resistor referenced in my previous post and noted the following, along with a few other possibly relevant points;
1) The former on which the tape resistance wire is wound appears to be made of a solid round piece of white ceramic material.
2) The end caps were strongly attracted to a permanent magnet.
3) Neither the tape resistance wire nor the lead-out wires were attracted to a permanent magnet.
4) The lead-out wires appear to be made of tin plated copper.
5) The tape resistance wire appeared bronze or dark copper in colour.
6) My analyzer has a maximum sensitivity of 88mV in order to reach ďset levelĒ which is the level that was applied across the 0R05 resistor. Therefore the dissipation in this resistor was limited to 155mW. The rated power for this resistor is not known, but from previous experience, it corresponds in size to those that are known to be =>3Watts.
7) These resistors were sent to me by a company in China, so I would think it likely that they were made in China.

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Peter
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