Distortion in low resistance wirewound resistors - diyAudio
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Old 9th January 2014, 11:46 PM   #1
PLB is offline PLB  United Kingdom
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Default Distortion in low resistance wirewound resistors

Many years ago, I thought I learnt all there was to know about resistors, but recently, I learned a little bit more about them. This came about while I was working on a new actively powered 2 way + subwoofer speaker system that used some excellent bass/mid-range drivers having quite a low Qts. When they were installed in the enclosure, the resulting Qtc was quite low. So to get the Qtc up to my target value, I added a small external circuit to the amplifier which forced the amplifier to have a positive output impedance of about 5 ohms. Testing the circuitry all went well right up until I measured the THD into an 8 ohm load resistor. The THD of the original amplifier measured 0.005% (30W/8R/1KHz), but with the added circuit, it measured 0.05%, about 10 times higher!!
The circuit was very simple with just a dual opamp and a 0.1 ohm sense resistor, plus a handful of resistors. After a few test measurements it quickly became obvious that the distortion was appearing across the 0.1 ohm sense resistor. This was removed from the circuit and tested separately by connecting it in series with an 8 ohm load resistor, which were then connected across the output terminals of the original amplifier. A distortion analyzer was first connected across the output of the amplifier to verify 0.005% THD and then reconnected across the 0.1 ohm resistor where the THD measured 0.05%.
I then replaced the 0.1 ohm resistor, a wirewound device, with a metal oxide type. The THD then went down to 0.007%, showing a remarkable improvement over the wirewound version. The distortion, using the metal oxide resistor, was slightly higher than the distortion at the output of the amplifier and its waveform was of a different shape. I changed the 8 ohm wirewound load resistor to a tight tolerance version and the THD went down even further to almost the same as that from the output of the amplifier. At this point I also noted that the distortion waveform had also changed to the same as the output, plus a very small amount of extra wideband noise.
I concluded from all this that (at least) very low ohmage wirewound resistors can be non-linear and be capable of causing measurable distortion in circuits as described above. Since I had not noticed this kind of behavior in a wirewound resistor before, I tried another 0.1 ohm wirewound resistor from another manufacture and got exactly the same results!!
Although the level of distortion in my system may be considered somewhat academic and in practice inaudible, my concern is that I had not come across this kind of problem before. Has anybody else come across this problem or have found a technical reference to it?

Peter
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Old 9th January 2014, 11:53 PM   #2
RNMarsh is offline RNMarsh  United States
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Good observation --
Many of the wirewound resistors are wound with a non-linear metal... a ferrous metal. So, that when current flows through a non-linear wire/metal distortion is produced.

Low values wirewound R's are also found in output stage emitter degeneration resistors and when testing amps -- the 4-8 Ohm loads and many other places the signal must pass thru. Metal film R's are often the best if you can get the wattage needed. But even here, there are ferrous metal end-caps on some film resistors... so be aware to keep all signals away from ferrous (magnetic) metals.

When in doubt... test with a magnet. If there is an attraction, it should not be used for audio use.


Thx-RNMarsh

Last edited by RNMarsh; 10th January 2014 at 12:00 AM.
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Old 9th January 2014, 11:56 PM   #3
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One has to be careful with low value resistors and thermocouple voltages generated at the end caps. Most of the resistance wires themselves are darn good, but combine some flavor of constantan with another metal and you have a strong thermocouple. That's why manganin is so desirable as it generates near zero voltage against copper. Very few low value resistors are manganin, though you can easily wind them yourself. Low value sense resistors generate small signals, on the same order as their thermocouple voltages. If you have power dissipation, it might matter. If not, I don't have a clue what the cause is! The various resistance alloys should be quite linear, especially the ones used for resistance standards. If they had a large voltage coefficient, they'd never be used for that.

Useless trivia- manganin apparently is pressure sensitive. They use it as a sensor for explosive blasts!
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Last edited by Conrad Hoffman; 9th January 2014 at 11:59 PM.
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Old 14th January 2014, 01:40 AM   #4
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Friendly bump as I'm quite interested in what anybody else has to say about this. I've seen output inductors innocently wound on metallic cores that increased THD, but have trouble believing the resistor would have enough inductance for it to be an issue. OTOH, if it were a higher value it might have a lot of turns. Could they have used a metal former for better heat transfer? I'm also not convinced that thermal emf could come into play at more than a few tens of Hz, probably way less. I can't imagine any wire used for resistors that wasn't linear at the ppm level, but stranger things have happened. Any other ideas or more votes for the above?
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Old 14th January 2014, 12:21 PM   #5
DF96 is offline DF96  England
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I think we need a lot more information before jumping to conclusions. Resistor make/type? Exactly what was done? Grounding details? What was the harmonic structure of the distortion - very important in seeking an explanation.
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Old 14th January 2014, 12:33 PM   #6
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Quote:
Originally Posted by PLB View Post
This was removed from the circuit and tested separately by connecting it in series with an 8 ohm load resistor, which were then connected across the output terminals of the original amplifier. A distortion analyzer was first connected across the output of the amplifier to verify 0.005% THD and then reconnected across the 0.1 ohm resistor where the THD measured 0.05%.
So was the 0.005 reading taken with the 8+0.1 ohm loading?
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Old 15th January 2014, 03:40 PM   #7
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One other interesting data point is that in Doug Self's Small Signal Audio Design book (very highly recommended) he says that an increase in THD can be measured just due to a steel frame relay! That suggests that some ferrous material in proximity or internally might likely be the cause.
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Old 15th January 2014, 05:14 PM   #8
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As it happens I've just measured ten 1k wirewound resistors (Ohmite WND1K0FET); distortion varies greatly from sample to sample, with the better ones having essentially unmeasurable distortion, and the worse ones having higher distortion than a quality metal film part. Distortion is independent from frequency and mostly 3rd harmonic, so a voltage coefficient effect. One or two samples also showed strange, popcorn-noise like excess noise.

Given the statistical observation, it can't be a systematic effect (magnetic material or thermocouples) for this part at least. I recall having read (in Cordells book?) that poor end contacts could be an issue.

I've also observed distortion in low value power wirewound resistors (1 W to 100 W), but haven't gathered detailed data.

On the other hand, a precision part (Rhopoint 8G16D) which I have evaluated (100r to 10k Ohm) showed no detectable distortion across a few dozen samples.

Samuel
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Old 15th January 2014, 06:18 PM   #9
DF96 is offline DF96  England
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In the absence of a better explanation I too would blame poor contacts. Thermal effects could only affect very low frequencies.
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Old 15th January 2014, 07:50 PM   #10
tvrgeek is offline tvrgeek  United States
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We used to have terrible trouble with small glass encapsulated caps and diodes as it was only light contact while in the autoclave that made the nail contact the chip or cap.

This almost means we have to measure everything, not easy without a big investment.
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