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
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
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
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
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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!
Useless trivia- manganin apparently is pressure sensitive. They use it as a sensor for explosive blasts!
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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?
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
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
Hi richie00boy,
Please excuse my late reply, I've been output town.
Yes, the THD of 0.005% was measured across the amplifiers output terminals with the 8R+0R1. When the THD analyzer input (Sound Technology 1701A) was reconnected across the 0R1 resistor the THD went up by a factor of 10 times to 0.05%.
Hi DF96,
In my measurements, I used the balanced input of the analyzer to avoid grounding problems. The 0R1 resistor has the following markings;
NTE5WD10
5W.1ohm5%
I also tried two 0R05 wirewound resistors in series. These resistors had no markings. They gave the same THD figures as the 0R1 resistor.
I don't recall the exact wave shape of the THD waveform, except to say that it was different from that of the amplifiers output and both spectra having predominantly low order harmonics. The amplifier used in the tests was an NAD906.
Hi Conrad Hoffman,
Both types of 0R1 resistor were attracted to a permanent magnet!
The resistive material of the 0R05 resistors appeared to have a tape rather that a round type of wire. I was not able to see if the 0R1 resistor had the same wire material since it was one of these square shaped white ceramic encapsulated type resistors.
Best regards
Peter
Please excuse my late reply, I've been output town.
Yes, the THD of 0.005% was measured across the amplifiers output terminals with the 8R+0R1. When the THD analyzer input (Sound Technology 1701A) was reconnected across the 0R1 resistor the THD went up by a factor of 10 times to 0.05%.
Hi DF96,
In my measurements, I used the balanced input of the analyzer to avoid grounding problems. The 0R1 resistor has the following markings;
NTE5WD10
5W.1ohm5%
I also tried two 0R05 wirewound resistors in series. These resistors had no markings. They gave the same THD figures as the 0R1 resistor.
I don't recall the exact wave shape of the THD waveform, except to say that it was different from that of the amplifiers output and both spectra having predominantly low order harmonics. The amplifier used in the tests was an NAD906.
Hi Conrad Hoffman,
Both types of 0R1 resistor were attracted to a permanent magnet!
The resistive material of the 0R05 resistors appeared to have a tape rather that a round type of wire. I was not able to see if the 0R1 resistor had the same wire material since it was one of these square shaped white ceramic encapsulated type resistors.
Best regards
Peter
Hi Ezavalla and Sbrads,
Perhaps if I advise you of my past experience with measuring THD it will dispel your concerns about measurement error.
1) I joined Dolby Laboratories in 1967 and worked in the R&D department for 9 years as a junior engineer where I had many occasions to make THD measurements with various analyzers like that made by B&K and Radford.
2) I joined Acoustic Research in 1976 and worked in the R&D department for 18 months as an engineer where I had many occasions to make THD measurements with various analyzers like that made by Sound Technology, Amber and HP.
3) I joined NAD electronics in 1982 and worked for 8 years as Director of Research where I had many occasions to make THD measurements with various analyzers like that made by Sound Technology, Amber, Tektronix and FFT analyzers.
4) I joined Weltronics Corp. in 1989 and stayed till 2010 (21 years) as Director of Research where I had many occasions to make THD measurements with various analyzers like that made by Sound Technology, Advantest R9211C FFT analyzer and several PC based analyzers.
In 2012, I attained the grand title of septuagenarian, so as you can see, I have many years experience, not one years experience many times over!! BTW, I’m still hoping that somebody will post a reference or application note etc.
Peter
Perhaps if I advise you of my past experience with measuring THD it will dispel your concerns about measurement error.
1) I joined Dolby Laboratories in 1967 and worked in the R&D department for 9 years as a junior engineer where I had many occasions to make THD measurements with various analyzers like that made by B&K and Radford.
2) I joined Acoustic Research in 1976 and worked in the R&D department for 18 months as an engineer where I had many occasions to make THD measurements with various analyzers like that made by Sound Technology, Amber and HP.
3) I joined NAD electronics in 1982 and worked for 8 years as Director of Research where I had many occasions to make THD measurements with various analyzers like that made by Sound Technology, Amber, Tektronix and FFT analyzers.
4) I joined Weltronics Corp. in 1989 and stayed till 2010 (21 years) as Director of Research where I had many occasions to make THD measurements with various analyzers like that made by Sound Technology, Advantest R9211C FFT analyzer and several PC based analyzers.
In 2012, I attained the grand title of septuagenarian, so as you can see, I have many years experience, not one years experience many times over!! BTW, I’m still hoping that somebody will post a reference or application note etc.
Peter
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That's a lot of experience!!! but I still suggest to you to apply a larger signal across the 0R1 resistor. You know that if you apply 10 Vpp across de 8.1R, the 0R1 only gets about 120 mVpp and a THD of 0.05% is about 60 uVpp... a bit on the low side for the measurement device.
Anyway, may be the resistor's wire material, may be the electrical joints. AFAIK I have never seen a wirewound resistor which can be attracted by a magnet, but may be just a bad resistor after all....
Nickel chromium alloy is a common resistive material in wirewounds, metal film and bulk foil resistors. Flat tape is a common construction in the better quality resistors. Low Tc is harder to obtain below 1 ohm and I look for 1% tolerance to be sure I'm getting reasonably low Tc, ie, 100 ppm or slightly better. The cheap sandcast resistors tend to have very much higher Tc, 200 ppm and up.
Hi RMJ1,
Page 4 under the heading “Polarities” and the resistor “diode” effect, seems to shed some light -- I wonder! Many thanks for the link.
Regards
Peter
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