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?