Conclusion
My ARCOL NHS300 (Non-inductive, 300W) 4Ω 1% power resistors are innocent and have been released by the parole board with a work permit on my bench on account of good behavior with secondary duties as a boat anchor.
I'm convinced that amp characterization using this resistor series (such as those published by ASR) can be trusted. I also have come to believe that even though I've found papers dating from the '50's on VCR and TCR distortion in resistors, this problem is still ignored in much of the amp testing you'll find online and largely explains why Amir's distortion results are often lower.
Summary
Today's tests used the AHB2 amplifier, which employs much less feedback than the NC400 to achieve its world-class distortion specs. As expected, this resulted in reduced distortion suppression of the poorly performing test load constructed from paralleled water heater elements.
Measurements
2 second continuous sweeps at 96k/24bit were used. Power levels were 1W, 5W, 10W, 100W and 200W. The (3) test loads were; (1) of the 4Ω ARCOL resistors, (4) of the 4Ω ARCOL resistors in series/parallel for a net 4Ω load, and a parallel array of (3) water heater elements with a net 4.2Ω load.
Heat
Neither of the ARCOL resistor setups showed any detectable warming. They remained as cool to the touch after the measurements as before. The water heater elements (rated at 4,500W each submerged) remained on the bench air-cooled. I did notice a very slight warmth to the touch after the 200W sweep.
Step One
Again, the major distortion component was the 3rd harmonic, and while the 2nd harmonic was again insignificant in most cases, it did start to rise at the 100W and 200W levels with the water heater elements. The measurements performed are the same as on the NC400; (5) sweeps using each of the (3) 4Ω load configurations. Again, higher orders than the 3rd harmonic were so far down they are had no material affect on THD.
2nd Order Harmonics
View attachment 155272
Step Two
Now we compare the single 4Ω ARCOL resistor to the quad series/parallel array that also nets to 4Ω. All (5) power levels effectively overlap. If some level of VCR (voltage coefficient) or TCR (temperature coefficient) distortion were within the range of my measurements, the quad array would have shown significantly lower distortion. The fact that the distortion traces overlap tells me that the resistors are innocent.
Level vs. THD
View attachment 155267
3rd Order Harmonics
View attachment 155271
Step Three
Now we compare the single 4Ω ARCOL resistor to the paralleled water heater elements. These traces do NOT overlap. The paralleled elements show significantly more distortion at each power level.
Level vs. THD
View attachment 155268
3rd Order Harmonics
View attachment 155270
God bless you and your precious family - Langston