Many experiments have been done to show why some resistors are "noisier" than others. But the only test that audio experts and audiophiles have agreed on is comparing the level of fidelity that results when different resistor technologies are used in actual audio systems. Overall resistor noise has several components. The most relevant for audio applications are thermal noise and current noise. Thermal noise is notable for being independent of the resistive material. In fact, the thermal noise level is the same for any type of resistor provided the resistances and temperatures are the same.
Current noise, on the other hand, has a direct relationship to the type of resistive material.
Resistors based on composition resistive materials such as carbon and thick film have the highest level of current noise. Thin-film resistors have a considerably more homogeneous structure and consequently are less noisy. Microhm Electronics' NAP series is a typical thin film resistor.
The lowest noise level is observed in resistors with bulk metal resistive elements: foil and wirewound. Wire is made of metal alloys similar to foil material, but additional noise may come from the junction of the fine wire of the resistive element and the comparatively coarse resistor terminals. In foil resistors, the terminals and the resistive element are parts of the same piece of the foil, so this problem is avoided.
But the major objection to wirewound resistors is their inductance, which results in chopping the signal peaks, and the significant dependence of resistor impedance on signal frequency. In addition, special attention must be paid to the following effects associated with reactance of wirewound resistors: The audio amplifier may oscillate spontaneously at 5 MHz to over 50 MHz affecting audio quality. Equivalent series inductance (ESL) can cause large phase shifts affecting audio tone. The wire coil may act as a pick-up of EMI that may surpass the level of usual current noise.
Foil resistors, such as MPR series, NMS4527, NMS2818, avoid these problems because they are structured using chemical etching of a flat bulk metal foil so that the current in adjacent current carrying paths runs in opposing directions, cancelling parasitic inductance of these paths. Also, path-to-path capacitances are connected in series, which has the effect of minimizing the parasitic capacitance of the resistor. These low-inductance/capacitance resistors are characterized by nonmeasurable peak-to-peak signal distortions.
