Generally speaking harmonic distortion occurs as a result of non-linearity. And a fuse is indeed a non-linear element, in the sense that the voltage drop across it is not in linear proportion (i.e., is not in direct proportion) to the amount of current it is conducting. That follows from the fact that the fuse’s resistance changes depending on how much current it is conducting.
It should be noted, though, that Teo’s explanation mentions the specific case of amplifiers. And indeed, in the case of most amplifiers the amount of current conducted by both mains fuses and fuses in internal circuitry will vary dramatically as a function of the dynamics of the audio signal. (Although in the case of mains fuses, amplifiers operating in class A are an exception to that). As I said in my lengthy previous post:
Almarg 4-4-2018
I suspect that the effects of the small but rapid variations in fuse resistance that occur in applications in which the current through the fuse fluctuates widely are somehow affecting circuitry that is downstream of the fuse. Presumably the fluctuations in resistance differ significantly among different fuse types. I can’t explain exactly how those effects on downstream circuitry may result, though.
However, I don’t see how Teo’s explanation would be applicable in the case of mains fuses that are used in preamplifiers and in most source components. In those cases the current being conducted by the fuse, rather than being subject to the “highly dynamic load” Teo refers to, is essentially constant.
Other questions also arise in connection with the harmonic distortion explanation. Under typical circumstances are the odd order harmonic distortion components that are referred to large enough to add significantly to the corresponding distortion components that may be present on the incoming AC, even if a power conditioner is being used? And in the case of components which provide tightly regulated DC voltages to their audio circuitry, such as most well designed preamps and source components, how would higher order odd harmonic distortion components that may be introduced by a mains fuse influence the audio signal, given that rectification (i.e., AC to DC conversion), extensive filtering, tight voltage regulation, and the typically very great “power supply rejection ratio” of the audio stages would all greatly reduce (and arguably eliminate) that influence?
I’m doubtful that any of us are in a position to provide definitive answers to those questions, in part because such questions cannot be readily analyzed from a quantitative standpoint.
Finally, to be sure it’s clear I am certainly not saying that fuses don’t make a difference, and I am not saying that anything in Teo’s post is incorrect. What I am saying is that the explanation he has provided for audible differences among fuses does not appear to apply to many and probably the majority of the circumstances in which benefits have been reported.
Regards,
-- Al