Millivolts...... If that was the reason, wouldn’t the same reasoning
hold true for the AC mains line voltage feeding the fuse? In other words
a small VD in millivolts on the mains feeding a piece of equipment
could/would have an impact on the SQ of the piece of equipment. Not
hardly....
You are correct in your first statement of this paragraph. An important question might be 'what is meant by 'small'?' In one of our preamps I measured a 158mV (0.158V) drop; in one of our amps I've seen a 2 volt drop cause a loss of 40 watts at the output of the amp. In the case of the preamp the power supplies are heavily regulated so I do not expect to hear anything from a voltage drop on the line nor do we in practice. So we can assume that some types of equipment are far more sensitive to this issue than others!
The line voltage determinately effects the sound of a lot of equipment. This should come as no surprise to many of us here who have experienced the system sounding like Nirvana on one evening and the next day falling well short. So much has to do with line voltage which fluctuates, but also with how easily the line voltage can sag as the equipment is running (a further issue is distortion of the AC sine wave which is not part of this discussion).
A fuse is one of those spots in that set of connections from the wall (AC power connectors, power cord and the fuse and holder) that is likely the 'softest' which is to say that the sag occurs easier at that point, due to contact area and conductor size (the effects being limited by the fuse being so small). Part of this is due to the nature of the fuse itself, the other part due to its connection into the system.
This ties directly into the power cord conversation (a very real and measurable phenomena) and its effect on the system sound.
On a different website I encountered an individual who was of the objectivity camp (measurements) who was adamant that power cords could not make a difference in system performance. When challenged on the subject, I discovered he had never even tried to measure the performance of a power cord and thus had no measurements! To individuals who live and die by measurement, I appear to be a subjectivist; I found it quite ironic that he lacked the measurements while I did not.
I'm not sure how I come off on this site but to be clear I like to know why I hear certain phenomena and so try to see if its measurable. Quite often it is.
In this case, its a simple fact that you can measure the voltage drop across the fuse. Depending on the equipment in use and your measurement technique, you can see that voltage drop vary somewhat with the audio signal. It could be construed that this could introduce IM distortion. IM happens to be audible to the human ear so it should come as no surprise that the effects of the fuse can be heard.
Our customers have been saying this all along as I have maintained elsewhere on this site. It took the obviously specious argument that the fuses were somehow directional to cause me to take a look at why people might think that. A simple DVM showed why and anyone who puts the DVM in AC voltage mode and simply measures across the fuse connections will see a number appear on the meter readout. This is not hard to do! Further measurement might show that by rotating the fuse in the holder the voltage drop can be altered. Sometimes you might have to reverse the fuse to optimize the contacts because the fuses we are dealing with are not always perfectly dimensioned so it might get better contact area in one direction as opposed to another. This does not mean that they are directional- it simply means they are not built precisely.