There are many possible pathways by which spurious frequency components riding on the incoming AC may couple into the signal path... Some of those spurious frequency components may instead completely bypass the power supply, and couple into the signal path through stray capacitances that will inevitably exist in a great many places in the circuitry, or they may couple into the signal path via EMI effects, or they may radiate into the signal path as RFI.Got it. In my last post, I was just trying to limit things as much as possible to a single scenario, so my brain doesn't hurt too much. :-) But I understand that spurii may originate from other sources, like EMI, RFI, stray capacitance, etc..
What particularly confused me was the concept of "frequency intermodulation," -- not so much the "intermodulation" part, but rather the "frequency" part, in light of the fact that we're talking about DC. My naive reasoning goes, "If DC is zero Hertz, then how does DC have a frequency to be intermodulated?" My (admittedly limited) understanding of "frequency intermodulation" is that it results in frequency components that are both the SUM AND DIFFERENCE of the original frequencies. So although I think I understand your comment that...
The net voltage at any instant of time will be the NUMERICAL SUM of the individual amplitudes (voltages) of each frequency component at that instant of time....but what about the DIFFERENCE part of intermodulation?
Wait a second. Maybe I just figured it out. (I'm typing as I'm thinking)
I was thinking that the "frequency intermodulation" you've been describing was some kind of intermodulation of the DC VOLTAGE with the NOISE. In other words...
IM = [DC voltage + noise] & [DC voltage - noise]
But maybe you've been talking about the intermodulation of the COMBINED DC VOLTAGE/NOISE with the SIGNAL. So...
IM = ([DC voltage + noise] + signal) & ([DC voltage + noise] - signal)
And maybe THAT is what you meant when you said...
The DC (zero Hertz) component of the combination of DC + noise has no relevance to the sonic effects we are discussing; it is just a possible pathway by which the noise may combine with the signal.
Am I anywhere closer to understanding this, or is it hopeless?
Bryon