If that's the case. How come the output doesn't show this "potentially contain many frequencies, all at the same time, and different amplitudes."? The output is all sine wave single frequency. The following is different bandwidth selected (main unit uses 2 pole Butteworth filters), and it's all uniform sine wave even for 20 minutes and all day.
I can guarantee that if you took your equipment down to the train station (electric train) and measure the EMI again, you will definitely get something different!
The 60Hz AC noises are only present if the main amp is not connected to any source (like a sine wave generator). See below. But if it connected to any source, there is no interference. So how do you characterize (or compute) the strength of interference before the sine wave input can be affected? I guess the 10 microvolt sine wave generator has same strength as the skin 10 microvolt, isn't it.. since they are 10 microvolt.
The following is when no leads were connected. I didn't disconnect the extension wire in table (so there is 60Hz AC capacitive coupling or electric field), gain of 10000, 1000Hz bandwidth
The following is when gain is adjusted from 10000 to 5000. Why is there less noise?
The following is when Audacity is zoomed to show the 60Hz AC waveforms
When 11 feet cable with open end was connected, the following is the result:
The following with sine wave generator inputted 10 microvolt, 50Hz, 5000 gain, 60 Hz AC on (extension plugged in), 1000Hz Bandwidth
The following when extension cable unplugged (no 60Hz AC (extension wire unplugged))
When there is sine wave generator input, the output is not affected anymore whether the extension wire is plugged (with capacitive coupling) and not plugged (with no capacitive coupling). So what can you say about any built in filters that activates when there is sine wave input??