almarg
7,433 posts 08-23-2017 6:08pm
Hi Steve,
You raise good questions, which get into some complexities that are not obvious.
"The signal," and the energy that it conveys, is conducted through neither of the conductors. It is conducted in the form of an electromagnetic wave, which propagates at a substantial fraction of the speed of light in a vacuum, and propagates through the dielectric which surrounds the conductors. The exact propagation speed is dependent primarily on what is known as the "dielectric constant" of the particular insulation.
Putting aside reflection effects that can occur mainly at RF frequencies as a result of impedance mismatches, and assuming that the load is essentially resistive, that energy propagates in just one direction, from the source of the signal to the load. However, that propagation of the signal and its energy is intimately related to movement of electrons within both of the conductors, which takes place in both directions (the direction alternating in each of the two conductors, assuming we’re not dealing with DC), and which takes place at an ***extremely*** slow velocity that is referred to as "drift velocity." In the case of electrical signals that are conducted via wires (as opposed, for example, to being radiated through the air or a vacuum), the extremely slow movement of electrons within the conductors and the near light speed movement of the signal and its energy are intimately related, as I said, and one would not occur without the other.
A way to visualize it is that at the instant a signal voltage is applied to the source end of a cable, a **very** slow movement of electrons will occur into one of the two conductors at that end of the cable, and out of the other of the two conductors at that end of the cable, corresponding to the +/- polarity of the signal at that instant. At the other end of the cable, and at all points in between, there will be a similar slow movement of **different** electrons, with the response of those electrons being delayed from the response of the electrons at the source end of the cable by the amount of time it takes "the signal" to traverse the corresponding cable length (at near light speed).
What can be referred to as "the current," as opposed to "the signal," can be considered as corresponding to the number of electrons traversing a given cross-section of a conductor in a given amount of time. One ampere of current, for example, corresponds to one coulomb per second, where one coulomb corresponds to the amount of charge possessed by about 6.2 x 10^18 electrons.
So assuming that only two paths exist between the source and the load, namely the two conductors in a single cable, "the current" being conducted by both conductors in response to an applied signal is in fact identical, except that when it is moving in one direction in one conductor it is moving in the other direction in the other conductor. And in the case of audio signals, or any kind of signal other than DC, the directions in the two conductors alternate between each half-cycle of the waveform.
So with the slight possible exception I mentioned earlier about RFI/EMI pickup, in the case of a speaker cable the two conductors are of equal importance. In the case of a line-level analog interconnect, on the other hand, IMO the "ground" or "return" conductor should if anything be considered to be **more** important than the "signal" or "hot" conductor. The reason being that the characteristics of the return conductor may affect susceptibility to ground loop-related high frequency noise or low frequency hum, depending on the internal grounding configuration and other aspects of the designs of the particular components that are being connected.
So given the foregoing it hopefully becomes clear that your statement that...
... when I think about speaker cables, the "energy" in the signal conductor must be very different from the neutral side simply because by the time the signal gets through the speaker voice coil, most of it has been converted into the movement of the driver, so the neutral must be quite different - doesn’t it?... is not a correct statement because the transfer of energy to the load goes hand-in-hand with current (movement of charge carriers, i.e., electrons) in **both** conductors. With that movement being equally important in the two conductors, and (putting aside the possible ground loop and RFI/EMI effects I’ve mentioned) being identical in the two conductors aside from being in opposite directions at any instant of time.
Hopefully that clarifies more than it confuses :-)
Best regards,
-- Al
Al,
Great post!
Jim
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