Wow, I must have been really tired when I wrote that one I've been pretty worn out lately...Dana's wake was yesterday evening (and I wrote that message late that night) and his funeral was today. I'll try and be a little more coherent this time around.

In and of itself, the power wire is the least problematic since it's generally carry DC currents (even if it's high current). Of course, it can also act as the largest antenna and pick up some of the more nasty EMI coming from the car, such as alternator stator crap. What you should note, though, is that the AC noise from the alternator (among other things) will be of the same level regardless of what wire carries it (i.e., it doesn't matter if it's on a 50A power wire or a 2A speaker wire, the flux is still the same).

Uhhh, I can't remember if I actually answered a question there or not...still have Dana on my mind.

 

[QUOTE]Originally posted by MacGyver
[B]Wow, I must have been really tired when I wrote that one

 

The engine bay is a madhouse of varying high-frequency noise with the cockpit being a bit better due to the firewall. Since you're running a conductor through that firewall (power wire), it is the logical carrier of most of the EMI coming from the engine bay. The noise along that wire will come both from direct connection to the battery/alternator connection with no filtering AND from the wire acting as an antenna. Which is the greater contributor I couldn't really say off hand, but it might be an interesting point for someone to check. It may start off as being worse due to the direct connection, then is overtaken after a few years by the antenna effect as the alternator connections get worn down (thereby acting like an old spark transmitter).

Since the speaker wire is further away (and has the added protection of the firewall), it will act as less of an antenna for the engine bay noise, but will instead pick up things external to the car and any energy reradiated by wires running through the firewall (case in point, the power wire). Hence the need to separate the audio wires from the power wires (reradiation). If the shielding on an RCA is of sufficient quality and of low resistance, I would have no issues bundling it right next to the reradiating power wire...but sometimes you get a bad RCA patch cable, then all hell breaks loose with the noise.