Yes, RG6 adn RG59 are both 75 ohm cables.
bcf, the MuxLab web site says the audio part of a 500001 is 600 ohms, but that doesn't tell the whole story.
Video has high frequency components to it. Whenever you run a signal on a wire for more than about a quarter-wavelength of distance, the impedance of the source, the wire, and the load come into play. For full transfer of power (note we are talking power here), all three impedances should be the same. If the impedances differ, signal gets reflected back, which results in lower output as well as ghosts from signals that reflect back to the beginning, then back to the other end...attenuating as they go, but still able to mess up the picture.
Audio used to be treated the same way, that is, dealing with power, back when Western Electric was making audio equipment. Audio stuff had outputs with 500 or 600 ohm impedances, and the loads at the other end were supposed to be the same. Reflections were much less of a problem because the wire would have to be miles and miles long to hear the effect.
Anyway, by at least the 1940s, it had occurred to someone that audio never went anywhere near a quarter-wavelength (which would be about 4 km at 20 kHz, the shortest audio wavelength), so maybe we could do it a different way...if the source had a low impedance and the load had a high impedance, then
the actual resistance of the wire would have less effect
you could actually run a few amps from one output without lowering the volume
input transformers could be done away with, as tube circuits have pretty high input impedance.
and it pretty much stayed that way. It is still typical for audio outputs to have output impedances in the range of a kilohm, but amps to have input impedances in the range of 10 to 50 kilohm. If you have ever y-connected a preamp's output without the volume dropping, you are dealing with this kind of system.
That means the impedance of the wire doesn't matter at all for audio.
Backing up a bit, if the preamp had a 1000 ohm output and you fed it to a 500 ohm load, the voltage would drop to one-third, about 5 dB down. Since that does not happen when you run signals through 500 ohm baluns, something else is going on. Even though the baluns are specified as 500 ohms, they are acting like they have a much higher impedance as they are not loading down the signal and lowering its level.
But baluns have transformers, and a transformer specified as 500 ohms will look like a much higher impedance if its output is fed to an input with an impedance higher than 500 ohms. It is then more important to look at the turns ratio between the primary and the secondary than just the impedance itself. Since we are not using baluns to try to multiply or divide the signal voltage, they are likely 1:1, that is, the primary and secondary have the same number of windings. A nominally 500 ohm transformer with a turns ratio of 1:1, connected to an amp with a 10K ohms input impedance will look like 10K!
Last edited by Ernie Bornn-Gilman
on March 21, 2006 17:51.
