In the last installment of this multi-part series I wrote about RCA connectors, their supposed impedance (50 ohms), and the fact that none of the applications for which they were either first or later used called specifically for a 50-ohm connector. I also said that only one company (that I know of) -- Canare -- has ever (again, to my knowledge), brought out a "true 75-ohm" RCA connector to meet the requirements of digital and video signal transmission. And even that connector, I said -- because it appears to differ from its 50-ohm counterparts primarily in its dielectric thickness and conductor spacing, may actually be a 75-ohm connector only if used with a very small range of cables having similar dielectric materials and conductor spacing.
To introduce and back up this conjecture, I mentioned that in a private conversation with the head of connector design for a very major military and aerospace contractor firm, she (gasp!) confided to me that they don't have special tools for measuring the R, L and C (resistance, inductance and capacitance) of the tiny metal parts and dielectric elements that go into their connectors, and that therefore their design procedure consisted partly of educated guessing (to come up with a basic design) and partly of often-repetitive "cut and try" to get each connector to where it matches the characteristic impedance of the particular cable(s) they try it with.
Where this all gets weirder is that in searching the Internet and all of the catalogs available to me, I have yet to find any 75-ohm RCA female "jacks" (the things that the RCA male plugs on the ends of cables plug into). With not even a single make or model of 75-ohm RCA female apparently available, the whole issue of 75-ohm impedance matching without the use of matching transformers seems to fall apart: If a 75-ohm RCA male is plugged into a 50-ohm female, it's no longer a 75-ohm circuit. And even if a 75-ohm BNC-to-RCA or RCA-to-BNC adaptor is used, it would still seem that, unless somehow compensated-for, the female RCA element of the adaptor set would preclude the possibility of true 75-ohm matching.
This issue of impedance matching applies, of course, only to digital or video signals. For audio signals, the issue of impedance matching is irrelevant -- either because the cables we use in our systems simply aren't long enough (even at 20 kilohertz, the top of the recognized audio spectrum, in order for a cable to produce characteristic impedance mismatch problems, it would have to be hundreds of meters long) or simply because the unbalanced lines that RCA connectors are normally used for are normally "loaded" (low output impedance to high input impedance) instead of being impedance-matched, so matching can never be an issue.
The list of things that can be issues, though, might surprise you. One of them is simply the metal that the connectors are made of. One of the problems that I found when I headed up XLO was that a significant percentage of the RCA connectors used even in the very most expensive cables were made in Taiwan, and that the "brass" that they were made out of was recycled from not-very-well-sorted-scrap and contained enough "iron triad" (iron, nickel, cobalt) metals to be quite distinctly magnetic.
Magnetism can definitely affect signal transmission (which may explain why some manufacturers actually put magnets or magnetic rings on their cables), but I don't recommend it. That's why, when we entered into a contract with a Taiwanese manufacturer to produce our own custom-made connectors for XLO, I required that much-more-costly, imported-from-Japan non-magnetic brass be the only conductor material used for the connectors for even our lowest-priced cables, and actually shipped Tellurium copper and the appropriate high-tech Brush-Wellman [now Materion] alloys to Taiwan for making the connectors for our UnLimited and Limited Edition cables. This was sufficiently important to us and to the sound of our products that I actually flew to Taiwan with a magnet to randomly test the connectors in the manufacture's first batch for us before accepting delivery and making payment. It was also sufficiently important that we continued to magnet-test every batch of connectors we ever got thereafter.
Having been out of the industry for more than the last dozen years (except to write these articles), I have no idea if the Taiwanese "magnetic brass" problem still exists, but if it does, it's still worth avoiding. You can check your own cables for yourself with any kind of a small hand-held magnet. Even one of the magnets off the door of your refrigerator will work. Just touch the magnet to the "working" parts of your connectors (the pin and the ground ring). If there's any magnetic attraction at all, your connectors have a problem: Brass isn't supposed to be magnetic. Ideally, it's made just of copper and zinc, neither of which has any magnetic properties at all, so if it does show signs of magnetism, there's something there that shouldn't be, and you may want to replace your cables, or at least their connectors.
Not only the kind of metal that a connector is made of, but the kind of plating that's applied to it can also have a perceptible effect on the sound. Right now, you can buy RCA connectors plated with tin, with cadmium, with gold, with silver, and with rhodium. You can also find them made anything from thin and flimsy-seeming to impressively heavy, and with dielectrics of any number of different kinds of materials -- including cardboard! Which of those sound best may surprise you, and we'll go into that next time.
I hope to see you then!