Broadcast & AV

AC DC and Cable

Steve Lampen

A week does not go by without a customer asking for a combination of power and signal, such as power + audio, or power + video, or power + data (or some combination of these). Now, as you can imagine, there's almost nothing that Belden can't make. We could easily make such a cable. The problem is not how to make it. The problem is liability. If this is 120VAC with any other kind of signal cable next to it, it is illegal in the USA. And by illegal, I mean the UL (Underwriter's Laboratory) and NEC (National Electrical Code) won't allow such a combination. The reason is simple. If a forklift runs over the cable, and mashes it all together, then you have (or could have) 120VAC on the audio, video or data cable as well. I'm sure you have read all those stories of someone picking up a mic cable only to get electrocuted. Well, this is one way it could happen.

But, we sure understand why people want to make this combination. Just think of all those powered speakers. All they need is power and audio in a single cable and they would be good to go. And, I'll guarantee you that you can find such a combo cable easily. They come in three different types. The first type is actually made in the USA. They will tell you that the power cable and the audio cable have been individually OK'd by UL (or some other testing lab), and then they just combine them. Will it work? Of course it will work. It's just that problem with the forklift....and then the lawyers get involved. Even if the chance is one in a million, that one incident will be very expensive, certainly more money than you could make on this one kind of cable. Your call.


The second kind is made offshore, where there is no UL and no NEC. Just tell them what you want and they would be happy to make it for you. But mysteriously after the forklift accident, the factory is no longer there. And getting a lawyer in that foreign country is another hurdle. Again, your call.


The third way you could get such a cable is to use a cable that looks like it would work. The picture shows a good example. This is Belden 1502R.


This cable has a shielded 22 AWG twisted pair, 100 ohms impedance, for data (could also run audio), and an unshielded 18 AWG power pair. So isn't this exactly what we were talking about? Well, no. This power pair is for DC power. You would have to work very hard to kill yourself with 12 volts or 24 volts or 48 volts. I’m not even sure it is possible. So DC power is fine. Well, you already knew that. Phantom power for your condenser microphones is DC power running down the same cable that the audio uses. No problem there. And, satellite downlink dishes use DC power on the same coax that the receive signal uses to power up the LNA in the dish. Works great (as long as you use an all-copper cable to carry the DC)


Interestingly, some of those powered speaker folks are now looking at supplying DC power with the audio in a combo cable. No problem there! After all, isn't DC what runs the power amp? All you have to do is plug the power supply in and have DC come out. A wall wart on steroids!


One last thing, if you have put signal cables with power cables in trays or conduits, you know you're supposed to keep them apart (and not just because of interference). We're all just trying to keep you safe!


If you had an "incident" like we're describing, tell us about it or just fill in the comment form below.