Last week’s tip on PA feedback (inSync 6/29/00) prompted one reader to inquire further about how to determine phasing and polarity in a PA system. While these points have been covered in past issues of inSync it is time to review.
First, when you are talking about cable wiring (as we were) the correct term is polarity. We’ve been down this path before, but using the word “phase” implies something happening to the time arrival of signals as opposed to simply reversing a couple of wires. I intentionally used the words “phase” and “polarity” interchangeably (and therefore incorrectly) in that Tech Tip because I wanted to make sure everyone understood. Some people don’t know what you mean by polarity in speaker systems. If you’re not clear, please look the words up in our WFTD archives.
There are a few ways to determine the wiring and polarity integrity of your PA or studio setup. One of the easiest ways to test an entire system is with a test device known as a “clicker.” A clicker generates a clicking sound with a specific waveform shape. At the other end you have a device that looks at this waveform and determines whether the polarity of it is reversed. It can’t really tell you what went wrong, or where it went wrong, but it can tell you if the “net” result is correct. Of course your signal’s polarity could be reversed a dozen times on the way, but if it ends up right the clicker is going to show that everything is fine. I’ll not enter into the debate about whether absolute phase should or shouldn’t be maintained throughout a system for sonic reasons, but I can say it’s usually simpler in the long run to maintain proper polarity throughout a system. It is also possible to accomplish most of these tests with a signal generator and an oscilloscope.
The best way is to be really thorough when setting up and wiring your system. If all of your wiring is correct and matched to your equipment then you know where you stand without having to take any exotic measurements. Have an understanding of what polarity is and then examine every connection in your system for a proper match to the equipment it is connecting. Speaker cables are an obvious example. Positive to positive and negative to negative. It’s very simple really. Mic cables can be checked with any decent cable tester or multimeter. If you find one with pins 2 and 3 reversed, fix it, throw it away, or mark it very clearly (it may come in handy later). Know or learn about the attributes of your gear. Most XLR connections have the positive signal on pin 2 (known as “pin 2 hot”), but sometimes you will find a device with hot on pin 3. Wire your cables accordingly. Anytime you use an XLR to TRS or Tip Sleeve cable you should think about whether it is wired with pin 2 or 3 hot. Getting this backwards is one place where we see a lot of people run into trouble.
Learn what your crossover is doing. Many older electronic crossovers do not output in phase signals. Some actually output signals that are 90 degrees out of phase with each other. Not much you can do about that but live with it. Some will throw a signal 180 degrees out of phase (yes, I mean to say phase here). Inverting the wiring connected to that output will get your signal back in proper polarity. Most crossovers do all this for you, but it never hurts to check it out (talk to the manufacturer).
If you take the time to make sure you maintain proper (or at least known) polarity and phase integrity throughout the cables in your system you will always be working from a solid starting point, which was the premise of the 6/29/ Tech Tip. Then you can try some of the tricks outlined such as reversing the polarity here or there to minimize feedback. In recording setups you are probably much more concerned with the accuracy of your signal than feedback. Again proper wiring practices from the beginning is the key.