Troubleshooting Audio Problems in Your P.A.

December 18, 2008

As with anything, eventually, you’re going to have something break, stop working, or just plain not “sound right” in your sound system. You will have to diagnose the problem, and find the easiest, fastest solution to the problem and get it fixed with the least amount of drama or attention being drawn to the situation.

The first step is to diagnose what the issue is in a broad sense. To do this, you’re going to need to hear it and be able to try certain things, so this part of the procedure will be behind the console. What is, and what isn’t working? Once you figure out what isn’t working, you need to figure out in what way isn’t it working? Is there just no sound? Is there sound, but it is distorted? Is there clean sound, but the level isn’t right? Etc. (It takes longer to read this paragraph than it should to perform this step.) This has to be done in a logical sequence to zero in on what piece of gear has broken in the least amount of time, and in a way that doesn’t waste effort. Start with the most likely cause, and if that isn’t it, move on to the next most likely cause. Eventually, you will find it.

Once you’ve narrowed down the result of the problem, it’s time to begin to narrow down the offending piece of equipment. You do this by swapping parts for known good parts, or connecting them to a known good piece.

Here’s an example: Let’s start with the easiest possible example – a vocal mic doesn’t work, and you are sending a monitor feed from the FOH console, so there is no splitter or monitor console. You solo that channel, and it’s dead – no sound at all. All of the other mics work and sound fine. It’s a dynamic mic, so it doesn’t need 48V phantom power, so that’s not an issue. There is no distortion in the FOH system or monitors. That narrows it down: you know that the basic system works – the speakers work, the amps are alive, the crossovers, EQ, and snake returns work. The thing that is not working is limited to one channel. The possible causes are (in order of signal flow): mic; mic cable; snake channel; console channel.

So, let’s re-arrange that to the order of likelihood of failure: mics rarely fail, and console channels don’t fail very often, and usually not total failure. That leaves the mic cable and that channel of the snake. From experience, I know that mic cables fail much more often than snake channels, so the first thing I would do is replace the mic cable with one that you KNOW is good. Then test the mic. If it works, great! If not, go to the next step. Still using the known good replacement mic cable, plug that mic cable into another channel on the snake, and test.
If it STILL doesn’t work, plug that snake channel into a different mic input on the console, and test. If it STILL doesn’t work, the only thing left in that chain is the mic.

It is this process of elimination that will help you find and fix a problem in the least amount of time, and with the least amount of effort. A problem like this isn’t a time to drag out cable testers or the like. You’d spend about as much time testing the cable in awkward locations (like beside the drummer, in the dark, with everyone waiting on you) with your tester, as it would take to swap it out.

As you might have gathered from this example, it’s easiest to fix things that don’t work at all – totally dead.

You WILL run into situations where things work fine . . . sometimes. Things will be going smoothly, and suddenly something breaks. Just as you start to troubleshoot, it starts working again! These problems can be the most difficult and frustrating problems to deal with, because you can’t find out what is broken while it is working. Sometimes it will only happen once in a great while. You can still go through the troubleshooting exercise to narrow it down to one particular group of parts.

Let’s say that all of the subwoofers cut out for a few seconds, and then just as suddenly, start working again. In your mind, you can do some troubleshooting. You know, for example, that it wasn’t a problem with any of the mics, snake, or console. It had to have been “behind” the crossover – and that chain would include the output of the crossover itself, the cable from the crossover to the subwoofer amps. the amps themselves, the cabling from the amps’ outputs to the connector on the panel of the rack, the speaker cables that go to the subwoofers, and the subwoofers themselves.

Since ALL of the subwoofers stopped, it also can’t be anything that is specific to one cabinet or pair of cabinets. It must be something that is common to all of the subwoofer cabinets, output of the crossover, cables from the output of the crossover to the input of the amps, or (if you drive all of your subwoofers with one amp) the amp.

Think about the rate of failure in a subwoofer system. Speakers are probably the first, amps next, then cabling, and lastly crossover. Since all of the subwoofers just stopped working, and didn’t make any nasty rattling noises first, and then just started working, I’d rule out speakers. The odds of you blowing every single subwoofer at the exact same time are extremely low, plus speakers usually give some sounds of distress first. Plus, blown speakers do NOT start working again. Once they die, they don’t resurrect themselves.

Some amps have thermal protection which can shut them off until the cool enough, and then they come back on. Check the amp’s fan to make sure it’s working, and the cooling fins are relatively clean and not obstructed. It could be the connection between the output of the crossover and the amp. You can swap that out with a known good cable to eliminate this cable as the problem. You can eliminate the crossover itself by connecting the input of the subwoofer amps to the mid-bass output of the crossover (this won’t hurt anything). If the subwoofers put out sound connected this way, but not when plugged into the low-frequency output of the crossover, it could be a crossover problem. Swap it out with your spare (you DO have a spare of all of the “show-stopper” parts of your system, don’t you??).

The key here, is that you can’t actually DO any of this until it fails again, but you will at least have a plan of action in mind, and may be able to have some of the swap-out parts handy so you can cut a few extra seconds off of your down time.

Using your knowledge of your sound system components, some very basic understanding of electronics, and a lot of logic, you can quickly and easily get your system back up and running in very little time. Once you have identified a bad part – MARK IT AS BAD!! I can’t stress this enough. Nothing will take the wind out of your sails faster than the next day when you intend to repair a bad mic cable you found, you remember that you didn’t mark it, and it looks just like the other 40 mic cables in your cable trunk.

If you find a bad mic cable, slide the XLR shell back and see if there is a broken connection where the wire is soldered to the connector. If so, fix it, and also re-do the other end. When one end fails, the other end may be close behind. If both of the ends look good, cut the cable in half, and use your ohm-meter to find which half is bad. Take the XLR connector off the bad half and use it on the good half. Throw the bad half away, unless you want to spend the time to keep cutting it in half to find a good part that’s still long enough to use for anything.

Pin 2, or Pin 3? Does it Matter?

November 26, 2008

If you run balanced lines, either mic level or line level, you are undoubtedly using 3-pin XLR connectors. They are much sturdier than 1/4″ connectors and provide a better electrical connection. However, there is one thing you need to be aware of when wiring your system, and that is which pin you are going to use as the ‘hot’ connector.

Pin 1 is ALWAYS ground. It never changes, and never will change. The other two pins are a little different. In the “old days,” (late 70s), the “standard” was Pin 1-ground, Pin 2-neutral, Pin 3-hot. It makes logical sense if you think that if pin 1 is ground, the pin numerically furthest was Pin 3, so that was hot. The pin in the middle, 2, was “neutral.” Most US and British-made equipment adhered to this standard. Then came the Japanese products spearheaded by Yamaha, and they all used pin 2 as hot, and pin 3 as neutral.

Everybody has gradually shifted to the “Japanese standard” and now pin 2 is hot with pin 3 being the neutral.

It should be noted that due to the physical layout of the XLR connector, it doesn’t make ANY difference at all, since all of the pins are equally spaced in a triangle shape. You just have to make sure when connecting pieces of gear together that they all use the same pin as hot, or you can run into trouble.

One of those problems is if you have a piece of gear that has Pin 3 hot, and you are feeding something that has Pin 2 as hot, the signal will be 180 degrees out of phase. This, in itself, may or may not cause any problems, but I am a firm believer that a positive signal voltage swing at a mic preamp should result in a positive voltage swing the rest of the way through the system, and should result in an outward movement of the speaker cones, and therefore result in a positive pressure wave being generated. It can make an audible difference on some instruments. Kick is a good example. The drummer’s beater hits the drum head, and the initial percussive impulse moves the head toward the listener. If the speaker cones move toward the listener, the drum sounds good, and percussive. If the initial impulse moves the cone back, away from the listener, it results in something that doesn’t sound nearly as percussive, since the initial result is to decrease the pressure of the air.

There are also times where you may need to tie neutral and ground together when connecting two pieces of gear together. If piece A has Pin 3 hot, while piece B has Pin 2 hot, and you are using a standard XLR-XLR connector, you will have trouble, since you will be shorting hot to ground, and connecting neutral to the ‘hot’ conductor of piece B.

Now, you could go through the entire system, checking every piece of gear for which pin is hot, and make adapter cables that swap pins 2 and 3 at one end (and ONLY one end), but then you have the problem of “special cables,” which I hate. If you are troubleshooting a problem, you constantly have to be aware of which cables need to be “special cables, and what exactly those “special cables do” while you are substituting. You can’t just grab any old cable and use it. Since having to troubleshoot a system as complex as a sound system is not trivial, and rarely happens during a lull in activity or under ideal conditions, this is one thing you really don’t want to have to worry about.

It would be MUCH better to go through the entire system, and rewire EVERY XLR connector on every piece of gear so that they all have the same pin hot. You can pick whichever one you want. As I said earlier, there is no electrical or structural advantage to either one. Personally, I would pick whichever one your console uses. Since it has the highest XLR jack/inch density. it would definitely be the biggest hassle to switch. While you are at it, it can be worthwhile to go through all of your mic cables and check them as well. You run into the occasional one that has been repaired at some point, and someone got Pin 2 and 3 switched on the end they fixed.

In fact, I would recommend that once a year, you go through EVERY cable you own, cut the ends off about 6-12″ back, and completely remake every cable. Nearly every cable failure I have ever seen has been at a cable end from repeated flexing or stresses on the solder joints. Just cut off the cable nearest the ends where it has suffered the majority of the flexing and twisting, clean out the solder cups in the connectors and flow new solder in them, strip and tin the conductors, and put them all back together. This would be a good time to add shrink-tube strain reliefs, if you don’t already use them. This will improve the reliability of your cables greatly, and may just save you the joys of crawling behind amp racks, in the dark, 10 minutes before showtime, with a Big Name Star breathing down your neck, trying to find out which cable has finally failed because it was bent, pulled, and twisted one time too many. It is just this kind of situation which can ruin your reputation as a professional-grade sound company, and can result in the loss of future jobs. In the market you have chosen, and in an economy such as we now have had thrust on us, you need every advantage you can get, and absolutely must avoid things like this.

Every situation is a learning situation. In this situation, you would learn that a couple of hours of tedious work each year can prevent 5 of the worst minutes of your life.

More Important Info on Grounding

November 24, 2008

A good electrical ground serves a couple of purposes.  As I pointed out earlier, all of the shields in your audio cables are connected to ground.  As the name implies, it “shields” the ‘hot’ conductor (in the case of unbalanced) or + and – conductors (in a balanced cable) from external electrical noise and shorts that noise to ground.  As long as the ground connection is a good one, and the current flow is not excessive, the noise goes away, and no problems are caused.

There is one aspect of grounding which you MUST take into account, and personally check.  This is the ground integrity of everything on stage.  Here’s why: Let’s say that there is a guitar player who defeats the ground on his guitar amplifier and manages to plug the AC cord in so that his chassis is connected to the “hot” side of the AC line instead of the neutral side.  Now, let’s say that he also sings, so you place a mic on stage in front of him, and your mic is properly grounded through its mic cable.  So, if the guitar player touches his guitar strings, which are at the same electrical ‘potential’ as the chassis of his amp (which is SUPPOSED to be at ground potential, but which in this care, is NOT!), his body is also at the same potential as his amp chassis (i.e.-‘hot’).  If he touches the mic at the same time, with either his hand or his lips, he can receive a fatal shock, or at the very least feel like he has been bitten on the lip by an angry badger.  Even if it’s not fatal, it’s not very conducive to his continuing to put on a good show, as he will spend several minutes trying to breathe.

This is very serious stuff.  There have been guitars players KILLED by this, and thousands of others shocked.  Check out all of the amps that the guitar players will use.  If you see any vintage amps, make sure that they have a 3-prong AC plug.  In all cases, I strongly suggest taking your AC Voltmeter and checking every guitar amp.  Plug in a guitar and, with your meter, measure from the guitar strings (or bridge) to something you know has a good ground (like the metal casing of a mic, or the metal case of an XLR connector, or, if you are close enough, a chassis ground lug on something in one of the PA or monitor racks.  It may take a few minutes, but it’s an important issue.  Safety is about the only thing I consider more important than sound.  I doubt that sound quality is very high on the priority list of someone who is riding in an ambulance to the hospital after getting shocked.  And since few musicians have health insurance, it can be a costly experience on top of everything else.

Just remember:  BE SAFE!!  MAKE IT SAFE FOR OTHERS!!!