What's Phantom Power?

Well, it goes like this...

ooooooOOOOoooooOOOoooOOOOOOOooooooopowwerrrrrrr.........

Or maybe not.

Okay, kidding aside. Phantom power is a 48 Volt DC voltage carried by the balanced microphone cable. Certain mics, such as the Beta 87 and the SM-81, require DC power to operate. The phantom power is a convenient way to power these mics without having to have separate power supplies or internal batteries. The 48V switch at the top of the X8 channel row turns this power on and off. You can use a mic that doesn't require phantom power (like an SM-57) and leave the phantom power on and it won't hurt a thing. The SM-57 will simply ignore it. However, there are circumstances where there must be no phantom power on the line. Our Behringer stage sub-mixer outputs would be a good example. If you have 48V enabled while connected to that sub-mixer, your signal will disappear.

If a mic doesn't work, the 48 Volt switch is the first thing to check. If you must turn it on, be sure and mute the channel or you will be rewarded with an obnoxious (and possibly damaging) thump.

Be aware that a patch cable connected to the soundboard side of the patch bay but not connected to the stage side will have 48 Volts on the exposed plug. You may see a slight spark if you then plug the stage side in. As a matter of good practice, if you have to change a patch on the fly, mute the appropriate channels, disconnect the board side first, then the stage side. Reconnect the stage side first, then the board side. This will avoid exposing the 48 Volt power.

Incidentally, this is why we cannot use wired mics anywhere near the baptistry because 48 Volts and water creates a genuine safety hazard.

Jim K.

So what do we mean by "balanced" and "unbalanced" lines?

The cabling that we use to connect devices (instruments, mics, media devices) to the system is going to have a minimum of two conductors. An unbalanced cable has one "hot" conductor and one grounded conductor (a shield return). A balanced cable has two "hot" conductors that carry equal but opposite signals and that "twisted pair" as it is often called is surrounded by a grounded shield. In professional audio systems, unbalanced cables are essential  because they can run for hundreds of feet without incurring any substantial loss or noise, whereas unbalanced cables can get noisy and flaky after about a 20-30 foot run. However, we still have to deal with unbalanced lines too.

All of your XLR cables are balanced. Your guitar cables are unbalanced. The DI boxes provide an unbalanced input and they convert the incoming signal (TS) to balanced output (XLR).

Now, what do I mean by "TS"? This is a term that applies to those 1/4 inch connectors that you see on the instrument cables. A "TS" plug means "tip/sleeve". The sleeve is grounded, the tip is hot. A similar-looking plug has an additional conductor called the "ring". It looks like a stereo headset plug, but for the purpose of interconnecting devices, it is a balanced cable and it is called a TRS (tip/ring/sleeve). TRS has 3 distinct conductive surfaces, separated by thin rings of plastic, whereas the TS has only two. A stereo headset uses the TRS plug in its unbalanced form: the tip for one channel, the ring for the other and the sleeve as the shield return. A balanced cable uses the tip for one "hot", the ring for the other "hot" and the sleeve for the shield.

The plugs at the patch bay are TRS but they use a slightly different type of plug that has its roots in military and telephone applications. For practical purposes, it's a TRS. Never uses an unbalanced TS cable at the patch bay as it will result in hum and noise.

Jim K.

Crackle, Crackle, Sizzle-Hissy-Thump, Thump, Thump...

We've all heard it at one time or another - noise, crackle, weird moaning sounds. In many cases, these intermittent noises are happening at the patch bay. As the connections get dirty or the brass plugs begin oxydizing, they create resistive connections that cause anything from annoying noise to complete failure of a channel. This is the purpose of the "brush test" on the checklist. The brush test is simply taking your hand and brushing it against the jumpers at the patch bay, looking for intermittent connections. If a channel pops or sizzles, you've got a connection that's bad enough to cause you headaches if it decides to act up during a service.

Very important!!! This brush test needs to be done at a relatively low level. Master fader only needs to be up about 25 percent at most, just enough to hear the problem, and keep the monitors off. If you have the volume up too high you will be treated to some nasty loud sounds and you risk damaging the speakers. A noisy channel will be pretty obvious at the board - you will see the telltale lights flickering on the offending channel. Usually a wipedown with iso-alcohol on both sides of the patchcord will restore normal operation. In more stubborn cases, a little Flitz metal cleaner on the plug, followed by an alcohol wipedown, should do the trick.

Incidentally, if you have a patch cord plugged into the board socket, be aware that phantom power exists at the tip of the exposed plug. It's generally best to remove the cord altogether, clean it, then reinsert both ends. Make certain that the channel is muted and preferably that the master fader is down as well prior to removing or inserting patch cables.

Jim K.

So what happens when I turn the key?

The key switch is the master switch for the entire system. Please be gentle when you turn it - if we lose that switch we've got problems! When you turn the key, a device call the sequencer begins turning the components of the system on in a predetermined order. This prevents "thumping" the speakers and it also prevents an enormous current inrush that would happen if all the amps were activated at once. When you switch off, it essentially reverses the process, making sure the speakers don't get "thumped".

It is good practice to make sure the master fader is all the way down prior to start-up.

Jim K.

Welcome!

This blog is primarily a resource for the audio volunteer crew of Spring Creek Church in Wisconsin. It will contain tips, tricks and general information about our sound system, how it works and how to work with it. It is not an "official" church blog, the opinions and guidance expressed here are my own and are subject to review and correction. However, I've been studying the system for awhile now and have a pretty good understanding of how it works. Hopefully this will be a useful resource for you.

So what do we have exactly? The centerpiece of our system is a Crest X8 soundboard with 40 mono channels, 4 stereo inputs, 8 submixes , 8 aux outputs and 4 matrix channels. It is an imposing beast with nearly 2000 controls. It is pretty logically laid out and once you understand the basics, it's pretty easy to use. The mixed audio is sent to the mezzanine through a single line to a device called the Audia Flex. This computerized device controls how the signal will be distributed to the amplifiers and speakers. We have a stack of amplifiers driving the house speakers, monitor speakers and the subs.

Inputs to the board are routed through a patch bay which connects stage inputs to specific channels on the board.

There is a 24 track recorder made by JoeCo connected to the soundboard. It can record 24 simultaneous tracks of audio to a USB device for later mixdown. There is also a CD recorder which records the main output of the board with the help of the Nexia device. Think of that box as a "sound man in a can". It mixes audio according to a predetermined program and is used to feed the CD recorder and the foyer speakers (not the ones in the TV sets).

There are graphic equalizers for the lavalier mic sub, the choir sub, for Monitor A and for Monitor B. There are also several compressors for various channels.

So, now that we have a simplified overview of the system, we'll continue with a few more descriptions next time.

Blessings,

Jim K.