RialTech.Com

A Good Club PA System
By Christine Rial #7529 L3 TAP

A good PA system is a must have for any club. The most dangerous thing on the field is not hearing a heads up when a rocket goes wayward. The RSO & LCO should be able to be heard the whole length of the flight line. By having a good PA system this also let's more of the public join in with the excitement of a good launch. But first a full disclosure here, I was a Radio Shack store manager so I am going to talk about what I know. There are other PA suppliers that have similar equipment for sale so shop around and you could find a cheaper system.

So the first thing you need for any PA system is a good PA amplifier. Currently our club uses an older Radio Shack 20 Watt amp. That unit has been replace with a newer 40-watt version. Which is nice since that will let you drive even more speakers now. The reason we went with this unit is that it is 12-volt battery power. So there is no need to have a noisy generator running in the background. The second reason with went with this amp's is that they have a 70-volt output, which makes hooking up multiply speaker easy. Now for a little tech background here. If you take two 8-ohm speakers and hook them up to one speaker output, the amplifier see is a 4-ohm load, since two 8-ohm loads hook in parallel equals 4 ohms. What happens is the amp tries to put out twice as much power due to the lower resistant an eventually it burns the amp out. If you hook up two 8-ohm speakers in series the amp now see 16 ohms and tries to work twice as hard to get the same level.

The way the PA installer's work around this in schools and office builds is that they have an output on the amp call the 70-volt output. You can think of this as the same way the electric company delivers power to your house. At the electric plant they step the power up to a very high level and at each house there is a transformer to step the power back down to a level used in the house. We do the same thing with the PA system. We step the voltage up to 70 volts and then run two wires all around the field. Then at each speaker we use a 70-volt transformer to step the level back down to the 8-ohm speaker level. By using this system we don't have to worry about ohms at all. You can split the wires at any point the amp is still very happy.

The final thing we did to make the hook up easy was to mount a little box on top of the PA amp that hooks into the 70-volt output. In this box we have three 1/4 inch phono jacks all tied together. We then put 1/4 inch phono plugs on the ends of each of our speaker's wires. This lets us just plug the speakers together. At each speaker we have a wooden board that we mount a small metal box to. Inside the metal box are two 1/4 inch jacks. One jack is for the input and the other jack is an output that lets you hook up another speaker. Coming off these two jacks is a tap for the 70-volt transformer. You take the center or tips side of the jack and hook that up to the 10-watt input connection of the transformer. Then you take the ring or ground connection of the jack and hook that to the common side of the transformer. On the other side of the transformer take the speaker lead and hook the plus side of the speaker to the 8-ohm output, and the negative lead to the common side.

For microphone you have several choices, you can use a plug in microphone or an wireless mic system. The plug in mic is a nice low cost way of doing it; if you use a regular handheld mic I would highly recommend that you get a foam windscreen for the mic. This is important due to the fact our launch sites tend to be windy locations. The more expensive way to go is to get a wireless mic system. Most likely you are going to pay over $80 for the mic and another $80 for the receiver that gets plug into the amp. The best feature of using a wireless system is that your not going to trip over any wires and lets you have more freedom to get out from under the tent.

The final thing we did to make our PA system better is to raise the speaker off of the ground. While the speaker will work quite fine at ground level by raising the speakers to a six-foot level lets it be heard further back at the second row of cars. To keep cost down we cut two-foot lengths of 1/2 inch rebar. Then we attach a six foot long pieces of electrical conduit using pipe clamps, at the top the speakers get attached to conduit. We even make that easier buy mounting the speaker and the 70 volt transformer box to a piece of wood which has a couple of conduit brackets on the back side. So the only hard part of setting up our PA is pounding the rebar into the ground.

So with our normal layout we have two speakers on each side of the range tent. And with a hundred feet of speaker wires between each speaker lets us have a span of 260 feet of sound facing the crowds. Now that Radio Shack came out with a 40-watt version of the PA amplifier we could have a span of over 600 feet of sound aim at all corners of the fight line. And if you have a generator you can use the AC power 100-watt amp and expand your speaker system out to a width of over 1200 feet.

These FM transmitters are great but if you step away from your radio you cant hear the RSO calling a heads up. Also at this year LDRS I got to see several cars needing a jump since they had to leave there car radio on all day just to hear anything from the range tent. Already a lot of clubs have the a PA system that puts out 70 volts, so all they need to add is a 70 volt transformer at each speaker and they can make their system more expandable. The first step of having a safe launch is to make sure that your RSO & LCO can be heard.

I will take some more pictures of our system, once the flying season starts.