When it comes down to purchasing new sound equipment for your church there are many difficult choices to be made. How can we best provide for audio in our services within this budget? What, if anything, needs to be “value engineered” to get the best “bang for the buck”? Does anybody make cheap sound gear that actually works?
Well, we’ve all faced those situations and it can be hard to decide where it is best to cut corners. Of course, in a perfect world we wouldn’t have to consider what we won’t buy this budget year, but instead, “How am I ever going to spend all this money??” Reality usually sets in rather soon and you realize that you can’t have the concert sound system you’ve always dreamed of.
That doesn’t mean that your system can’t be very good. In fact, if you carefully consider your options you just might be surprised at what you end up with.
One of the hardest parts of your system to understand can be your amplifiers. You know, those heavy boxes that take up space in your rack and heat up your equipment room. The ones that just make things louder, right?
Well, there are some things that you should be aware of when shopping for an amplifier. If you look at the spec sheet for your next potential amplifier you are likely to see some terms that have very little meaning to you. Terms like slew rate and damping factor. You will also see ratings for wattage at various impedances, THD, and input sensitivity. All of these ratings and measurements can help you determine which amplifier is right for your situation. While we don’t have time to hit on all of the terms you might run across we will take the time here to give a brief primer on what to look for in a good amplifier.
Let’s start with wattage. You will often find wattage measured at impedance loads of 8 ohms, 4 ohms, 2 ohms, and occasionally 1 ohm. While anyone could build a loudspeaker that would measure any impedance theoretically let’s just assume that most loudspeakers will show an impedance of 8 ohms. If you run two in parallel you average 4 ohms. Add another and you are down to 2. Most manufacturers would recommend that you not run your amps anywhere below two ohms or risk damage to your gear. It is probably a good idea to stay within the 4 ohm range just for safety’s sake.
So, we have approximately one amp for every two loudspeakers in our system. This is not taking into consideration any speakers that you might be running on a constant voltage system. 70V and 100V systems are common for distributed systems such as you might find in your fellowship hall or in a meeting room or even in an under or over balcony system. “In a 70 Volt distribution system, a transformer is added to each speaker which increases the speaker’s impedance significantly. This allows you to connect a large number of speakers in parallel to the amplifier. The amplifier sends a higher voltage than a regular 4 or 8 ohm amplifier (up to 70 volts) in order to compensate for the high speaker impedance. By connecting the speaker transformers at different wattage levels, or by adding 70 volt attenuators, each speaker can be set to the desired volume independent of the others. In a 70 volt system, you can add as many speakers as you desire as long as the total power requirement of all the transformers added together does not exceed the power output capability of the distribution amplifier.”1
In a conventional sound system, then, we know that we will need an amplifier for every two loudspeakers that we have in the system. One important consideration is total output wattage. This will be measured at different impedances on most spec sheets. We are concerned with the 8 ohm and 4 ohm measurement. It is important to note that different manufacturers will occasionally use different methods to reach this number. For instance, you want to be sure that power rating is measured at full range and not just at 1 KHz. Also pay attention to the impedance. If the manufacturer is measuring impedance at 6 ohms then you know something is up!
Wattage is an important measure to take into consideration because it can help determine not only the volume of sound that will come from your system, but also can help determine the longevity and reliability of your system. You will want to choose an amp with a power rating higher than you will need, but not much higher than the rated wattage of your speakers. Choosing an amp with a much lower wattage than your speakers can actually serve to damage them. You will cause more damage by underpowering a speaker than you will by overpowering it.
It is important that your amps are powerful enough to drive your system to the volume levels that you require. If you try to make up for weak amps elsewhere in your system you risk driving your amplifiers into clipping or overdriving which can damage your amp and your speakers. More power is good in an amp – just be careful not to go too far.
Another spec to which you should pay close attention to is frequency response. The commonly accepted frequency response of the human ear is 20 – 20,000 cycle per second or Hertz (Hz). Make sure that your amp of choice can evenly reproduce everything you are likely to hear. If you choose an amp that is lacking in frequency response you will certainly not like the sound of your system.
Every electronic device will introduce a certain amount of noise into your system. Your amplifier is no exception. A well built amp will have a Total Harmonic Distortion (THD) and Intermodulation Distortion (IMD) of less than one five-hundredth of one percent (<0.05%) at 1 KHz. THD and IMD are part of what is often referred to as coloration of the sound. A low ratio in these categories means that these amplifiers will reproduce the sound put into them with greater fidelity. One five-hundredth of one percent doesn't sound like much, but why have more noise in your system than you have to?? Many professional amps rate much better than this. Now let's move on to a couple of my favorite terms! If you have perused an amplifier spec sheet you have most likely come across the term "slew rate". (Contrary to popular belief this has no direct correlation with "slaw rate" which is a measure of how much coleslaw one can consume at your church's homecoming covered dish lunch!) The Slew Rate is the "fastest rate at which an amplifier can change the amplitude of its output signal measured in volts per microsecond with a higher figure being better (meaning that the amplifier can change more voltage in a given period of time, one microsecond). An amplifier with a good slew rate can quickly alter the amplitude, or power/volume, of a signal. This is an important characteristic for reproducing sudden, extreme changes in a signal such as those developed with an explosion or similar loud, powerful, sudden sound."2 Unfortunately, and amplifier with a good slew rate will reproduce that loud feedback very well! However, that never happens to you I know. The quicker your amp can respond to sudden changes the more impact your music in worship, effects in drama, and hallelujahs in the message will have. Another confusing yet important measurement is the damping factor of the amplifier. Damping is the ability of a power amplifier to control loudspeaker motion. It's measured in Damping Factor, which is load impedance divided by amplifier output impedance. Let's explain. If the speaker impedance is 8 ohms, and the amplifier output impedance is 0.01 ohms, the damping factor is 800. That's a simplification. Since the speaker impedance and amplifier output impedance vary with frequency, so does the damping factor. Also, the impedance of the speaker cable affects damping. Thick cables (with low AWG) allow more damping than thin cables with (high AWG).The lower the amplifier's output impedance, the higher the damping factor, and the tighter the sound is. A damping factor of 1000 or greater is considered high. A high damping factor will give you bass that is more precise and has a greater impact. It will also affect the overall clarity of the music. One last technical consideration is protection circuitry built into the amplifier. Many amps today have overload and overheating protection as well as limiters to prevent clipping. These are good things to pay attention to if for no other reason they can save you money in the long run on service calls. Some manufacturers are also including protection circuitry for things like Electromotive force (essentially an electrical surge back into the amp from the speaker) and power surges. As of yet, to my knowledge, there is no protection circuitry for the spilled cup of coffee. It may be wise for you to also take a look at the back of your amplifier and your speaker to make sure that the connectors match. If you are working with an A/V integrator this will not be much of an issue, but if you are putting together a portable system that you will have to assemble and disassemble frequently it is important. Most amps provide at least two different types of connection for getting signal into and out of your amp. The problem is that there is no standard per se. If your amp has binding post outputs and your speakers have 1/4" inputs you may have to make a trip to the local music store before you can hook them up. Amplifiers are certainly one of the more important components in a church sound system. The technical specs can be a bit confusing, but they are worth a good look. It won't take long for you to begin to see the differences in the really good products and those that are not so good. Choose right and you will have good sound for a very long time.