Speakers & Amplifiers for Houses of Worship
Seating Less Than 1,000

Blake A. Engel, All Church Sound
edited by Joseph De Buglio, JdB Sound, Acoustics

Part 4

Amplifier Quality
In the last 10 years the quality of amplifiers has excelled. To the audience, there’s less and less of an audible difference between units. You do have to pay attention to the power output and the impedance however. Some amplifiers are advertised with a specific power rating that sounds really good, until you see that it’s into 4? load. For example, if an amplifier is rated for 800 watts into a 4? load, when you connect it to a typical 8? speaker, you’ll really only have about 400 watts driving the speaker.

Amplifier Inputs
Another issue with amplifiers is the input stage. Some amplifiers don’t have a proper differential/balanced input; they only have a quasi-balanced input. Such inputs can allow noise and ground problems into the system. This type of input is typically found on less expensive models–even from the same manufacturer. Such inputs can’t ignore noise picked up on the audio cable very well. If the cable run is short and interference isn’t a problem in your area, you might get away with it. If, however you have a long cable run, the area is prone to interference, or if the sending piece of equipment (most likely an equalizer) also has a quasi-balanced output, you’re going to have problems.

What’s interesting to note is that many people will say it’s no big deal to have quasi-balanced inputs (or outputs) and that you can get away with it. To an extent, this is true. The problem is, when there is a problem, many people don’t know exactly what’s causing it, and it’s rarely blamed on the input or output stages of the equipment, wherein the real problem lies. Thus, some will say they’re never heard of a problem with using such equipment—simply because the blame was never put where it should have been.
(here's a nice article on interconnction issues with balanced and unbalanced circuits from Sony, and here's a great one from Rane)

It’s NOT a Volume Control!
Keep in mind the control knob found on the amplifier is not a volume control but an input attenuator. What it does is determine how high the input signal must be to drive the amplifier to full output without distortion. Even with the input attenuator set to a very low level, a signal that’s high enough will drive the amplifier to full output and often with distortion. The distortion can come from the input stage of the amplifier being overdriven (in an attempt to make the output level louder).

What Size?
So how do you know what size amplifier to get to power a specific speaker? The best option is to use an amplifier that has an output rating higher than the speakers’ power handling rating. One of the biggest reasons speakers get blown up is because they’re underpowered. If you use an amplifier that’s too small, it can often cause distortion due to lack of control. This distortion creates heat, and before long the speaker fails. To be safe, the amplifier should be able to provide twice the power of the maximum power handling of the speaker. If you can’t get this close, multiply the maximum power handling level of the speaker by 1.8 and 2.5; look for an amplifier that has an output within this range.

Output Specifications
Are you familiar with the way output specifications are listed for power amplifiers? You’ll see that they list the power output based on the impedance of the speaker connected. Most ratings are given for loads of 8Ω, 4Ω and 2Ω. Now, ohm’s law states that if you have a power rating into a specific impedance, cutting the impedance in half will result in twice the amount of power flowing. When it comes to power amplifiers, we would then expect the following to take place, assuming an initial rating of 100W (100 watts) into an 8Ω load:
8Ω - 100W
4Ω- 200W
2Ω - 400W

Unfortunately, this isn’t the case when it comes down to real life. The power conversion in an audio power amplifier isn’t quite so true. Here’s a real life example from a few different amplifier manufacturers:

The first three listings for each amp are the specifications given for each channel when the amp is operated in stereo mode. The last two listings are the specifications given when the amp is operated in bridge-mono mode.

This is what happens with EVERY audio power amplifier, regardless of who designed it, who sells it, or whose name is on it. The power conversion doesn’t follow ohm’s law.

What this means is that if you have a speaker rated for 500 watts at 8Ω and you connect it to an amplifier rated for 500 watts at 8Ω, you’ll be just fine. What many people do, however, is connect two of those 500 watt speakers to the amplifier channel, thinking that with twice the impedance (now 4Ω if they’re in parallel), they will each get 500 watts of power from the amplifier. In reality, they each only get 325 watts or so—again, due to the power conversion not following ohm’s law. This is really cheating yourself when it comes down to it; instead of trying to find a larger amp that can run two speakers per channel with the proper output power, you should really just get smaller amplifiers and connect one speaker to each amplifier channel. Remember, it takes doubling the power to increase the sound pressure level by 3dB; you need to increase the power level 3 times (9dB) to double the (perceived) loudness of sound (though in a closed room, this rule doesn’t always apply).

Here's some more examples to look at in regards to connecting stage monitors. Click any of the images below to open them in a different window.

Bridge-Mono Mode
What is it? Using a normal two-channel amplifier, bridging is done by using only one of the input stages to drive both channels of the amplifier. The two amplifier channels work together, one side pushing, the other pulling. This creates an output with more power than the single channels alone could provide. This of course turns your two-channel amplifier into a one-channel amplifier (which may or may not be an issue). When using an amp in bridge-mono mode you also limit the lowest impedance you can connect to the amplifier. Most quality amplifiers can support a load of only 4Ω, sometimes as low as 2Ω when used as a two-channel amplifier. When used in bridge-mono mode, these same amplifiers often suggest no less than an 8Ω load, sometimes as low as 4Ω. This usually isn’t a problem (since you’re only connecting one speaker to one amplifier channel), but you do need to be aware of the issue to ensure you don’t create a problem.

The lower the impedance an amplifier has to drive, the harder it has to work, the more heat it generates, and the short its life span is.

Amp Mounting Locations
If the amplifiers will be located in an equipment room, a typical fan-cooled amplifier is a fine choice. However, if the amplifiers are located within the sanctuary or in a noise sensitive area, you must use convection cooled units. These of course, cost more than the more common fan-cooled units.

How close should the amplifiers be to the speakers they’re driving? The closer, the better, but you don’t need to go overboard by mounting them up in the attic! Most churches have no trouble keeping the speaker cable run less than 100 feet, this is just fine. Remember, the longer the cable, the more signal loss will occur. This means you need to use heavier cables which cost more. More often than not, amplifiers are located in an equipment closet in a balcony or behind the platform. Just ensure this room doesn’t experience high temperatures, especially in summer. Many such equipment closets never get HVAC vents put in them (why bother?) and thus the rooms are very cold in winter (fine for the amplifiers) and very hot in the summer (not good for the amplifiers). The life expectancy of electrical components is directly related to the temperatures they’re exposed to both long and short term. The warmer it is, the shorter life they have. For some components, this means that for each increase in temperature by 10-degrees Fahrenheit, their lifespan can be cut in half. Thus, if a component is rated to operate 10 years at 90ºF, it may only last 5 years at 100ºF or 2.5 years at 110ºF. If a piece of equipment such as an amplifier is in a high-temperature environment, it can’t be cooled properly and thus wears out much sooner than it should.

chart showing capacitor life vs. Temperature

Cables & Speaker-to-Amp Ratio
Finally, when it comes hooking it all up, for the best performance, each main speaker in the system should have its own amplifier channel. No amplifier channel should drive more than one speaker. By default, each speaker should have its own dedicated speaker cable run from it to the amplifier. Don’t share speaker cables or amplifier channels. This ensures the system can be set up and adjusted properly. It also aids in troubleshooting if there’s ever a problem.

Floor monitor speakers can afford to share amplifier channels if needed, but you need to ensure you don’t overload the amplifier. Such connections are set up so the speakers are wired in parallel with each other. Never wire speakers in series, this results in poor audio quality and can lead to overheating and damaging your amplifier.

Don’t skimp on the size of cable you use. One of my associates reminds people to “buy the largest piece of copper you can afford”. This is good to a point, of course, but you don’t need to go to extremes either. 12-guage speaker cable is easily found at a low price. Unless your speaker is rated less than 100 watts or is closer than 50 feet, use it. Otherwise, 14-guage is a good choice for those smaller speakers or short runs. Be sure to use twisted cable. This is cable where the two conductors are twisted around each other. Using flat zip-cord style cable can mean problems with noise in the audio system. Every amplifier has what’s called a feedback loop which sends a portion of the output back into the amplifier. If you use flat non-twisted conductor cable, it acts just like an antenna and can feed noise and or RF right into the feedback loop. This results in hearing all sorts of noises – or not hearing anything, but wondering why your high frequency drivers are being blown out all of the time (if the frequency of noise is very high, the amplifier will amplify it and the speaker will TRY to reproduce it). Some will suggest using normal THHN wire as used by electricians for wiring electrical circuits. This wire is fine to use, however you need to twist it and then ensure it’s well labeled as being the audio system and that future electricians shouldn’t tap into it. The extra efforts required to do these simple things ends up taking too much time and energy and makes standard off-the-shelf speaker cable a much better choice. (If you’re wondering, yes, there have been cases of electricians tapping into the cables used for amp to speaker wiring when THHN is used in conduit.)

Powered Speakers
What about powered speakers? If you’re doing a self installation, you need an electrician to install the AC and then run another set of cables for the signal lines. Before installing it, fire up the whole speaker system while it’s still on the ground. Make sure you don’t have a Pin-1 problem or other AC problems. (Powered speakers are well known to have Pin-1 problems.) If there are problems once it is installed, there’s not much you can do besides take it down and send it in for repair. If you use a separate speaker and amplifier, you can still swap amplifiers to determine if it’s a faulty amplifier or speaker. I feel powered speakers are fine for touring groups who set up and tear down every few nights, but for now, I don’t feel comfortable suggesting their use for permanent systems in churches. If inspected on a regular basis and easily swapped out for repair, (as in a temporary portable system) they can be a good option. But for a church who wants to install it and forget about it, it’s not worth the convenience.

Conclusion
The selection of an amplifier is based on the power handling of the speaker it will be used with and the requirements of the system as a whole. For the most part, you’ll do well with any of the higher-quality amplifiers available on the market. This doesn’t mean they need to cost a lot. Expect to spend $500 to $800 for an average amplifier capable of 400 watts into each channel at 8?. Speaker selection depends on the acoustics of the room, the size and shape of the room, the coverage required, the sound pressure level required, and the fidelity desired. There are just too many variables to consider and too many speaker manufacturers and products on the market for the average person to determine what’s best for their sanctuary. If you find this too daunting, get professional help or an independent consultant. Speaker prices range from $500 to $1,500 for an average “main system” speaker. Larger, more powerful speakers or those suited better for rooms with poor acoustics can cost $1,800 to $4,000 and more. Smaller speakers used for filling in or a delayed system can cost $100 and up. If you have acoustical problems, a low ceiling, or need a great deal of pattern control or sound pressure level, you will need to make a bigger investment if you want to do it right.

Don’t use what another local church used or what other churches in your denomination have used just because it worked well for them and you like the sound. Ten churches can look the same, be of the same size and use the exact same sound system design, yet each church may need different models of the same speaker line, different amplifiers and be installed differently even by a few feet for all of them to sound good. You need to choose the product that fits the situation, not the hype. Speaker types and locations are the most important parts of a good audio reinforcement system. The amplifiers that drive them are nearly as important; you need to ensure you’ve made a good match. The design of your system is forever and the equipment will need to be upgraded as it ages. Designing your system right is the single most important step to good sound after acoustics. The less you compromise here, the healthier your church will be.

© 2003 Blake A. Engel, All Church Sound