How to Choose a Power Amplifier
What do you need in a power amplifier? Combine the right amount of wattage with the right features, and that’s the right power amp for your live PA system. Sounds easy, huh? But there are so many aspects to consider when purchasing a power amp. That’s why we created this Power Amplifier Buying Guide, to walk you through all the factors involved in choosing the perfect power amp for your needs.
- Matching Amps to Speakers
- Impedance: It’s All in the Ohms
- Class D Amplifiers – Smaller, Cooler, Lighter
- Operating an Amplifier in Bridged Mode
- What Does a Limiter Do?
- What to Look For…
Matching Amps to Speakers
When you’re matching a power amp to a PA speaker setup, a good rule of thumb is to pick an amplifier that can deliver power equal to twice the speaker’s program rating. This means a speaker with a “nominal impedance” of 8 ohms and a program power rating of 350 watts will require an amplifier that can produce 700 watts into an 8-ohm load. For a stereo pair of speakers, the amplifier should be rated at 700 watts per channel into 8 ohms. A quality professional loudspeaker can handle transient peaks above its power rating if they occur.
Using an amp with some extra “headroom” will help assure that only clean, undistorted power gets to your speakers. Headroom is the difference between the normal operating level of an amplifier, and the maximum level that the amp can pass without distorting. Music has wide variations in dynamic range; without enough headroom, you’ll find your gear clipping (distorting) far too frequently! Some professional amplifiers are designed so they have additional headroom. These amps can cleanly reproduce transient peaks that exceed their rated power. In this case select a model with an output power rating equal to the program power rating of the speaker. Consult the amplifier manufacturer or owner’s manual to learn more.
In some applications, such as critical listening in a studio environment, it is important to maintain peak transient capability. For these applications, use an amplifier that can deliver two- to four-times more power than the speaker’s program power rating.
If budget restraints or legacy equipment force you to use an amplifier with less power, extreme care should be taken to see that the amplifier is not driven into clipping. It may surprise you to learn that low power can result in damage to your speaker or system.
Impedance: It’s All in the Ohms
Ohms are measures of resistance. Audio amplifiers are commonly designed to work with 4, 8 or 16 ohms of resistance, and optimum system performance can be obtained if the total ohm load of the loudspeakers is exactly correct for the amplifier. If the total loudspeaker impedance is too high, the power delivered to the loudspeakers will be reduced. If the total loudspeaker impedance is too low, the power delivered to the loudspeakers will be higher, which can overload your speakers and damage the amplifier. You can connect any amount of speakers to one amplifier provided that they are correctly wired and don’t collectively fall below the specified output impedance of the amp.
Multiple loudspeakers can be connected together. Dual speaker connections whether on an amplifier, a mixer/amplifier, or a speaker enclosure are all wired in parallel. The following general rule will help you match the impedance of PA speakers to power amplifiers for optimized performance (avoiding overloads and other issues). Don’t worry, it’s an easy one to use and remember.
To keep life as simple as possible, most people put enclosures of the same impedance in a parallel circuit. If you do this it’s all just a matter of dividing that impedance by the number of speakers.
Take the ohm rating for the speakers and divide by the number of speakers.
If you have four speakers that are rated at 16 ohms, you would take 16/4 to get the overall rating of 4 ohms. (Similarly, two 8-ohm speakers in parallel = 8/2 = 4 ohms.)
The following is a quick reference listing of some commonly used parallel loads: (Avoid the ones that go lower than output impedance rating of your power amp.)
- Two 16-ohm speakers = 8 ohm
- Two 8-ohm speakers = 4 ohm
- Two 4-ohm speakers = 2 ohm
- Three 16-ohm speakers = 5.33 ohm
- Three 8-ohm speakers = 2.67 ohm
- Three 4-ohm speakers = 1.3 ohm
- Four 16-ohm speakers = 4 ohm
- Four 8-ohm speakers = 2 ohm
- Four 4-ohm speakers = 1 ohm
Class D Amplifiers – Smaller, Cooler, Lighter
Power amplifiers feature a variety of circuit designs categorized into several classes (Class A, B, AB, etc.). Historically, Class A and Class AB designs dominated the market. Although they deliver the best sound quality, their components make them large and heavy, and their inefficiency results in high power consumption and heat output. Over the past decade, Class D power amplifiers have taken over the live sound market. This class of amplifier produces as much power as Class A or Class AB amps, but far more efficiently, all while maintaining sound quality that is sufficient for sound reinforcement and dramatically reducing the size and weight of your amplifier rack.
Operating an Amplifier in Bridged Mode
A method of configuring a two-channel amplifier so that the two channels can be “ganged” or bridged to be used together on one load. The purpose of this is to take a two-channel amp and create a larger single channel amp that can deliver more power. The result is more power to the speaker than would be possible from either channel alone. For example, a 100-watts-per-channel amp may output a single channel of 300 watts after bridging. Most modern amps have a special switch to enable mono bridge operation.
Typically, amplifiers operating in bridged mode can only do so with speakers that have twice the impedance of the minimum rating load on the amp. For example, an amp rated at 4 ohms running in normal mode will generally require 8 ohms in bridged mode.
What Does a Limiter Do?
Many amplifiers feature a built-in limiter for maximizing signal levels while preventing distortion, preventing overload in a signal chain, setting a maximum volume level to protect users of in-ear monitors, protecting speakers and amplifiers from clipping, and so on. Any time you want to establish a maximum gain setting and prevent signals from passing it, a limiter is your tool of choice!
What to Look For…
How do I choose the right amplifier power for my speaker system? When it comes to choosing a power amplifier there are a two important factors to consider.
Generally you should pick an amplifier that can deliver power equal to twice the speaker’s program/continuous power rating. This means that a speaker with a “nominal impedance” of 8 ohms and a program rating of 350 watts will require an amplifier that can produce 700 watts into an 8 ohm load. For a stereo pair of speakers, the amplifier should be rated at 700 watts per channel into 8 ohms.
Using an amp with some extra “headroom” will help assure that only clean, undistorted signal gets to your speakers. Headroom is the difference between the normal operating level of an amplifier, and the maximum level that the amp can pass without distorting. Music has wide variations in dynamic range; without enough headroom, you’ll find that your gear will clip and distort.
If you’re unclear about all of the options available, the best thing to do is call a Sweetwater Sales Engineer at (800) 222-4700 to help you determine which power amplifier is best suited to your needs.