PA amplifiers buying guide

PA amplifiers are designed to withstand the rigors of live performance situations. That means they can handle a certain degree of unintended abuse. Most models have robust protection circuits, trouble-sensing signal limiters, and other safeguards. When pushed too hard, they protect themselves, save your speakers, and prevent audible distortion.

Pro amps are versatile, too. That means they can safely adapt to a variety of speaker system configurations. Models with digital signal processing (DSP) let you tailor the amp’s output to suit your speakers and the space they’re in.

How many amps (or amp channels) do you need?

Most PA amplifiers have two channels. You'll find four- and eight-channel amps, mostly in permanent-installation sound systems or large-scale touring PAs. In any case, each channel corresponds to a single set of speaker connections.

Say you want to power your two main, audience-facing speakers, one on each side of a stage. A single two-channel amp is a great choice.

You might want a second two-channel amp to power two monitor speakers. Monitors are the wedge-shaped speakers that sit on the stage floor and fire sound up toward the performers so they can hear themselves.

If you need a lot of bass, your system should include one or more subwoofers. You can use a two-channel amp in “bridged” (one-channel) mode to power a subwoofer. Bridged mode combines the power of both channels, sending more power to a single output.

For more information about system configurations, read our intro to live sound equipment.

How much power do you need?

Remember: PA amplifiers and passive PA speakers depend on each other to sound their best. That's why the truest answer might be that your amp needs to have as much power as your speakers can handle.

PA speakers typically have three power ratings that you want to understand: "continuous" (or "noise"), "program," and "peak."

The continuous/noise rating is the minimum power required to power a speaker in real-world situations. Peak power is the maximum the speaker can handle in short bursts. It's kind of like your car. The speedometer might show you that you can get up to 120 miles per hour, and you probably can. But if you drive 120 mph for an extended length of time, you'll burn up your engine.

Program power is twice the continuous/noise rating. And program power is what you want to look at when you're choosing an amp to match speakers. If you underpower your speakers, you'll have to crank up the amp's volume, which can cause unpleasant distortion.

Your speakers' manual will give you their peak and program power ratings, and most likely even a recommended amplifier power range.

Amplifier power and speaker impedance

You’ll get the best performance if your speakers' total ohm load — or impedance — matches that of your amplifier.

An ohm is a measure of electrical resistance. But what is "resistance"? Think of the power an amp delivers to a speaker sort of like water flowing through a hose with an adjustable spray nozzle.

You can screw the nozzle down so far — increase the resistance — that you choke the water off completely. Or you can take the nozzle off and remove all resistance — plenty of water, but splattering on the ground instead of in a focused stream.

If the total impedance — or ohm rating — of your speakers is too high, your amp can’t deliver enough power. If it’s too low, then the amp could over-deliver, overloading your speakers and possibly damaging the amplifier.

Impedance matching formula

Most PA amplifiers are designed to work with four- or eight-ohm speaker loads. Check your amplifier and speaker manuals to figure out their impedance rating.

You can connect any number of speakers to one amplifier, as long as their total resistance doesn’t fall below the amp’s impedance.

The different ways you can wire speakers can get pretty techy. But in almost every case, you'll be connecting PA speakers to an amplifier in what's called a parallel configuration.

You use a simple — if not completely intuitive — formula to calculate the total resistance of your speakers, depending on whether you connect one, two, or more to a single channel

You’ll get the best results when you use cabinets with the same ohm rating. Then to calculate the total ohm load, you divide that ohm rating by the number of cabinets you’re connecting.

Getting the most out of your PA amplifier

The amount of power an amplifier can deliver depends on the impedance (or resistance) of the speakers it's driving. So you might see an amp that puts out 1,000 watts into eight ohms and 1,500 watts into four ohms.

Keep this in mind when you’re shopping for speakers. Most PA speakers are eight ohms. Choose four-ohm speakers like the JBL PRX425, and you’ll get more power out of your amp.

Amp classes

When shopping for PA amps, you may notice that they come in different "classes." The class designations tell you something about the design of their output stages. Check out our article on amp classes to learn more.

Most of the PA amps we offer are Class D. This is the most efficient class of amplifier. Class D amps are much lighter and run cooler than other types.

We also offer a few Class AB amps. They are heavier than Class D amps and run noticeably warmer. Watt-for-watt, you are unlikely to hear the difference between the classes, especially in a live sound situation.

Headroom

“Headroom” is a term that sound pros use a lot. A system with good headroom has all the power the speakers can handle, not just the bare minimum they need to get by. Having comfortable headroom gives you several benefits.

With ample headroom, you should have all the volume you need when you're around five-to-seven out of 10. Pushing an amp to its limits — running it on 10 — can cause "clipping" distortion. The tops and bottoms of the signal waveforms are cut off (or “clipped”).

Clipping can damage your speakers. Some PA amplifiers have indicator lights to warn you when the signal is approaching the clipping point — these tiny lights have saved many a speaker. But having plenty of headroom is the best way to avoid clipping.

With robust headroom, your system will also have better dynamic range. The loud parts will sound just as good as the quiet parts. The bass parts will hit a lot harder without distorting.

Don't forget line-conditioning power protection

You'll get cleaner, clearer sound and protect your gear from power surges if you plug everything that needs AC power into a power conditioner.

What you need to know about the connections

Power amplifiers typically offer XLR and 1/4" inputs for connection to a mixing board or other audio source component.

Most amplifier manufacturers recommend using balanced XLR connections throughout the signal path. Balanced connections reject noise. They’re especially helpful where long cable runs are necessary.

You may also find RCA connections, which are more common on home audio components.

Speaker connections

You'll find three different kinds of amp output jacks on the back panels of most PA amplifiers:

• Neutrik NL4 Speakon
• Banana/binding posts
• 1/4" TS (Tip/Shield)

Speakon plugs offer the added security of a twist-and-lock connection.

Banana plugs/binding posts are more versatile. They accept bare wires, if the need arises.

When using 1/4" connections, make sure you use pro speaker cables with 1/4" plugs. Don’t use instrument cables, which have the same kind of plug, but aren’t designed to carry amplified audio signals.

THRU or line level outputs

Planning to use multiple amps in the same system? You’ll need amps that have line-level preamp “link” outputs. They send the mixing board’s signal on to the next amp in the chain.

These outputs are usually 1/4" and can also be used as inputs.

All about the knobs, buttons, lights, and display screens

Some PA amps are pretty brainy. You may find knobs, buttons, and indicators for the following:

Gain settings

Typically, the front of the amplifier will have a gain/level knob and signal level indicator for each channel. Colored lights indicate when the amp is clipping.

Frequency filters

Frequency filters let you tailor the output of an amp to suit the type of speakers it’s driving.

A high-pass filter (also known as a low-cut filter) lets you reduce the output of a speaker below a certain frequency. It lets the amp focus its power on the higher frequencies and not waste energy on the power-hungry bass frequencies.

This kind of filter helps to eliminate audio distractions such as bass rumble, wind noise, and microphone thumps. When you have a separate subwoofer handling the bass in your PA system, you’ll want to remove the deep bass from your main speakers. They’ll play louder, and without distortion.

A low-pass filter removes the high-frequencies from the amp’s output. Use the low-pass filter when the amp is driving a subwoofer.

A band-pass filter removes both lows and highs. Band-pass filters are good for fine-tuning the performance of your subwoofers.

Limiters

Limiters are protection circuits that can help keep your amplifier from clipping and to prevent distortion in the sound. They help prevent distortion caused by an overdriven signal, a dropped microphone, or a short in an input jack.

Time alignment

Churches and theaters often have speakers along the side walls. These allow people in the middle and the back of the room to hear things just as well as the people up front. And you don’t have to turn the front speakers up too loud.

For your side speakers, you’ll need an amp with the ability to delay the output signal. A little delay will “time-align” the side speakers with the front speakers. The sound remains crisp and highly intelligible throughout the room.

Parametric EQ

Some amps have very sophisticated equalization capabilities to help you fine-tune your sound. These adjustments can be made via the front-panel buttons or (in some cases) with a connected computer. Features like this eliminate the need for racks full of outboard signal processors.

Questions?

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