Pro audio power amplifiers buying guide
How to choose a power amplifier that's right for you
Dominic J. DeVito has been a member of the Crutchfield A/V writing squad since 2006. He was born and raised in Staunton, Virginia, and attended the University of Notre Dame, where he earned a degree in chemical engineering. During that time he developed a passionate obsession for experiencing music, both live and recorded, which he parlayed into a 15-year stint in record retail (much to the chagrin of his very patient parents) and a long-running tenure as a rock DJ at WTJU. His expositions can be found in back issues of Plan 9's 9X Magazine as well as Schools That Rock: The Rolling Stone College Guide. He's been to more concerts than he can remember.
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The power amplifier receives the audio signal from the mixing board or signal processor and magnifies it, giving it the power it needs to drive your speakers and entertain your audience. In this article, we'll discuss the things you need to know when choosing an amplifier for your pro audio system.
The amplifier is the heart of a pro audio sound system, providing the power that speakers need to create music.
What to look for in an amplifier
Most amplifiers have similar features. The differences come in the amount power it produces, the number of channels it has, the types of connections it offers, how the controls are set up, and whether it has any built-in processing or effects.
How much power do I need?
This can feel like a bit of a minefield since amplifiers and speakers will often come with multiple power ratings. For best results, pay attention to the amplifier's RMS power rating. That's a measure of how much power it puts out consistently.
Speakers, on the other hand, tend to list a program power rating for their handling capabilities. This is the amount of power a speaker needs in real-world situations.
Compare the amplifier's RMS power rating (how much power it puts out consistently) to the speakers' program power handling (how much power the speakers need to sound good).
- As a general rule, the amplifier should be able to provide up to twice the speaker’s program power rating.
- Also important to note is that it’s generally better to overpower a speaker a little than to underpower it. It is much easier to damage a speaker by giving it too little power than by giving too much.
For example, for a speaker with a program rating of 200 watts, you want an amplifier that'll deliver between 200-400 watts RMS. The closer you get to the higher number, the better the speaker will sound.
Other power ratings
The peak power rating gives you an idea of the maximum, instantaneous short-term power an amplifier can deliver or that a speaker can handle, typically for intervals lasting less than a second. It’s good to know, but not very helpful when planning a system.
Amplifier power and resistance
The amount of power an amplifier generates depends on the impedance (or resistance) load of the speakers it's driving. It'll put out different amounts of power to different impedance loads. So you might see something like an amp that’s rated 1,000 watts at 8 ohms, but 1,500 watts at 4 ohms.
Problems arise when the amp's output meets very little resistance (low impedance) and it tries to put out more power than it was designed to produce. This leads to the amplifier overheating and shutting down — not good in the middle of a performance. In most cases, power amplifiers are rated to work best against 4- or 8-ohm loads.
Just remember that the amplifier you choose must be able to provide an adequate amount of power to your speakers at the impedance they present to the amp's output. For example, connecting two 8-ohm speakers to one channel presents the amp with a 4-ohm load. Make sure the amplifier can handle that load before adding the second speaker. In this case, there's no problem.
Using our example above, our amp puts out 1,500 watts at 4 ohms. This power is divided among the two speakers, so each will get 750 watts. If that's enough power for the speakers, then this amp will be a good match for them.
How many channels do I need?
Power amplifiers are categorized by the number of channels they offer: mono (single-channel), stereo (2-channel), and multi-channel (usually 4). The vast majority of amps are 2-channel. They're the most popular because of their flexibility. You can use one as a 2-channel stereo amp, two single-channel amps, or a more powerful, single-output amp.
How many channels you need depends on how many speakers you need to power. A simple system with two speakers (left and right) is perfect for a 2-channel amp. If the amp has enough power, you can add more speakers on each channel, so long as the impedance load we talked about doesn't drop too low.
And that leads us into...
Stereo, parallel, and bridged modes
Most amplifiers can be operated in stereo or bridged modes, which determine how they’ll handle the signals. Some amplifiers also offer a parallel mode (sometimes referred to as parallel mono).
Stereo is the default mode for 2-channel amplifiers, and is used for powering a pair of speakers, one right and one left. In stereo mode, each channel is receiving a signal independent of the other. A 2-channel amp in stereo mode can also be considered the equivalent of two mono amplifiers — for example, one channel can power PA speakers while the other powers the stage monitors.
Parallel mode allows a single input signal to be sent to both channels, and the second input channel is not used at all. This can be used to route a mono input such as a microphone or mono output from a mixer into both channels of an amplifier. This mode can be useful when powering an odd number of speakers. If you want to use three speakers, using your amp in parallel mode would send the same signal to all three speakers.
Bridged mode combines the power of both channels, sending more power to a single output. In many cases the output power may be more than double the amplifier’s normal power rating. This mode is typically used to send a signal to a subwooferor. In most cases, the impedance of the speaker should be double the minimum impedance of the amplifier in stereo mode. It’s important to note that the manufacturer’s instructions should be followed when bridging an amplifier, which may involve using a “jumper” cable or connection between the output channels on the back of the amplifier.
Headroom is a term that gets bandied about quite a bit when discussing power amplifiers, and it's the reason everyone suggests having more than just enough power to satisfy the RMS requirements of your speakers. Having that extra power gives you several benefits:
- It lets you turn up the amp without distorting the music on those occasions when you need to play louder than usual, like when your band is playing a larger venue than normal.
- It allows you to play music with a wide dynamic range (music that has quiet parts and loud parts) and have those loud parts sound just as good as the quiet parts.
- When that bass hit drops or that high note screams, the headroom in your system keeps them both clear and free of distortion.
Having lots of headroom helps you avoid...
Clipping of an amplifier occurs when you try to get a larger output signal out of an amplifier than it was designed to provide. This is a fancy way of saying that your amp doesn't have enough headroom. It usually happens when an amplifier is already performing near maximum power output and the volume is increased sharply. The tops (and sometimes the bottoms) of the signal waveform are cut off (or “clipped”), resulting in a highly compressed signal that results in distortion, which can damage your speakers.
Power amplifiers have indicator lights to warn you when the signal is approaching the clipping point. These tiny lights have saved many a speaker.
Heat and distortion
So what happens when you don’t stay within the recommended parameters? Typically you’ll introduce heat and distortion into the signal path, and those are both enemies of good sound. Heat can build up within your amplifier’s internal circuits, causing a shutdown or — even worse — a meltdown. Distortion is caused by heat and irregular sound waves, and it’s bad for both your speakers and your listeners’ ears.
Input and output connections
Power amplifiers typically offer XLR and 1/4" inputs for your incoming audio sources. You’ll need to keep an eye on your amplifier’s output level when switching between sources using different inputs because XLR inputs are much lower in voltage than 1/4" jacks. If you can help it, avoid using two different kinds of connectors when plugging into a power amplifier.
This output panel has both locking outputs and binding posts
Amp outputs will usually be a combination of Neutrik SpeakON, banana/binding posts, and 1/4" outputs. SpeakON plugs offer the added security of a twist-and-lock connection, while banana/binding plugs are more versatile. Banana/binding plugs can also accept bare wires if the need arises. 1/4" plugs connect to cables that can be confused with instrument or patch cables, which is less than ideal because instrument or patch cables are not designed to carry amplified audio signals. 1/4" plugs can also short out if they are touched to metal. Be sure to check which types connections your speakers accept when shopping for an amplifier and cables.
Most amplifiers feature very basic controls. 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 or when thermal compression or other signs of overheating have begun. In many cases, there will also be a light signaling the amp’s entry into protection mode, which is automatic shutdown to protect the amp’s interior circuitry. Some amps may have a small LCD screen that provides status updates or can be used to set crossovers, amp mode, and limiter configurations. Switches for stereo, parallel, and bridged modes will often be found on the rear of the amplifier.
The LCD display on this Crown amplifier helps you dial in the settings
Filters and crossovers
Some amplifiers offer a high-pass filter, also known as a low-cut filter. This lets you reduce the output of a speaker below a certain frequency (typically between 20 and 150 Hz). 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, and is also helpful when you have a separate subwoofer handling the bass in your PA system.
Amplifiers can also offer more sophisticated crossovers, which are filters that divide the incoming signal into multiple frequency ranges. These are especially handy for multi-speaker setups or for systems which involve a subwoofer. With a crossover network, you can dictate which frequencies go to which speakers, maximizing the efficiency of your amplifier’s power by ensuring it won’t work harder to power speakers that have a difficulty reproducing certain high or low frequencies.
Limiters are protection circuits that can help keep your amplifier from clipping and to prevent distortion in the sound. A limiter lets you set a maximum level setting and prevents a signal from going above it. Limiters help prevent distortion caused by an overdriven signal, a dropped microphone, or a short in an input jack.
Getting the most out of your power amplifier
Connections and cabling
Improper connections can result in unwanted noise or interference, so be sure to follow your amplifier’s owner’s manual regarding plugs and cables. This is especially true regarding balanced and unbalanced connectors — see our guide to choosing the right cables.
- Most amplifier manufacturers recommend using balanced connections throughout the signal path, which will insure the least amount of outside interference.
- Whenever possible, avoid power strips and try to plug your power amplifiers directly into an AC outlet or power conditioner.
- A good rule of thumb for speaker cable is the shorter, the better. Use the same length of wire for each speaker connection, even if one speaker is closer to the amplifier.
Avoid overheating and distortion
Once everything in your sound rig is properly connected, the amplifier should be the last thing you turn on, and the first thing you turn off when it’s time to shut down. When trying to find the best point to set the levels (gain) for your amplifier, make sure you listen closely for distortion and pay attention to the amp signal levels for clipping. If you see indications of clipping or hear the sound distorting, dial the levels back.
When attempting to send audio through multiple speakers at the same time, be sure to consider the advantages and disadvantages of using multiple amplifiers, or even a multi-channel amplifier. Look at the impedance load implications of wiring multiple speakers to an individual channel, and determine whether one amplifier is up to the task. If you want more than two speakers in your system, it's often best to add another amplifier. And subwoofers almost always need their own amplifier.
When dealing with a subwoofer, you’ll find that using your amp's crossovers can make a big difference in the sound. The crossover lets you send the low frequencies to the subwoofer and the middle and high frequencies to the main speakers. External (or "active") crossover networks are great for single or multi-amp systems because they are efficient and have a wide dynamic range. In the signal chain, external crossovers are usually found between the mixer and the power amplifier. You can separate signals into high-, low-, and sometimes even mid-frequency bands and send to the proper speaker for optimum response.
The right amplifier for you
Matching an amplifier to your speakers is an important step, but it's just one of several on the road to great sound. Our experts are here to help walk you through the gear, so don't hesitate to give us a call if you have any questions. We can assist you in choosing the gear you need for a great sounding system.
When you purchase your PA gear from Crutchfield, you get access to our free lifetime tech support. We can advise you on the best ways to set up your PA system so you can enjoy memorable live events at the club, church, auditorium, or wherever you like.