Power Amp Buying Guide

Table of Contents

Passive speakers or powered?
Power amp specifications to consider
What’s in a watt?
Amplifier modes
Channels
What does Class D mean anyway?
The ins and outs
Input types
Output types
Built-in DSP
Headroom
Matching your amp and speakers
The impedance factor
Still not sure?

No matter the style of music or the size of the venue, wherever amplified music happens, speakers and power amplifiers do the heavy lifting, converting electronic signals back into sound waves. And it’s the power amp that has the toughest job: it has to take the low-level output of instruments, mics and other sources and up the voltage significantly to drive PA speakers and stage monitors.

In this guide, we’ll help you simplify your search for a power amp by covering the critical specs and features you’ll encounter and explain their meaning in real-world terms. We’ll also cover matching power amps with your speakers as well as the connection options.

Improve your live-sound IQ with our PA System Buying Guide

Passive speakers or powered?

When it comes to gear like active (powered) PA speakers and monitors as well as guitar combo amps, the speaker(s) and power amp are all housed in one unit. Those integrated designs are certainly a convenient option. But in PA systems, one or more standalone power amps used to drive passive (unpowered) speakers and monitors make sense if you want to grow your sound system over time. Also, if either the speaker or onboard power amp in a powered speaker system goes bad, you have to replace the entire unit—a costly proposition.

If a power amp is a good option for your rig, you’ll find plenty of models to choose from in the Musician’s Friend selection of live sound power amplifiers.

Power amp specifications to consider

In evaluating power amps, specifications can help you narrow down your selection. But as we note below, manufacturers calculate those specs in different ways. Understanding what the numbers mean and how they’re calculated will help you shop smart.

What’s in a watt?

The power amplifier’s single most important spec is how much power it puts out. This output is measured in watts, the standard unit of electric power, and output among power amps varies widely.

When it comes to determining whether an amplifier is powerful enough for your needs, it’s important to understand that amp manufacturers use different ways to measure wattage. The two most common are:

• Peak watts refers to the absolute maximum amount of power an amplifier can momentarily put out. As such, it’s not a useful, real-world specification.
• Continuous or RMS watts are calculated using a continuous sine wave or other signal to measure the Root Mean Square (RMS) voltage at the point of stated distortion. It can loosely be thought of as the “average” watts the amp can put out at a maximum. Audio engineers feel the RMS rating offers a more realistic indication of an amp’s performance.

When comparing amplifier output, be sure the specs are comparable. Comparing one amp’s continuous/RMS output with another’s peak output is like comparing apples with oranges. In some cases manufacturers don’t specify what type of wattage is being used in their marketing materials. If so, the user’s manual usually discloses how wattage is measured. You may also find this information on the manufacturer’s website.

Another power-related rating you’ll run into is program watts, which is usually used in reference to the power handling capability of speakers. As with amps, a conservative RMS power handling rating is more useful. See the section titled Matching your amp and speakers below for more details.

The amount of power an amp puts out is impacted by the impedance (resistance) of the speaker loads attached to it. For example, an amplifier might produce 1,100 watts with an 8-ohm load, but 1,800 watts at 4 ohms. We’ll address impedance further in a bit.

Consider how much power you’ll actually need for the venues you plan to play as well as the genre of music you play. Obviously, a folk duo that sings and plays guitars needs far less power than, say, a death-metal quartet.

Coming up with wattage requirements involves a lot of variables such as the venue’s acoustics, the size of the crowd, inside versus outside performance spaces and a many more factors. That said, we offer a very rough guide to get you started. The guidelines here are given in continuous or RMS watts:

• 25 to 250 watts for folk music in a small venue like a coffee shop or for practicing at home
• 250 to 750 watts for pop music in medium-sized venues such as jazz clubs or auditoriums
• 1,000 to 3,000 watts for rock music in medium-sized venues like concert halls or local festivals
• 4,000 to 15,000 watts for a full-scale arena rock or metal performance

Clearly, this is a wide range of power levels. The good news is there are models across the spectrum to match any need. For instance, delivering 185 watts per channel at 8 ohms, the QSC RMX850a offers pro performance at a modest price. On the other end of the power range, the Crown XTi 6002 can generate a massive 6000 watts in bridged mono mode at 4 ohms. The Peak X Plus limiting system provides speaker protection plus complete threshold, attack, and release control.

Ultimately, the amp you choose should be able to handle the speaker loads you plan to connect to and generate enough power to drive those speakers to volumes that will fill the spaces in which you play. (But read the section titled Headroom below—you need enough reserve power to handle momentary peaks without serious distortion.)

Amplifier modes

Browsing through different power amp models, you will see that many amps have per-channel power ratings. The channels on these amplifiers can be operated in a number of different configurations, depending on your needs.

• In stereo mode, two sources such as the left and right outputs on a mixer are connected to the amp, each using one of the channels. Those channels are then connected to the speakers through their output connections creating a stereo soundstage.
• In parallel mode, a single input source is connected to both of the amplifier’s channels. This sends an identical signal to each of the speakers through the outputs.
• In bridge mode, you can turn a stereo amp into a more powerful mono amp that harnesses the power of both channels.

Amplifier specs usually indicate output running in both stereo and bridged modes. When running an amp in bridged mode, be sure to observe any warnings in the manufacturer’s manual to avoid damaging your gear.

Channels

When you consider how many channels you need, your primary concerns is the number of speaker cabinets you want to connect and how you want to wire them.

Most power amps have two channels, allowing you to connect two speaker cabinets either in mono or stereo mode.

Some power amplifiers have additional channels, typically up to a total of four but sometimes as many as eight. QSC, for instance, offers a number of four-channel options, such as the PLD4.5 Multi-Channel System Processing Amplifier, and extend its designs to eight-channel amps in models like the CX168 8-CH Low-Z Power Amplifier. These units are designed for sophisticated, multi-channel sound systems.

Multi-channel models give you the convenience of connecting additional speaker cabinets to a single amplifier. However, these designs will usually come at a higher cost than two-channel amps with similar output due to their greater complexity and versatility.

What does Class D mean anyway?

Power amplifiers are classified according to their circuit designs. You’ll see amps designated as Class A, AB, C, D, etc. The latest generation of portable sound system amplifiers are mostly Class D designs that deliver a lot of power relative to their weight and size. Due to their efficiency, they run much cooler and offer greater reliability than other amp types.

The ins and outs

In shopping power amps, pay close attention to the connections on the back panel. Count up all your sources and speaker cabinets to determine if the amp has sufficient connectors of the right types.

Input types

At a minimum, the most basic PA amps are equipped with XLR connectors. More often, multiple connection options are available, including ¼ inch, XLR, TRS and sometimes RCA inputs. The Crown XTi 4002 Power Amplifier gives you substantial sound-shaping options using its 8 parametric filters per channel with adjustable Q.

Note that balanced connections such as an XLR cable are usually best when long cable runs are involved. However, in DJ systems, home audio, and some live sound setups where the cable length is shorter, RCA connectors may be fine.

Output types

Here are the five most common output connector types you’ll run into when shopping for a power amp:

1. Binding posts - As is common in older home audio systems, bare speaker wire ends are wrapped around binding posts that hold the connection in place. This is a sturdy and reliable connection, but it takes time to set up and isn’t a good option for gigging musicians who connect and disconnect gear frequently.
2. Banana plugs - Available in both male and female versions, they provide a fast and secure connection. Some combine the positive and negative conductors in one unit.
3. Speakon - A sturdy and secure connection type that plugs in quickly. Originally introduced as a proprietary feature of Neutrik, it is now used across the industry. Adapters are available for speaker without Speakon connectors.
4. XLR - XLR outputs are balanced, meaning they reduce or eliminate interference noise. They are easy to plug in and lock securely in place.
5. ¼ inch - A sturdy and straightforward connection type, particularly for low-wattage applications. ¼ inch plugs may be less reliable in high-wattage situations.

You’ll find loads of additional info about inputs, outputs, connectors and cables in the Musician’s Friend Audio Cable Buying Guide.

Built-in DSP

DSP, or digital signal processing, is available on some power amps. It converts the input analog signal to a digital bitstream, allowing you to control and shape the signal in several ways. Some DSP processing you’ll find on amps includes:

• Limiting - A means to control the peaks of strong input signals, usually to prevent overloading the amp or damaging the speakers.
• Filtering - Some power amps offer filtering features such as low-, high-, or band-pass filters to emphasize certain frequencies and/or prevent damage caused by very low-frequency signals.
• Crossover - A circuit that divides the output signal into frequency bands for delivery to specific driver types such as woofers, midrange drivers, and tweeters. (The internal passive crossovers in multispeaker PA cabinets can usually be overridden when using the amp’s DSP-based crossover.)
• Compression - A method of limiting the dynamic range of an audio signal, for audio enhancement or to avoid distortion.

With 32-bit DSP and a slate of EQ presets, the Peavey IPR2 DSP 5000 Power Amp is easy to set up and control. It excels at handling large speaker loads and delivers up to 1500 watts at 4 ohms.

Note that these signal processing tasks can also be handled by specialized standalone units, and many professional audio engineers prefer using them.

Headroom

In live music performance, levels vary constantly. Momentary musical peaks can put a lot of demand on your PA. Be sure your amplifier and speakers can handle these momentary peaks without distorting. To do this, the amp needs enough reserve power or “headroom” as audio engineers refer to it. We’ll discuss this in a little more depth next.

Matching your amp and speakers

Matching power between your amp and speakers can be a little baffling. Don Boomer of Peavey offers some general guidelines in his excellent article on the subject. For the best performance, he advises that your speakers’ “program” watts rating be the same as the amp’s RMS/continuous output wattage rating. But he hastens to add that does not grant you license to mic a kick drum without limiting its huge low-frequency output.

As Boomer explains, clipping—the flattening of waveforms that produces ugly-sounding distortion—is what really destroys speakers. A high pass filter in your signal chain is essential to protect your amp and speakers from meltdowns caused by their attempts to reproduce bass and drum frequencies beyond their intended range. Keep an eye on your amp’s clipping indicators and believe them. If you’re seeing a flickering red light on anything other than the kick drum beat, it’s likely your speakers are being pushed beyond their limits.

The impedance factor

Ohms measure the electrical resistance of a circuit, and PA amps are typically designed to operate with speakers having 4, 8, or 16-ohm ratings. You’ll usually get the best performance and minimize risk of gear damage by exactly matching your amp’s’ impedance with the speaker load you connect it to.

If the the total impedance of your speakers is too high for the amp, the power delivered to them will be insufficient. If the speaker load is too low on the other hand, the amp may overload your speakers damaging them and itself in the process.

How speakers are connected to the amp effects their impedance. There are basically two ways of connecting speakers to your amp:

Parallel: Each speaker cabinet is independently connected to the amp via its own circuit. This is is the simplest and safest kind of connection, especially if your speakers vary in their impedance ratings.

Series: Multiple speaker cabinets share the same connection from the amp. (Think Christmas tree lights.)

Without getting into the theory, it’s generally best to choose speaker cabinets that have the same impedance rating and to connect them in parallel to your amplifier.

Still not sure?

After reading this guide, if you’re still not sure which power amp is right for you, we invite you call to one of our expert Gear Heads at 877-880-5907.