What Kind of Amp Do I Need for a PA System?

Selecting the right PA Amplifier

Selecting the correct PA amplifier is paramount in crafting an audio system that not only meets but exceeds expectations. The journey to finding the perfect amplifier involves understanding various technical aspects, from power ratings and amplifier classes to the digital vs. analogue debate. Let's break down these concepts to ensure your PA system delivers outstanding sound quality for any event or venue.

The Basics of Power Ratings

The power rating of an amplifier or speaker is a critical factor in determining its suitability for your needs. It's essential to distinguish between RMS, Peak Power, and Max Power, which are the most common ratings used. You can read about this in more depth in our ‘guide to matching speakers and amplifiers’, or in ‘speaker power ratings explained’, but we will cover the basics here too.

• RMS Power

  This represents the continuous electrical power the amplifier can deliver without distortion. It's the most reliable indicator of an amplifier's performance under a consistent load and operation time.

• Max Power

  The maximum power an amplifier can produce, taken as a signal measurement over a set period until the amplifier falls into heavy distortion or fails completely. Its usually two times the stated RMS figure.

• Peak Power

  This is the maximum power an amplifier can produce in ultra-short bursts. It measured in the same way as Max but is even more extreme. Peak is generally four times the RMS value, though some manufacturers go as far as six times the RMS. While it might seem an impressive number on paper, it's irrelevant as a real-world figure concerning sound quality and consistency.

For a balanced and clear audio output, your amplifier's RMS power should match or ideally just slightly exceed the speaker's RMS power handling capability. This matching ensures that your system can handle dynamic audio content without distortion, providing a more immersive listening experience.

Matching Amplifiers to Speakers

Properly matching your amplifier to your speakers is about more than just power; it's about synergy. Here are a few guidelines:

• Power Matching

  Aim for an amplifier that can deliver 75% to 150% of the speaker's RMS power rating. This range ensures that the amplifier can drive the speakers effectively without the risk of damage.

• Impedance Matching

  Amplifiers and speakers have rated impedances (measured in ohms). This is a measurement of their resistance to the flow of an electrical signal. Ensuring these values are compatible is essential to maximise efficiency and sound quality. An amplifier will have an impedance range (usually 4-Ohm to 16-Ohm), and the load presented to it by any attached speakers must fall within that range to avoid serious damage.

  It's important to understand when connecting multiple speakers to an amplifier that their impedance is halved in value - so two 8-Ohm speakers wired in the standard fashion will present a 4-Ohm load to the amplifier's output transformer. You can run into impedance loading problems quickly when wiring larger speaker installations so it's a really important factor to understand.

Mono or Stereo Amplifiers

It's worth noting that a stereo power amplifier is two mono amps sharing a box. They are separate entities. In a hi-fi style amplifier, they are combined at the output stage and sent through a master volume control, giving you a simple level adjustment of the left and right channels together.

In a PA amplifier, however, the channels are left uncombined as separate mono channels, and each will have its own volume control on the front of the amplifier. They will also usually have their speaker outputs labelled as channel A and channel B, rather than left and right.

Many will also include a ‘bridged’ mode, which combines both channels into a single mono output, utilising the full power available, which is ideal for subwoofers and large audio installations in clubs.

Amplifier Classes

Amplifier classes signify the internal operation and efficiency of the amplifier, each with its unique characteristics. PA amplifiers have been Class A/B for decades as it was always the best option for the most energy-efficient operation. Most DJ amps and power amplifiers still use this configuration as it is a tried and tested design that gives reliable results, with its main downside being excess heat production.

Many newer power amplifiers can be found using Class D amplification, which use an extremely efficient circuit design of current control and signal pumping that allow them to produce a significant power output from slimline electronics. They do away with the bulky transformers and required cooling of traditional Mosfet amps, offering huge energy savings over the older designs.

• Class A

  These amplifiers are known for their exceptional audio quality but are extremely inefficient due to their circuit design. They generate far more heat than any other amp so are only seen these days in boutique level guitar or Hi-Fii amplifiers.

• Class A/B

  Amplifiers balance sound quality and efficiency, making them the most popular choice for both Hi-Fi and PA amplifiers and active speakers since the 1980’s. They have gone by many names, with ‘solid-state’ or ‘Mosfet’ being the most common, but its worth noting that the A/B design is also employed in valve-driven guitar amplifiers. The class of an amplifier represents the circuit design, not the components used.

• Class D

  These are highly efficient electronic pulse amplifiers that generate near-zero heat and incredible output power from slimline designs. They have been critiqued for slightly lower audio fidelity in high-end audio applications though this is changing all the time as the technology improves. While PA amplifiers and DJ amplifiers have stuck mostly to the A/B design, Class D is now the standard for active speakers due to its significant weight and heat benefits. It’s worth noting that the ‘D’ does not stand for digital as many seem to believe, and these are still analogue amplifiers.

Amplifier Inputs and Outputs

Versatility in connectivity is crucial for accommodating various audio sources and speaker setups. Look for amplifiers with a comprehensive range of input and output options, including:

• Balanced Inputs

  XLR or 6.35mm TRS jacks allow for noise-free signal transmission over long distances.

• Unbalanced Inputs

  These are highly efficient electronic pulse amplifiers that generate near-zero heat and incredible output power from slimline designs. They have been critiqued for slightly lower audio fidelity in high-end audio applications though this is changing all the time as the technology improves. While PA amplifiers and DJ amplifiers have stuck mostly to the A/B design, Class D is now the standard for active speakers due to its significant weight and heat benefits. It’s worth noting that the ‘D’ does not stand for digital as many seem to believe, and these are still analogue amplifiers.

• Speaker Outputs

  PA amps and DJ amplifiers will generally feature spring terminals on lower-cost models, with 4mm bare-wire/binding posts as a common standard, or 6.35mm jack sockets. Higher power amplifiers will offer NL2/NL4 locking Speakon sockets as a professional option.

DSP (Digital Signal Processing)

Some modern amplifiers come equipped with DSP, allowing for precise control over the audio signal. Features to consider include:

• Equalization (EQ)

  Used for adjusting the frequency response to suit the room acoustics or speaker characteristics. These will either be manually controlled or will be a set of selectable presets to suit different situations.

• Crossovers

  Used to direct specific frequency ranges to the appropriate speakers, handy for systems with passive subwoofers. Some amplifiers allow the outputs to be adjusted for frequency cut-off, so the connected speaker will only receive bass to a certain point, or will only get midrange etc.

• Limiters and Compression

  Many power amplifiers include anti-clipping to protect against damage from an over-driven signal, and some will include a compressor-limiter which will remove unwanted spikes in the audio signal, ensuring consistent sound quality and protecting the speakers from unwanted burst frequencies which can take out tweeters.

Amplifier Setup and Maintenance

• Rack Mounting

  Securely mount your amplifier in a rack or case if possible to protect it from physical damage and improve ventilation. Amplifiers may look tough, but they contain fairly delicate electronics so should always be treated as such.

• Ventilation

  Ensure adequate airflow around the amplifier to prevent overheating. Avoid stacking equipment directly on top of the amplifier. Class A/B amps will often be fan-cooled and need good airflow to allow them to operate correctly.

• Cable Management

  Use quality cables and organize them to prevent interference and accidental disconnections. Be especially mindful when using bare-wire connections to speakers as stray strands of copper can easily short across terminals. Power amplifiers require a load against the output to operate, and sudden disconnection of a speaker at high volume can cause severe damage to the amp's output stage.

Maintenance and Troubleshooting

• Dust and Debris

  Keep the amplifier clean from dust and debris, which can obstruct cooling and lead to overheating. Fan-cooled amps are sucking in air, and filth at the same time. Clean them regularly and ensure none of the ventilation is blocked.

• Connections

  Regularly check and secure all connections to prevent issues related to poor connectivity. This is especially important for mobile work and touring setups where equipment is constantly connected and disconnected, as it puts a strain on plugs and sockets which can lead to intermittent issues.

• Troubleshooting

  Familiarise yourself with common issues such as overheating, distortion, and channel imbalance. Learn about signal feeds and the difference between volume and gain when using a mixer. Knowing how to identify and resolve these issues can save time and prevent damage.

By understanding these critical aspects of PA amplifiers, you are well-equipped to select and maintain an amplifier that will serve your PA system's needs reliably and effectively. Remember, the right amplifier not only powers your speakers but also enhances the overall sound quality, ensuring every performance is heard loud and clear.