How to choose the correct power amplifier for your PA system - an easy guide

Author: Brent  Date Posted:5 February 2021 

Selecting the correct power amplifier for your application and particular speakers can sometimes be difficult for people without a background in pro audio. 

Here we're going to help!

When shopping for a power amp, there are a few factors we need to consider in order to select the right one.

  1. How many speakers you need to power
  2. If it's a mono or stereo setup
  3. The power handling capacity of your speakers
  4. The nominal impedance of the load presented to the amplifier (this is measured in ohms, and ties in with point 1)

So how many speakers do you need to power?

What we need to do first is find out how many speakers you need to power in total. Most setups involve a speaker for the left side of the pa, and one for the right. Pretty basic! 

So if it's one speaker you need to power per side - easy. 2 per side, usually not an issue. More than 2 per side? That's where we have to start to do the maths.

We'll come back to that once we work out the configuration of the speakers.

Most applications call for a stereo configuration, which means you'll need at minimum 2 channels of amplifiication - one for left, the other for the right side of the PA. Stereo sources contain a different signal for each side, and cannot be combined to mono, amplified and split back to stereo.

Most amps are capable of taking a mono signal and sending it to both the left and right outputs, but this is and always will be mono. To get a stereo sound you need 2 channels of amplification to keep left and right independent of each other.

Now for the power rating of the speakers.

You may of noticed everything is rated in "watts". This can be a bit of a minefield, as the cheaper speakers usually quote their "peak" wattage. This is basically a useless number, quoted by manufacturers to help sell the speakers.

Don't expect that $500 speaker that has a big sticker saying "1000 Watts" across the front to actually be able to handle that much power. It might do OK for a second, and then it's going to be off to be repaired. 

As you get into the more prestigious (expensive) brands, you may notice the power rating given is lower than the cheaper ones - this is simply because the wattage on the more expensive speakers are usually given in "RMS".

Note: RMS (root mean square) isn't actually the best way to measure the power handling of speakers, as the power handling of any speaker will vary depending on the frequency the speaker is producing. But, we'll work with what we're given, and in the case of speakers it's RMS. We'll get into the topic in more depth another time.

If your speakers are of the less expensive variety and claim to be 2000 watts each, I'm confident in saying that is it's peak power. Don't know the RMS value? 

An easy way to get a fairly accurate answer is to simply divide the peak power rating by 4.

2000 watts (peak) divided by 4 = 500 watts RMS.

Note: If your power rating is quoted as "program", instead of dividing by 4, divide by 2 to get the RMS value. (1000 watts program would equal 500 watts RMS).

Now we need to find the nominal impedance of the speakers.

Note: The resistance of a speaker will vary (just like it's RMS value) depending on the frequency and there is just no single number. This is why we say "nominal impedance" - because that number is an average (ish) measurement of the speaker's resistance across it's frequency range.

Each speaker will have a nominal impedance (measured in ohms) stated on the rear. If not, just check the manual or ask google. Impedance is the measurement of the speaker's resistance to current, which equals load on the amplifier.

The ampifier is designed to be presented with this load, and if it's too low, you'll end up with a amp that's gone into protect mode, or worse, cooked and needing repair. This is because if the load is too low, there is less resistance to current - therefore more current draw on the amplifier. Too much current draw = unhappy, very hot amplifier - since it's now operating out of it's designed parameters.

Scenario number 1: So let's say you have two speakers rated at 500 watts RMS each, one speaker for the left and another for the right side of the PA. The speakers are both rated 8 ohms each. So you need a stereo amplifier rated to output 500 watts at 8 ohms per channel right? Wrong.

Now, this may make you think I'm telling you to do something that will hurt your speakers. Don't worry, I'm a professional.

To drive your speakers correctly and acheive the best performance from them, you should choose an amplifier that will output twice the speakers stated power handling (in RMS).

This is an average number and will vary depending on the material the speakers are reproducing, and it should be greater if you're powering a subwoofer, but in general it's a good rule of thumb.

The reason we need to provide so much power to the speaker is because we need to allow for peaks - music is dynamic and we should make sure we can reproduce the loudest parts of the music correctly and without clipping anything.

Remember we quickly spoke about peak power?

The speakers are designed to handle peaks momentarily and reproduce the material as it should be - dynamic. We refer to this extra power as "headroom", and if you want your PA system to sound good and operate correctly, you need it.

Most people ask: "Won't I blow up my speakers if the amplifier is rated at double the speakers capacity?" 

Nope. Most speakers are damaged from being underpowered.

How?

If you don't have headroom in your system, you'll find yourself pushing it hard to get the desired volume and may end up "clipping" the console or amplifier. Clipping is basically a distorted waveform, and you really want to avoid it. Clipping will lead to amplification of this distorted waveform and will be passed along to your speakers, causing damage. Having this extra power in your amps reduces the chance of accidentally producing a clipped signal, which usually happens in the peaks of the source material. 

Note: Correctly establishing  proper gain structure is vital to the performance and longevity of your equipment. We will discuss this in another article, so keep an eye out for that. It is simply the most important part of deploying any PA system and is crucial to success.

Now that's out of the way, let's revisit scenario 1. We have a single speaker rated to handle 500 watts RMS and with a nominal impedance of 8 ohms per side. You'll be needing a stereo amplifier that can produce around 1000 watts continuous per channel at 8 ohms to drive them correctly.

So let's look at powering multiple speakers per side. We'll stick with the most common method of connecting multiple speakers - connecting them in parallel.

Yes, you can connect them in series, but we'll look at that another time.

Scenario number 2: You have 4 of the same speakers - 500 watts RMS each with a nominal load of 8 ohms each. You want to use 2 speakers per side for FOH. 

The simplest way to connect them is to simply "daisy chain" them, usually using the link output on the first speaker and connecting that to the input of the second.

When wiring speakers in parallel, every speaker you add reduces the load on the amplifier. Since your speakers are rated at 8 ohms each, when you connect them both together the resistance halves and in turn you cut the load presented to the amplifier in half. (2 x 8 ohm speakers = 4 ohms). 

Calculating the impedance of two speakers in parallel is easy, simply take the speaker's nominal impedance and divide it by the number of speakers you've connected together. 3 x 8 ohm speakers in parallel = 2.66 ohms. 4 x 8 ohm speakers in parallel = 2 ohms, and so on. 

So in scenario number 2, the two speakers per side equal 1000 watts RMS total with a nominal impedance of 4 ohms. This scenario calls for a stereo amplifier rated to produce 2000 watts per channel at 4 ohms.

The amp from scenario number 1 would in most cases also be fine for scenario number 2 - since the resistance is halved in scenario 2, the amplifier has half the load and will usually produce around double the power. 

Now if you need to go lower than 4 ohms, you need to make sure the amplifier is capable of doing it. The majority of power amplifiers found for sale will need a minimum of 4 ohms per channel load to operate without being damaged. There are amps that can run below 4 ohms, but these are more expensive (usually by a lot).

Click here to see our range of power amplifiers.

As always, we are more than happy to help should you need it. All our staff are the best in their fields and are happy to take as much time needed to make sure you get the correct amplifier for your situation.

Remember, our staff are engineers - not sales people. Give us a call on 07 5628 2200 if you need advice!


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