PRO AUDIO
Chris

Chris
July 16, 2012

The Ultimate Amp Matching Guide

THIS INFORMATION IS BASED ON MANUFACTURER SPECIFICATIONS AND KNOWLEDGE FROM PROFESSIONAL SOUND ENGINEERS. ALWAYS CONSULT THE EQUIPMENT’S MANUAL OR CONTACT THE MANUFACTURER TO MAINTAIN THE INTEGRITY OF YOUR EQUIPMENT.

If you don’t have much experience setting up powered amplifiers with passive PA speakers then you have come to the right place. First I’m going to explain how to setup a traditional PA setup with one stereo amplifier and two speakers. Next I will explain how to add speakers to that system. Finally, I will explain bridge-mono, when you use it, and how you set it up.

Two Speaker Setup:
There are two simple rules of thumb you must follow when you are choosing an amp for your PA speaker setup.

1. Get an amp that’s 50% more powerful than your speakers.
2. Match your speakers’ ohm rating to your amps ohm rating

Let me take a moment to explain watts and ohms.

Watts are a measurement of electrical power. The more watts you have, the more electrical power you have.

Ohms are a measurement of electrical resistance. The more ohms something has, the more it will resist the electrical current. For example, the rubber casing around a wire has a lot of ohms. It is very resistant to electrical current. However, the copper wiring inside the rubber has very few ohms. It conducts electrical currents very well. For our purposes today, let’s also say that impedance is the same thing as resistance. Both of these are measured in ohms.

If I want to invest in a PA system I generally look for speakers first. If I’m not sure how loud my speakers need to be I usually go with this rough formula: 10 watts per person indoors or 20 watts per person outdoors. So if I’m playing at an indoor venue that can hold 100 people, I would want a system that could push 1000 watts. If it was an outdoor venue I’d want a 2000 watt system. Once I’ve got the right speakers, I just have to find an amp to match. There are two specs that you must get from your speakers before you go on the hunt for the ideal amplifier.

1. Nominal Impedance
2. Power capacity
Getting the nominal impedance is straightforward. But there are a couple things to note when searching for the power capacity. First, your spec sheet might call it something else. Continuous power capacity is the same thing as RMS power handling and program watts. But it is NOT the same thing as peak power capacity. If you play your speakers at peak power capacity you will destroy them. Hypothetically, the JBL JRX125 in the spec sheet above could survive an audio spike up to 2000 watts, but I would never put that rating to the test intentionally.

Now let’s say you’ve decided that the JBL JRX125 is the perfect speaker for you. The nominal impedance is 4 ohms and the power capacity is 500 watts. Let’s take a moment to recall our 1st Rule of Thumb:

Get an amp that’s 50% more powerful than your speakers

Right about now some of you may be thinking, “Hmm, isn’t 50% more power too much power? I don’t want to blow out my speakers. Shouldn’t I just get an amp with the same power rating that my speakers have?” The answer is no. Look at it this way. You have a Honda Accord with a speedometer that goes all the way to 120mph. It’s not a good idea to do 120mph on your way to work every day. Your car would wear out pretty fast. However, if you had a Maserati with a  speedometer that goes all the way 185mph you could drive 120mph on your way to work every day no problem (except for speeding tickets). Amps work the same way. You don’t want to make your speaker perform at its maximum capability all the time or you’ll wear it out.

OK, now you are ready to find an amp. Well, you’re almost ready. You still have to do some math. You’re JBL speaker has a power capacity of 500 watts and 50% of that is 250 watts. What’s 250 more than 500? Right, 750 watts. You didn’t make it past the 5th grade for nothing. So, we are looking for an amp capable of pushing 750 watts.

Now let’s take a moment to recall our 2nd Rule of Thumb:

Match your speakers’ ohm rating to your amps ohm rating

Our speaker has a nominal impedance of 4 ohms. So according to rules 1 and 2 we are looking for an amplifier capable of pushing 750 watts in a 4 ohm load. Amps are usually designed to work with different impedance ratings. As you can see, the Crown XTi 2002 amplifier in the spec sheet below can handle 2, 4, and 8 ohm speakers. You’ll also notice that it’s capable of pushing 800 watts in a 4 ohm load per channel. This amp is a perfect match for our two JBL PA speakers because it meets and even exceeds our criteria by 50 watts per channel.

Multiple Speaker Setup:

Some of the resources you find online on this subject can be a little complex. They generally do a good job of covering all the different type of speaker connections, but I think its generally bit much for beginners. And a lot of times they’re talking about car speakers not PA speakers (same concepts but different applications). Connecting multiple PA speakers to a single amplifier can actually be very simple. The technical term of the wiring method I’ll be covering today is wiring in parallel. In fact, any speaker that has a ¼”or speakon cable input is wired in parallel by default. You can chain together as many of these speakers as you want, and they’ll still be wired in parallel. If your speakers have a positive and negative terminal it takes a little more know-how to wire them in parallel, but not much. Regardless of your speakers connection type, you will have to do some simple math to ensure that they won’t hurt your system when added.

But before we get into that keep mind that all speakers you are connecting in parallel should have the same nominal impedance. If my first two speakers are the JBL JRX125’s, which have a nominal impedance of 4 ohms, every other speaker I add to the system should also have an impedance rating of 4 ohms. This ensures that power from the amplifier is divided equally among the speakers. Otherwise the speakers will play at different levels putting unnecessary stress on the ones getting the most power, which can shorten their lifespan.

Wiring the positive and negative terminals of your speakers is very straightforward. All the right channel speakers have their positives connected to the amp’s right channel positives and their negatives connected to the amp’s right channel negatives. The same setup applies to the left channel.

When you add more speakers in parallel, you lower the overall resistance of your system. It’s very important to know exactly how low that resistance is because you don’t want to fry your amp. Use the following formula to calculate the overall resistance of each channel.

R = resistance of each speaker
Rtot = Total resistance of my speakers

1/Rtot = 1/RA + 1/RB …etc

Example:

If I’ve got two 4 ohm speakers on the right channel what’s the overall resistance?

R = 4 ohms
Rtot = ?

1/Rtot = 1/4 + 1/4

1/Rtot = 2/4

1/Rtot = 1/2

Rtot = 2 ohms

Recall the PA system I was talking about earlier with my two JBL JRX 125 speakers and my XTi 2002 amplifier. As you can see from the calculations above, I can add two more 4 ohm speakers to the system (one extra speaker per channel), and I will still be in the clear. The XTi 2002 can handle a 2 ohm resistance.

Now I would probably choose 200 watt speakers to add to the system so that we observe our 1st Rule of Thumb. Remember we want the 1000 watts that comes in the 2 ohm load to be 50% more powerful than our speakers.

Combined Speaker power: 500 + 200 = 700 watts

50% x 700 = 350 watts

Power needed for our speakers: 700 + 350 = 1050 watts

As you can see the amp falls 50 watts short of the power we need, but that’s ok. We’re close enough, and the XTi 2002 will live a long healthy life in this speaker setup.

Bridged-Mono:

Some amps have an option to switch into bridged-mono. When you switch your amp from stereo to bridged mono you are just combining the power of the left and the right channel into one powerful channel. In bridged-mono you only have one input and one output, hence the mono part of the term (mono means one).
Normally, you would only use this feature to power a subwoofer, but you wouldn’t use it to power your higher frequency speakers. There are exceptions in very large speaker setups, but that’s beyond the scope of this article. In order to power your subwoofer properly in bridged-mono you need a device called a crossover. A crossover is a mechanism that separates audio frequencies. In this case, your crossover would eliminate the mid and high range frequencies leaving you with nothing but the bass frequencies.

Below you can see the normal stereo setup. On the back of the amp two inputs run from the mixer to the amp. The amp switch is set to stereo, and two outputs run to the speakers. On the front of the amp both volume knobs for channel 1 and channel 2 are turned all the way up.

In bridged-mono the setup looks a little bit different. Your mixer runs two inputs to the crossover, but there’s only one output from the crossover to the amp (blue arrow). Our amp switch is set to bridge-mono (green arrow). Since both channels are bridged, channel 2’s positive post is now the negative post for the bridged amp (yellow arrow). Channel 1 is now the positive post for the bridged amp. We only have the amp connected to one speaker which is our subwoofer. The volume knob for channel 1 becomes the volume knob for the bridged channel. Since the volume knob for channel 2 isn’t doing anything we have that turned all the way down (purple arrow).

Hopefully, this helps you understand how your PA speaker setup works with the traditional two speakers, with multiple speakers, and in bridged-mono mode.

Video Transcript:
Hey guys this is Chris here with Uniquesquared.com, these are my hands, and this is my markerboard. Together we are going to show you some of the basics of setting up your PA system with speakers and an amplifier. I’ll be talking about watts, ohms, bridged-mono and what it all means. Now I’m going to try to keep things simple because this can get really confusing.

But before we begin you need to know about ohms and watts. Now Ohms are all about resistance to electricity. Everything has a ohms including you. If something has a lot of ohms it means electricity has trouble getting through. If it has less ohms that means electricity runs through it really easily. A thick piece of rubber, for example,  has a lot more ohms than a metal pole. Also, for our purposes today, resistance and impedance mean the same thing and they are both measured in ohms. Ok, now watts. Watts are all about power. More power equals more watts. Enough said.

Alright let’s get started. I recently got a pair of JBL JRX125’s from uniquesquard.com. They each have a horn and two 15” speakers and I think they sound really good. But they’re unpowered or passive so I had to find an amp before I could get any sound out of them. The first thing I did was find the specs of the speakers. With the specs pulled up I was only interested in a couple of numbers. The nominal impedance and the power capacity. With that information I was ready to choose an amp.

Now there are two main rules of thumb when choosing an amp.

1st rule of thumb: Get an amp that’s 50% more powerful than your speakers.

When your amp sends watts to your speakers, it’s making them work. Their job is to pump out the sound. The more watts they get, the more work they have to do. Your speaker can only do so much work over a long period of time. Remember our spec sheet? There were two numbers. One was the power capacity which was 500 watts for the JRX  125. Other spec sheets might call it continuous power capacity, RMS power handling, or program watts. But they all mean the same thing. You’re gonna want to ignore the peak power capacity.  If I tryed to play 2000 watts out of my JBL’s I would probably blow them to smithereens. It’s way too much work for them to do. But 500 watts? They can play 500 watts 24/7 no problem.

Now your amp needs to be 50% more powerful than your speakers for the same reason. Well, basically the same reason. Because your amp is also doing work. It’s job is to pump watts to your speakers. Think of it this way. If you are on foot, you can walk a lot further than you can run before you are completely exhausted and have to stop. So, if my amp is rated at 800 watts, pushing a 500 watt speaker will be equivalent to an easy paced walk. But if it’s rated at 500 watts, pushing a 500 watt speaker would be more like  the equivalent to a strenuous run. You would be pushing it to its limits, and it would wear out pretty quickly.  Now, whatever you do, DO NOT, I repeat, DO NOT underpower your speakers unless you want to ruin your amp. Bottom line.

So, since my pair of JRX125’s each had a power capacity of 500 watts in a 4 ohm load, I was looking for an amp that could push at least 750 watts per channel in a 4 ohm load. Now we’re getting back into ohms and so that brings me to the 2nd rule of thumb.

Match your speakers’ ohms to the ohms your amp can handle.

Amps are generally designed to work with 4, 8, and 16 ohm speakers. But it’s very important to know exactly what your amp can handle. If your speakers’ ohms are too low for your amp, you run the risk of blowing out your speakers and frying your amp. Please, don’t make that mistake. Now if you’re speakers’ ohms are too high for your amp, you won’t get much sound out of your speakers. It’s not as bad but it’s still not good.

So since I’ve got two speakers with a nominal impedance of 4 ohms, I need an amp capable of handling 4 ohms. But remember the 1st rule of thumb. I also need 750 watts to power my speaker. So I need 750 watts in a 4 ohm load.

That’s why I decided to go with a XTi 2002 from Crown to power my JBL speakers. As you can see on the spec sheet, it’s capable of pushing 800 watts in a 4 ohm load per channel which means per speaker, left and right. And honestly I couldn’t ask for a more perfect match. I’ve even got 50 extra watts of power per speaker. And as you can see on this spec sheet it’s even capable of handling an impedance of 2 ohms, which is handy. Why? Well, so far I’ve only talked about a two speaker setup. But what if I wanted to add more speakers?

Well, before I do that I need to make sure that it has the same impedance rating as the speakers I’ve already got. Since my JBL’s have an impedance rating of 4 ohms, any speakers I add must also have a 4 ohm impedance rating. Let’s say I add one extra speaker to each channel in parallel giving me a total of two speakers per channel. Now to calculate the new impedance load of your speakers it’s really easy. You just divide you’re speakers’ impedance rating (which is 4 ohms in our case) by the number of speakers you have. Now the new load per channel comes to 2 ohms. Now that’s a low number of ohms, but it’s cool because I’ve checked the spec sheet of my amp I know that I’m in the clear. And I know that it can handle an ohm load that small.

Now a lot of amps also have an option for bridged mono. What is bridged mono? Well, rather than taking two inputs and sending amplification to the left and right channels like you would in stereo mode, you are getting one input and combining the power of the left and right channels into one super powerful channel.

Ok so I’ve been talking about amp setups in stereo. Here I’ve got my mixer board, the back of my amplifier, my two speakers, and the front of my amplifier. As you can see I’m in stereo mode. I have two inputs coming from the mixer. And I have two outputs as well. Each output has a red and black post: red for positive and black for negative. Both the left and right channels should be at the same volume and as you can see, I have them cranked all the way up. But that’s just my personal preference, you can have them at whatever volume you want.

The bridged-mono setup looks different. Now the first thing I’ve done is  flipped the switch from stereo to bridge mono which is very important. I still have my mixer, but this time I’m  running through a crossover. Why? You don’t usually want to run your high frequencies speakers in bridge mono unless you’ve got a really big setup. I’ve got it running to my sub. Now what’s a crossover? Well it’s basically a device that lets you separate different frequencies. In this case its singling out the bass and completely cutting out the mids and the highs. You can see that there is only one input from my crossover to my amp. There’s no second input because we’re goin in mono which means one channel. Now we’re also bridging channels 1 and 2 into a powerful channel like we said, so the outputs are going to be different as well. The positive post on output 2 becomes the negative post for your powerful ouptut and the positive post on output 1 becomes the positive post for the powerful mono- bridged channel. Now the volume control on channel 1 becomes the volume control for the entire thing. And I’ve got that cranked up all the way. Since I’m not using the volume knob for channel 2, I turn that all the way down.

So there you have it. Now there’s a lot more to this subject that I didn’t have time to go over and for some of you who know a lot about speakers and amplifiers already, this video may have been a little bit pedestrian but hopefully for some of you beginners this will help you get your PA system up and running. For great prices on speakers and amplifiers check uniquesquared.com. Thanks for watching, my name is Chris we’ll see you next time.