stagepasNow here’s a tricky one…….What’s all this about WATTS? I have been asked by many people about power, watts, RMS, impedance and the like and why they don’t seem to be getting the kind of power they expect out of their sparkly new PA system.

To put all this into perspective you really do need to know some of the physics involved in sound reproduction but before we get to that lets have a look at your PA. We can split a sound system into three components. 1. Pre-amp (and that would include mixers) 2. Power amp and 3. Speakers.

The pre-amp part of your setup is the bit with all the whizzy knobs and lights. It takes in the signal from your instrument or microphones and boosts the weak signal from those instruments and mics up to a level that the power amp can deal with. It also gives you control over the sound of the final product by letting you add or take away bass, treble etc.

The first thing that you need to know about this section is that just because you are presented with an impressive array of knobs….you don’t have to use ‘em all. If you are using decent mics- and if you aren’t….why not?- then you shouldn’t need to apply much tonal variation. Rule one…..you don’t get nothin’ for nothin’. If you whack up the bass or treble, or anything else too much you are going to run into feedback and distortion problems later on. Aim to have all the equalisation flat. If you don’t understand what you’re doing….leave it alone, and if you do understand what you’re doing you will have left it alone already. I know all the knobs with all their range of boost and cut are inviting but you should always start off with everything flat….and then leave it alone. Turn your volume up and down by all means but leave the rest alone.

The power amp section is interesting. It boosts the signal from your pre-amp to a level where it can drive your speakers. It is designed to be clean. What you put in is what you will get out….but louder….much louder. Amplifier power is complicated. It is quoted in Watts, which is the electrical unit of power. There are various flavours of watts, many of them merely designed to make it appear that you are getting more of them than you actually are. I have seen amps quoted as having 1000 watts peak music power (PMP) and such. Don’t mean a thing. It is an arbitrary term that gives you no idea at all how loud it is going to be but the 1000 watts bit will have got you and sat in your brain. Don’t listen to it. RMS is the only way to go. 100 watts RMS is a quantifiable value. I won’t go into technical details as to what RMS means- just believe me that if it don’t say RMS then you won’t know what you’re getting. Right, now, your 100 watts RMS will be quoted into so many Ohms. The Ohm is the electrical unit of resistance. Loudspeakers present themselves to amplifiers as a resistance because they are made with coils of wire which have electrical resistance. (disclaimer to come-watch out for it).

 

Every speaker has a quoted impedance...i.e. An 8 Ohm speaker or a 4 Ohm speaker. There are basically three main impedances 4, 8 and 16 Ohms. So, and we are still on power amps here, you will have a rating printed on the back of your power amp of something like 100 Watts RMS into 4 Ohms. Great, here we have numbers that actually mean something. If you apply that amplifier to a 4 Ohm speaker you will get 100 Watts of sound out of it. But.…and here’s where they don’t always tell you….if you apply it to an 8 Ohm speaker that power will halve, and if you apply it to a 16 Ohm speaker it will halve again.  Disclaimer time- I told you to watch out for it- All these figures are not totally correct because other factors come into play but for all practical purposes you can believe it that….if you double the speaker impedance you will halve the power and alternatively if you halve the speaker impedance you will double the power. So…our 100 Watt RMS amplifier, if used to run a 16 Ohm speaker will only produce 25 watts (halved when run into 8 Ohms and halved again into 16 Ohms).

 

So what is the point of all this? MATCHING is the answer. Take the trouble when buying your equipment to make sure that all the numbers match up. It is totally pointless to go for a fabulous 500 watt PA and then buy 16 Ohm speakers because the maximum power output of the amp is always quoted into 4 Ohms, or sometimes 2 Ohms if it is good quality and pricey. So, a 500 watt amp driving 16 Ohm speakers is only going to put out 125 watts. Pretty much exactly the same as a 100 watt amp running a 4 Ohm speaker. Which brings us to the speakers.

If you connect two speakers on parallel (across each other, red to red, black to black, + to +, – to – etc.) the effective impedance will halve so that two 8 Ohm speakers in parallel will make 4 Ohms. Great you say, we’ll just keep adding speakers, halving the impedance and doubling the power…cracked it!……well no, blown it actually because the amp will not be happy below it minimum rating of 4 Ohms and the output will blow. Also if you connect speakers in parallel each speaker must be able to handle the full output. Now some amps may have impedance switches so you may have more control over it all. Valve amps will always have impedance matching switches but you are much more likely to be dealing with solid state amps which will not. So…if you are having problems with your PA the things to check are….feedback and distortion are pretty likely to be over twiddled knobs……..low power is pretty likely to be mismatched speakers.

About the author

Our guest blogger Alan Kensley has been playing guitar, harmonica and singing for 54 years. He ran his own business making, customising and repairing electronic music gear and guitars for many years and was a staff writer and consultant for 'International Musician and Recording World' magazine and its sister magazine 'What Keyboard?'. He has an in depth knowledge of electric music from a technical background and a player's point of view and has a great reputation for his honest, no nonsense approach.

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