Can someone explain Ohms when buying an amp?

[bentleybloke]

Hi Folks,
this is my first post so sorry if its in the wrong section!
I am having a mid life crisis and trying to start a band. I am buying some PA equipment to compliment my electronic drum kit and also will use it with the other band members.
So far I have two 500 Watt 8 Ohms speakers, a Soundcraft Spirit folio SX 20 mixer but now need to buy an amp.
I have looked on the net but I am very confused. What is the recommended wattage amp I need to buy with my speakers? Some people say I need more whats than the speakers so the amp does not work hard. Also what Ohms value do I need to look for if I have 8 Ohms speakers and I want to run them in stereo?
Can someone please explain the pitfalls or difference between 4 and 8 Ohms? Sorry if this is a basic question but I only have limited funds and do not want to buy the wrong amp or worse damage my equipment. Manufacturers names and model numbers for recommendation would be most welcome too. Thanking you all in advance. Mark

[SY]

"Watts" on a speaker just indicate how many it will nominally take to burn it out (but of course, things aren't quite that simple...). It doesn't indicate how loud the speaker will play or how much amplifier power it will take to achieve that volume.

The amplifier is the bit that supplies the power (measured in watts). The speaker has an impedance measured in ohms- the lower the impedance, the easier it is for electricity to flow through it- N.B.: lower ohms does NOT mean louder sound.

The important points to consider are:

1. Will the amplifier be happy driving the impedance of the speaker you want? The lower the impedance, the harder an amp will have to work for a given voltage into the speaker. Some amps are perfectly happy at 2 ohms, some will choke, smoke, and die at 4 ohms. Most high quality amps will be fine at any reasonable (2-16 ohm) speaker impedance.

2. Will the combination be loud enough? This is a function of the speaker's sensitivity or efficiency (usually measured in dB/W at 1 meter distance, sometimes dB/2.83V at 1 meter distance). The bigger this number, the louder the speaker will play at a given power, as long as the power is below the point where the speaker coughs and dies.

There are a bunch of other finer points (voltage versus current swing, for example), but this will at least enable you to go past the marketing specs and look at the important numbers- output power of the amp and efficiency of the speaker.

[AndrewT]

Hi,
find out if that speaker power rating of 500W into 8ohms is continuous or music programme or peak or short term transient power.

If it is a continuous power rating then you can use an amplifier from half to double the speaker rating, i.e. between 250W into 8r0 and 1000W into 8r0. The 1000W amplifier will allow unclipped transient peaks to be about 6dB louder than the 250W amplifier. This in turn will allow average SPL levels to be about 6dB louder. Check the sensitivity of your speakers. If they are <90dB/W/m then they are not PA and pushing them to get adequate SPL may damage them.
If they are >96dB/W/m then you can drive them to pretty high peak SPL and they should survive your gigs.
If they are between 91dB/W/m and 95dB/W/m, then seek advice on what duty they are intended for.

Since you are into PA, many times you will bump into 4ohm speakers. You have more chance of being able to use your amplifiers if they are rated for 4ohm speaker use.

Your power rating changes to:
500W into 4r0 to 2000W into 4r0.

Do you have a vocalist?
Then dedicate at least one channel of amplifier and speaker to the singer alone.

[Nigel Goodwin]

It's really pretty simple - PA gear is sensibly rated, with proper RMS power specs.

As a 'rule of thumb', an amplifier will deliver half as much power into double the impedance - so an amplifier which will deliver 1000W per channel into 4 ohms, will deliver 500W per channel into your 8 ohm speakers. Pretty well all PA amps will handle 4 ohms, and many will go as low as 2 ohms, with some restrictions.

It would be VERY rare to find an 8 ohm only amplifier in this day and age.

Something like this should be fine for what you need, and if you later add a second pair of 8 ohm speakers will cope with that as well.

http://www.bluearan.co.uk/index.php?...e=manufacturer

If you can find the older EP2500 in stock somewhere, that would be fine as well, but as it's only £10 less, it's probably not worth bothering.