The first thing to do when selecting which power amplifier you need for your passive speakers is to make sure the resistance is the same in both the speaker and the power amplifier. This means if you have a speaker that is rated @ 8 Ohms you want to make sure the specs for the power amp you are buying is also @ 8 Ohms. This speaker may advertise being 1,000 watts, but this is useless if you don’t know what resistance it is. Most amplifiers are capable of outputting power at multiple resistance ratings such as the example below.
Watts/Side @ 8 ohms 215W Watts/Side @ 4 ohms 350W Watts/Side @ 2 ohms 550W
Now another important thing to remember is that power amps are stereo. Which means they output power in 2 sides. This is important because most of the time a PA system consists of 2 speakers in stereo. So each side of the power amp will provide independent power to each speaker. So make sure these specs are listed as per side. Here’s another example of a power amp spec:
Provides 440W/channel at 8 ohms, 775W/channel at 4 ohms
They will display this as either Per side, Per channel or 775W/Channel. You can run your power amps in bridge mode which combines both channels or sides to give a combined power rating. This is useful if you are only using one speaker or if you have a speaker that requires an extreme amount of power.
Watts Bridged 700W @ 8 ohms or Example 4: 1550W at 8 ohms bridged
In example 3 you have a power amp rated at 215W Per side @ 8 ohms. In bridged mode it would be around 700 watts @8 ohms. Every amplifier manufacturer develops their design independently and therefore no two amplifiers may have the same per side to bridge mode relationship. But the idea being that combining both sides gives you more than double the power for your speaker in most cases. Now lets talk speakers, this is where a lot of musicians get confused. There are 3 different power ratings for a speaker.
The first is RMS (Roots means squared)/Continuous : This rating tells you the average operating power the speaker can output without harming it. This is what you would want to run your speaker at normally. The manufacturer establishes this rating by running a continuous test tone for a long duration, making sure that the speaker stays perfectly intact.
The second is Program : This rating is always double the RMS and is what you use to determine what power amp you need. If your Speaker is 400 Watts @ 8 Ohms program, then you need a power amp that is 400 watts per side @ 8 ohms. By far the most useful power rating.
The final is PEAK : Now this rating is what almost always is advertised as the power rating of the speaker, and is by far the MOST USELESS piece of information. Peak power is how much power the speaker can output for mili-seconds right before it blows up. Definitely not something you want to use as your standard operating power. This rating is used as a selling tool to trick people into thinking that they can operate their speaker at this rating. If there is one thing you take away from this, it is to never use this rating. Most of the time this rating is highly inaccurate anyway, do not fall for this advertising trap.
If you have any questions about power amps – or any pro audio gear – feel free to give me a call or drop me an email – Evan.