Amplifiers Shopping

When you are purchasing a power amplifier for a PA Speaker, a good rule is to pick an amplifier that can deliver power equal to twice the speaker's continuous IEC power rating. Secondly, using an amp with some extra "headroom" will help assure that only clean, undistorted power gets to your speakers. Thirdly, you need to be considerate about the damping issue. Damping is the ability of a power amplifier to control loudspeaker motion. It’s measured in Damping Factor, which is load impedance divided by amplifier output impedance. Since the speaker impedance and amplifier output impedance vary with frequency, so does the damping factor. Also, the impedance of the speaker cable affects damping. Thick cables with low AWG allow more damping than thin cables with high AWG. The lower the amplifier’s output impedance, the higher the damping factor, and the tighter the sound is. As you might suspect, damping factor is most important at low frequencies, say 10 Hz to 400Hz. High damping factor equals tight bass. In short, the loudspeaker damps itself through the amplifier output circuitry. The lower the impedance of that output circuitry, the more the back EMF can control the speaker’s ringing. Fourthly, you need to know that it is all in the ohms. Ohms, is a measure of resistance. If the total loudspeaker impedance is too high, the power delivered to the loudspeakers will be reduced. If the total loudspeaker impedance is too low, the power delivered to the loudspeakers will be increased, which can result in speaker overload and damage to the amplifier. In this regard, multiples of loudspeakers can be connected together by three different methods, termed Series, Parallel, and a combination of the two, Series/Parallel. In the case of PA sound, calculating parallel loads is an important capability for two main reasons; first, because dual speaker connections whether on an amplifier, a mixer/amplifier or a speaker enclosure are all wired in parallel. Some people think that if you run separate speaker cables from each speaker output on the amp or mixer/amp to the enclosures you somehow "avoid" putting the speakers in a parallel circuit.

To keep life as simple as possible, most people put enclosures of the same impedance in a parallel circuit. If you do this it's all just a matter of dividing that impedance by the number of speakers. You can see that for the same number of speakers, the Ohm load presented to the power amp is significantly reduced. Use this formula to insure that the impedance of your total number of speakers matches the output impedance on the amplifiers.

Slew rate does also really matter regarding the shopping of the amplifiers. It is a measure of an amplifier's ability to follow its input signal. The term is used to define the maximum rate of change of an amplifier's output voltage with respect to its input voltage. As far as slew rate having an effect on perceived sound, the real issue is slew rate limiting, which relates to an amplifier’s ability to pass complex waveforms without clipping them, resulting in an open musical sound. Slew rate is measured by feeding an input signal that is too fast for the amplifier to cope with. So slew rate is an overload condition, and it should not happen at all for an audio amplifier.

Generally you should pick an amplifier that can deliver power equal to twice the speaker's continuous IEC power rating. For a stereo pair of speakers, the amplifier should be rated at 700 watts per channel into 8 ohms.

A quality professional loudspeaker can handle transient peaks in excess of its rated power if the amplifier can deliver those peaks without distortion. Using an amp with some extra "headroom" will help assure that only clean, undistorted power gets to your speakers. Some professional amplifiers are designed so they have additional headroom. Consult the amplifier manufacturer or owner's manual to learn more. If budget restraints or legacy equipment force you to use an amplifier with less power, extreme care should be taken to see that the amplifier is not driven into clipping.