Choosing an Amplifier
Choosing and understanding audio power amplifiers and amplifier types can be a challenge and is subject to a wide range of differing opinions. Nevertheless there are certain basic elements that you can be sure of and this can help make your decision. The notes here relate specifically to power or slave amplifiers for band PA, Installed commercial music systems or DJ systems. Most amplifiers are two channel, although multi channel types are readily available and will see a variety of options in our amplifier section.
Amplifier Classes
Amplifier classes are often mentioned when you read the technical notes on an amplifier and generally, for PA power amplifiers Class AB, H and D are the most frequently used. Digital or class D amplifiers are becoming the most popular for portable equipment, since they are light weight, very efficient and low internal power dissipation. This is less of a problem for fixed instillations and there are many who feel more traditional class AB types offer better performance, especially at low frequencies. This may have been the case during the introduction of class D amplifiers, but this no longer holds true, with improved design techniques. Modern class D amplifiers operate at very high switching frequencies and have excellent filters and power supplies giving efficiencies well over 90%.
Amplifiers that have a 100 volt line output can also be considered for some applications, these are useful where large numbers of speakers are distributed over a wide area, such as a warehouse or restaurant. There are some separate technical notes on line amplifiers.
Power Ratings
High power amplifier output ratings can be very misleading. Peak power or Music power output are not very good guides to amplifier performance and are often quoted by disreputable manufacturers to mislead customers. This practice has become less common with professional suppliers, but beware when making an amplifier purchase. When checking specifications, always look for the R.M.S. or A.E.S. output. The former RMS (Root mean squared) when quoted as continuous RMS output will be a good guide to the long term output capabilities of an amplifier. This was the old school way of rating amplifiers and gives the equivalent DC power into any given load with a continuous sine wave, usually 400Hz or 1KHz.
AES (Audio Engineering Society) is a more recent way of rating amplifiers and is a little more sophisticated, since it uses pink noise as the sound source, filtered between 125Hz and 8KHz for a period of two hours. This is arguably a better way to define power amplifier or Loudspeaker rating, since it represents a more likely signal type and duration. All said either is a good guide. If the power rating seems unbelievable for the price, then it probably is!
Distortion
Used within their rated output power most amplifiers with have negligible distortion. The problems start if they are overdriven so the output waveform clips, causing all types of harmonic and intermodulation distortion. The output no longer follows the input signal faithfully and the result is obvious poor sound quality. This is to be avoided, since the amplifier will be under electrical stress and the speakers will start to see a DC element in the signal, heating the voice coil. This can cause speakers to fail even if the speakers are will within their rated power.
Output Impedance
Amplifier output power will often be different depending on the loudspeaker load. Most modern audio amplifiers will work with a 4 or 8 ohm load with many larger types having enough drive current for 2 ohms also. Correctly matching the amplifier power to the loudspeaker system will give the best results and remember that an amplifier rated at 200 watts into 4 ohms will only give around 100 watts when connected to an eight ohm load. You can see from this that the power delivered to the speaker is quite dependent on correct matching.
Bridging For More Power
Some power amplifiers can be bridged and this can be useful for driving bass speakers, that require a mono signal and lots of power. Something worth considering is using a four channel amplifier and bridging two of the channels. This configuration will then give two channels for the mid top speakers and a further bridged channel for the bass cabinet.
Stereo Image
While stereo imagery can off a great enhancement to your sound system, it wont be relevant if there are large number of speakers distributed around a bar of club. In this case its better to switch or wire the amplifier in mono mode, some designs have ready made features to switch in to mono or multi zone mode.
Matching To An Audio Source
Most power amplifiers will match directly to mixers or other sound sources. Audio signals from mixers and other equipment are usually sufficient to drive the amplifier to maximum output If you have a long run of cable consider using the balanced inputs on the amplifier, or use a DI box either end of the cable. This will reduce induced noise in the cable, you can see more about in our DI technical notes.
What Cables Do I use
To achieve the best sound quality and efficiency, use good quality cables to connect your amplifier to the loudspeakers and try to keep the speakers as close to the amplifier as possible. Long cable runs with 4 ohm systems can result in significant power loss and reduced speaker damping. Use a 100 volt line amplifier if a large number of loudspeakers are to be used at some distance from the amplifier; cables can then be thin.
The Power You Hear
Due to the characteristics of the human ear, doubling the amplifier power will not make the output sound twice as loud. A 200 Watt amplifier will only sound about 50% louder than a 100 Watt amplifier. This is only a very rough guide, but worth remembering when selecting a system.