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There are a few good brands in the professional power amplifier field, and they need to be your first consideration. It's important that each amplifier is judged on a level playing field. Power ratings are sometimes confusing to the inexperienced buyer. Terms such as RMS, peak, and continuous power aren't equal, but they can all be useful when considered thoughtfully and when they're assessed fairly: Always combine the implications of multiple specifications. For example, 2 amplifiers might claim an RMS rating of 500 watts; however, one is rated at .05 percent distortion (THD) and the other at 5 percent distortion (THD). In this case, the amplifiers sound like they're very similar at first glance; however, in reality, they're quite different. You would not want the second amplifier. Here are some specifications terms that need to be compared with each amplifier choice.
Peak power is the highest instantaneous power potential in an amplifying circuit. It is essentially the absolute maximum amount of power that the amplifier can generate in a mere instant in time. Sustained peak power levels result in amplifier failure. Providing peak power ratings offer little to no value in the assessment of a power amplifier.
RMS stands for root mean square; amplifiers are tested using sine waves as audio sources. Practically speaking, the RMS rating resembles an average power rating in that it closely resembles the average metering of the typical VU meter. RMS power closely equates to perceived volume because, like the human ear, it tends to ignore instantaneous peak levels. RMS power is a close representation to an actual live human sound source. In reference to a square wave rather than a sine wave, the RMS value is the equal to the peak value.
Continuous power represents an amplifier's continuous reliable power output potential and is often equated to the RMS power rating. This rating indicates the power level for continuous duty.
THD (Total harmonic Distortion)
THD is the addition of harmonic distortion during the amplifying process. The lower this number is the better. It is understood that the average human ear can not detect distortions less than about 2 percent. If the THD is less that 1 percent this is acceptable. A THD rating is typically displayed as THD < .06% @ 4 ohms at 20 HZ – 20,000 Hz.
SPL, Power and Speaker Sensitivity
SPL or “Sound Pressure Level” is a measure of the sound pressure that reaches our ears and we perceive it as music or speech. Power we have discussed above. Speaker sensitivity is a measure of the efficiency of the speaker. For example a sensitivity rating might assume the amplifier delivering a 1000 cycle tone at one watt (2.83 volts @ 8 ohms) the SPL is measured with a microphone one meter from the speaker. In this example, SPL can be very important when considering how much volume (or loudness) you need in the room containing the speakers. A 200 watt speaker with a sensitivity of 95 dB will produce 98 dB when given 200 watts. If this speaker has 3 more dB of sensitivity it will take only 100 watts to produce the same 98 dB of sound.
Matching the power to the Speaker Cabinet
Match the speaker cabinet power capability with the amplifier. A power amplifier with more power than the speaker ratings could damage the speaker. An amplifier that is rated at less than the capability of the speaker system may work too hard to achieve the desired SPL. The result is distorted sound.