To help you select an audio amplifier, I will explain the term "signal-to-noise ratio" which is commonly used to describe the performance of audio amplifiers.
After you have narrowed down your search by looking at some basic criteria, such as the amount of output power, the size of the amp and the cost, you will still have quite a few models to choose from. Now it is time to look at some of the technical specs in more detail. One important criterion of power amps is the signal-to-noise ratio. To put it simply, the signal-to-noise ratio describes how much hum or hiss the amplifier will add to the audio signal. This ratio is usually described in decibel or "db" for short.
One way to do a simple test of the noise performance of an amp is to short circuit the amplifier input and then crank up the amp to its maximum. Then listen to the speaker which you have connected. You will hear some amount of hissing and/or hum coming from the speaker. This noise is generated by the amp itself. Then compare different amps according to the following rule: the lower the amount of noise, the better the noise performance of the amp. However, keep in mind that you must set all amps to amplify by the same amount in order to compare different amps.
To help you compare the noise performance, amp manufacturers publish the signal-to-noise ratio in their amp spec sheets. Simply put, the higher the signal-to-noise ratio, the lower the amount of noise the amp generates. There are several reasons why power amps will add some form of noise or other unwanted signal. Transistors and resistors which are part of every modern amp by nature generate noise. Mostly the elements which are located at the input stage of an amp will contribute most to the overall noise. Therefore manufacturers typically will select low-noise components when designing the amplifier input stage.
Most of today's power amps are digital amplifiers, also known as "class-d amps". Class-D amps use a switching stage which oscillates at a frequency between 300 kHz to 1 MHz. This switching frequency is also noise which is part of the amplified signal. However, modern amplifier specifications typically only consider the noise between 20 Hz and 20 kHz.
The signal-to-noise ratio is measured by inputting a 1 kHz test signal 60 dB below the full scale and measuring the noise floor of the amp. The amplification of the amp is set such that the full output power of the amp can be realized. Then, only the noise between 20 Hz and 20 kHz is considered. The noise at other frequencies is removed by a filter. Then the amount of the noise energy in relation to the full-scale output power is computed and expressed in decibel.
Often you will find the term "dBA" or "a-weighted" in your amplifier spec sheet. A weighting is a method of expressing the noise floor in a more subjective fashion. In other words, this method attempts to express how the noise is perceived by a human. Human hearing is most sensitive to signals around 1 kHz while signals below 50 Hz and above 14 kHz are barely heard. Therefore an A-weighting filter will magnify the noise floor for frequencies that are easily perceived and suppress the noise floor at frequencies which are barely heard. Most amplifiers will have a higher A-weighted signal-to-noise ratio than the un-weighted ratio.
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