It is easy to be puzzled by the terminology which amplifier manufacturers utilize to explain the performance of their models. I am going to clarify the meaning of a frequently used amplifier spec: “signal-to-noise ratio” in order to help you make an informed choice when buying a new amplifier.
You can make a simple comparison of the amp noise by short circuiting the amplifier input, setting the volume to maximum and listening to a speaker attached to the amp. You will hear some amount of hissing and/or hum coming from the speaker. This noise is produced by the amp itself. Then compare different amplifiers according to the following rule: the smaller the level of noise, the higher the noise performance of the amplifier. Yet, keep in mind that you have to set all amps to amplify by the same amount in order to compare different amplifiers.
One method in order to do a simple check of the noise performance of an amplifier is to short circuit the amplifier input and then to crank up the amplifier to its utmost. Next listen to the speaker which you have connected. Generally you are going to hear 2 components. The first is hissing. In addition, you will frequently hear a hum at 50 or 60 Hz. Both of these are components which are generated by the amplifier itself. After that compare different amps according to the next rule: the smaller the amount of noise, the better the noise performance of the amp. Yet, bear in mind that you must put all amps to amplify by the same level to evaluate several amplifiers.
The majority of today’s power amplifiers are digital amps, also known as “class-d amplifiers”. Class-D amps make use of a switching stage that oscillates at a frequency in the range of 300 kHz to 1 MHz. In consequence, the output signal of switching amplifiers have a fairly large level of switching noise. This noise component, however, is typically inaudible given that it is well above 20 kHz. However, it may still contribute to loudspeaker distortion. Signal-to-noise ratio is typically only shown within the range of 20 Hz to 20 kHz. As a result, a lowpass filter is used when measuring switching amplifiers in order to remove the switching noise.
The signal-to-noise ratio is measured by inputting a 1 kHz test tone 60 dB underneath the full scale and measuring the noise floor of the amp. The volume of the amplifier is set such that the full output power of the amp can be realized. Subsequently, only the hiss in the range of 20 Hz and 20 kHz is considered. The noise at different frequencies is removed by a filter. Then the amount of the noise energy in relation to the full-scale output wattage is computed and shown in db.
Time and again the signal-to-noise ratio is shown in a more subjective manner as “dbA” or “A weighted”. This technique attempts to examine in how far the amp noise is perceived by human hearing which is most responsive to signals at frequencies at 1 kHz. An A-weighted signal-to-noise ratio weighs the noise floor in accordance to the human hearing and is generally larger than the unweighted signal-to-noise ratio.
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