Sunday, February 23, 2014

Several Suggestions To Help Understand The Signal-To-Noise Ratio Of Today's Digital Amps

By Sherry Lambert


It is easy to be confused by the language which amplifier suppliers utilize in order to describe the performance of their products. I am going to clarify the meaning of a regularly used amplifier spec: "signal-to-noise ratio" to help you make an informed decision whilst purchasing a new amp.

A technique in order to do a simple test of the noise performance of an amplifier is to short circuit the amp input and then to crank up the amp to its maximum. Then listen to the speaker that you have connected. You are going to hear some amount of hissing and/or hum coming from the loudspeaker. This noise is generated by the amplifier itself. Make certain that the volume of the amps is set to the same amount. Otherwise you will not be able to objectively evaluate the level of hiss between several amplifiers. The general rule is: the smaller the level of static that you hear the better the noise performance.

When looking at the amp spec sheet, you want to look for an amp with a high signal-to-noise ratio figure which indicates that the amp outputs a small amount of static. One of the reasons why amplifiers make noise is the fact that they use elements like transistors and resistors which by nature produce noise. Typically the elements that are located at the input stage of an amplifier will contribute most to the overall noise. Consequently manufacturers generally are going to pick low-noise components whilst developing the amp input stage.

Most of recent power amplifiers are digital amps, also called "class-d amps". Class-D amplifiers utilize a switching stage that oscillates at a frequency in the range of 300 kHz to 1 MHz. Consequently, the output signal of switching amps exhibit a moderately big amount of switching noise. This noise component, though, is typically impossible to hear given that it is well above 20 kHz. Though, it may still contribute to speaker distortion. Signal-to-noise ratio is normally only shown within the range of 20 Hz to 20 kHz. For that reason, a lowpass filter is used when measuring switching amplifiers in order to eliminate the switching noise.

Makers measure the signal-to-noise ratio by setting the amplifier such that the full output swing may be realized and by feeding a test signal to the amp that is typically 60 dB below the full scale of the amplifier. Next the noise-floor energy is calculated in the frequency range between 20 Hz and 20 kHz and compared with the full scale signal energy.

Often the signal-to-noise ratio is expressed in a more subjective method as "dbA" or "A weighted". This technique attempts to evaluate in how far the amp noise is perceived by human hearing which is most sensitive to signals at frequencies at 1 kHz. Therefore an A-weighting filter is going to amplify the noise floor for frequencies that are easily perceived and suppress the noise floor at frequencies that are barely heard. Most amplifiers are going to show a higher A-weighted signal-to-noise ratio than the un-weighted ratio.




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