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What Is the Slew Rate?

Oscilloscopes are frequently used in measuring the slew rate.
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  • Written By: Malcolm Tatum
  • Edited By: Bronwyn Harris
  • Last Modified Date: 21 December 2014
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The slew rate is a term that is used to identify the efficiency of some type of audio device to accurately reproduce the changes made in amplitude within a specified period of time. The idea is to assess the rate of change both in terms of strength and continuity, and how much distortion occurs during the sound reproduction. For the most part, a higher slew rate is desirable, since slower rates are usually characterized by increasing amounts of sound distortion.

One of the main purposes of determining the slew rate is to identify the maximum amount of input frequency that the amplifier is capable of sustaining without the creation of any distortion in the output. Most manufacturers provide guidelines that identify an acceptable range for the rate when the amplifier is generating both low and high frequencies during operation. Consulting those guidelines is important, since the scale or range that is considered acceptable may vary somewhat from one manufacturer to another.

Identifying the slew rate requires identifying the capability of the amplifier in terms of the amount of signal voltage the device is capable of producing. Generally, an acceptable rate would be a tenth of whatever amp rating the device possesses. For devices equipped with higher amps, the slew rate may be closer to a fifth of that amount if the amplifier is to produce the highest quality of sound reproduction.

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A low slew rate leads to distortions in the sounds produced by the amplifier as the levels of the sound change. For listeners, this means that the sound quality may suddenly become somewhat fuzzy or muffled for a time, or possibly begin to cut in and out if the rate is unusually low. The end result is that the reproduction of the sound is of relatively low quality, unintelligible to a degree and typically considered unacceptable.

There are several different strategies used to measure the slew rate. Equipment like an oscilloscope and some type of function generator is often helpful in assessing the current status of the rate. Testing equipment only provides measurement of the rate, but also provide some clues as to what is causing the distortion in the first place. This is very helpful, in that the data makes it easier to determine the origin and take the proper steps to make repairs. Once the repairs are completed, testing the slew rate a second time to make sure the distortion is resolved and the amplifier is now functioning at full capacity.

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