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What is 'Slew rate' ?

Discussion in 'AV Receivers & Amplifiers' started by avanzato, Sep 22, 2003.

  1. avanzato

    avanzato
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    I've seen 'slew rate' in the specs on some amps and I have only a little idea what it might be.
    Can anyone technically minded tell me what it actually is, why it's important and if big or small numbers are best.
     
  2. NicolasB

    NicolasB
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    It's a measure of how quickly the amplifier output can change its value. If you imagine the input signal instantaneously going from the minimum possible voltage to the maximum possible voltage, how long does it take the output to go from minimum to maximum and match it? (Well, strictly speaking that's slew time - slew rate would be be measured in maximum achievable voltage change per second).

    Slew rate or time is one indicator of how accurate an amplifier will be in use. If the slew time is high (i.e. the slew rate is low) then the output is relatively unresponsive, and this introduces distortion, especially to high frequency signals

    Very few amplifier manufacturers actually quote this figure, because it is actually of some use in determining the amplifer's performance, unlike (say) Total Harmonic Distortion, which is always measured in such a way as to be more or less meaningless, and is thus safe to mention.

    The other number which it would be useful to know (but which even fewer manufacturers quote) is the "settling time". Typically if you do actually feed a rapid change to the input signal an amplifier output won't zoom up to the correct level and then stop dead, it might actually overshoot, and then oscillate to and fro slightly until it settles on the correct value. The time taken to settle down to a steady value is the settling time, and, again, the faster this happens the more closely the output signal will match the input signal.
     
  3. avanzato

    avanzato
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    Right cheers that's what I thought it was.
    I asked because I saw on some Arcam Power amp specs something like 100 and a PA amp quoted 50. Naturally I assumed 100 was better but then you never know until you know
     
  4. EvilMudge

    EvilMudge
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    You can deduce the slew rate from the amplifiers upper frequency response limit, or rather you could if they gave you the frequency response and the maximum peak->peak voltage at that frequency. No wonder they don't give out this information.:devil:
     
  5. avanzato

    avanzato
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    Why is the world of audio never easy? :rolleyes:

    I shall look upon those that do publish such figures in a more favourable light than those that don't. Not that it will stop me buying something if it looks nice :D
     
  6. nathan_silly

    nathan_silly
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    Also read up on channel seperation, signal to noise, dynamic range, damping factor.
     
  7. avanzato

    avanzato
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    Is there anywhere that references what good, bad, or indifferent figures for the various specs are?
     
  8. alexs2

    alexs2
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    In many ways,these figures are somewhat outdated....in the 70's and 80's,the Japanese manufacturers and some US ones also,went to town on this,and produced some amazing measuring amps,that sounded awful,like the Crown DC300A and some of the Accuphase amps of that time.
    If you remember that some of the best sounding amps of today(like Audionote's Ongaku,and other SET's)have awful lab figures,but sound beautiful,you can see the point.
    By all means look at the figures,but listen to the amp also,with your own speakers,and don't be blinded by the figures.
     
  9. avanzato

    avanzato
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    I know specs don't tell the whole story but they are extra knowledge and I like to work from the widest base possible.

    I also remember the story from a lecturer in my college electronics course of how the first Japanese transistor amps measured so badly that they invented complete new measuring systems to make then look good. I don't know what the terms are cos that's about the only electronics lesson I went to.

    I have since learnt that going to classes is a good idea. :lesson:
     

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