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  • marx
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Slew-induced distortion
I briefly touched on this in another thread. Thought I might as well continue this topic in a new thread.

Quote
In the design of amplifiers there is more to consider than just attending to the frequencies perceivable by humans.

If frequencies that are beyond that of the human hearing are taken out there is now a void in the frequency range that needs to be filled. If I remember correctly this phenomenon is called slew-induced distortion. Happy to be corrected.


I should have noted that, that the void ends up getting filled with harmonics that we can hear. I would be interested to hear other peoples views on this as what I have written above is about all I know on this subject.

Do you think it would be possible to do DBT on this phenomenon? Would there be any point given that it can already be measured?

  • Woodinville
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Slew-induced distortion
Reply #1
I briefly touched on this in another thread. Thought I might as well continue this topic in a new thread.

Quote
In the design of amplifiers there is more to consider than just attending to the frequencies perceivable by humans.

If frequencies that are beyond that of the human hearing are taken out there is now a void in the frequency range that needs to be filled. If I remember correctly this phenomenon is called slew-induced distortion. Happy to be corrected.


I should have noted that, that the void ends up getting filled with harmonics that we can hear. I would be interested to hear other peoples views on this as what I have written above is about all I know on this subject.

Do you think it would be possible to do DBT on this phenomenon? Would there be any point given that it can already be measured?


This "void" in the frequency range does not need to be filled, and "slew induced distortion" does not necessarily "fill" any particular part of the spectrum.

Slew rate limiting, and internal-state-clipping as a result, are both simple issues dealt with in the 1940's in analog computer amplifiers, for which the knowlege evaded the audio industry for many years beyond the demise of 6V6GT's.

Any effects from far out of band signals in this 'void' that isn't a void will arise from nonlinearities in some part of the reproduction chain, including the atmosphere.  People have made that mistake enough times already.
-----
J. D. (jj) Johnston

  • marx
  • [*]
Slew-induced distortion
Reply #2
Most of my knowledge.... perhaps all of my knowledge has come from Randy Slones; High Power Audio Amplifier Construction Manual. If what you are saying is true then perhaps I should start looking for more accurate resources.

  • DVDdoug
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Slew-induced distortion
Reply #3
I remember reading about slew rate limitations in early op-amps...  As I understand it, a slew rate that's "too-low" can create two problems:

1.  It will limit your high-frequency voltage-output.  A preamp might have perfectly flat frequency response at low signal levels, but at high-levels, the high-frequencies will roll-off.    Or, a power amp might be able to put out 100 Watts at 100Hz, but only 10 Watts at 20kHz.  And again, the frequency response might be flat at 10 Watts.  (That's an exaggerated example, of course.  I'm sure there are no real amplifiers that are that bad.)

2. It will generate (or affect) harmonics.  A sine wave will begin to look more like a triangle wave, and a square wave will also begin to look like a triangle wave.  (This would be Slewing Induced Distortion.)

If you're designing amplifiers, this is probably important to know (it might help explain why you're getting frequency response or distortion problems).  But from a user/consumer point of view, I wouldn't worry about it.  A properly designed amplifier should be designed & tested for flat frequency response at full-output, and it should be tested for distortion across the full audio spectrum.

P.S.
There is a definition of slew rate on Wikipedia, and a diagram showing how it distorts a square wave.
  • Last Edit: 22 October, 2008, 05:03:10 PM by DVDdoug

Slew-induced distortion
Reply #4
The symptoms Doug describes above have been common knowledge amongst amplifier designers for at least half a century and, when using modern op-amps as part of a power amplifier design, slew-rates are now so fast and the available surplus loop gain so high as to make the effects negligible regardless where you set the upper cut-off frequency of the overall amplifier design, assuming that the designer knows how to design a negative feedback loop properly.

Cheers, Slipstreem. 

  • marx
  • [*]
Slew-induced distortion
Reply #5
Thanks for the info guys it's all useful for me.

I have only been into Hi-fi for about a year and I have recently turned my focus to DIY, so my knowledge is limited, asking questions and receiving straight answers like this is a very helpful way for me to learn.

To be fair to Randy Sloan, I had a look back through the book I mentioned and the slew-induced distortion was in the basics section of the book. No doubt it was put in there to give the reader a very basic understanding of some the considerations, past and present that are needed to design amplifiers.