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Slew Rate
Slew Rate
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Old 21st June 2017, 08:05 AM   #21
mchambin is online now mchambin  France
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It tells that, The maximum voltage slew rate of an undistorted 20kHz sine wave with 40V amplitude is just over 5V/us.
This is about a 100 watt rms sine signal in 8 ohm.
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Old 21st June 2017, 08:18 AM   #22
mchambin is online now mchambin  France
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[QUOTE]The topology of the amp causes its distortion to rise significantly as it approaches its slew rate limit. Some margin is needed. This will depend on the particular circuit of the amplifier.[\QUOTE]
I see no reason for this. The topology doesn't.t know slewing is going to happen.
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Old 21st June 2017, 08:19 AM   #23
Kay Pirinha is offline Kay Pirinha  Germany
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Ok, got it. Thank you!

Best regards!
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Old 21st June 2017, 08:25 AM   #24
bimo is offline bimo  Indonesia
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Quote:
Originally Posted by mchambin View Post
It tells that, The maximum voltage slew rate of an undistorted 20kHz sine wave with 40V amplitude is just over 5V/us.
This is about a 100 watt rms sine signal in 8 ohm.
Undistorted? I think you can not make a 100W amplifier, 5V/uS slew rate, with THD below 0,01% at 20kHz, 40V peak, 8 Ohm load. Even in simulation....
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Old 21st June 2017, 09:39 AM   #25
DF96 is offline DF96  England
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Quote:
Originally Posted by mchambin
I see no reason for this. The topology doesn't.t know slewing is going to happen.
It does. In almost all amps the slew rate limit is set by some signal current (which has a limit) charging a capacitance. As the signal current increases, but below the limit, it is likely that distortion increases in whatever active device is providing that signal current. How this happens depends on the amp topology, which is why people with different amps argue about what slew rate limit is needed.

If 5V/us is enough for an undistorted 20kHz at 40V peak, then if you could make an amp which could do this with low distortion then it would be fine for audio. It is difficult because of the distortion generated on the approach to slew rate limiting because of the extra signal current needed. Hence we go for more slew rate - and then some people mistakenly think that their amp/ears/speakers are capable of appreciating 50V/us. You may need a design level of 50V/us in order to ensure that the 5V/us actually needed by the signal can be delivered with low distortion. In reality you may need less than this as the maximim slew rate seen in any real audio signal corresponds to full amplitude sine at some kHz, not 20kHz.

So you have a 50V/us amp to deliver 5V/us cleanly, but in reality your music never has more than 2V/us in the signal - but bigger numbers look better in marketing or bragging.
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Old 21st June 2017, 10:13 AM   #26
jcx is offline jcx  United States
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and its usually much cheaper to have 10x the required slew rate than 10x the required output I or V
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Old 21st June 2017, 10:14 AM   #27
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Quote:
Originally Posted by mchambin View Post
It tells that, The maximum voltage slew rate of an undistorted 20kHz sine wave with 40V amplitude is just over 5V/us.
This is about a 100 watt rms sine signal in 8 ohm.
Do signals we listen for our pleasure ever exceed the slew rate of a full amplitude of a 2 kHz sine wave ? Are the volume of an audio system often set such as the maximal output of a power amplifier is reached ?
For the lab, a low harmonic distortion at 10 kHz is a largely sufficient clue showing an adequate slew-rate handling. It's a criterion used by Bob Cordell, I think.
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Old 21st June 2017, 10:21 AM   #28
bimo is offline bimo  Indonesia
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Originally Posted by DF96 View Post
....
So you have a 50V/us amp to deliver 5V/us cleanly,....
Agree.

But if you can make 90V/uS amp with low THD, and stable, why limit it for lower slew rate?
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Old 21st June 2017, 10:41 AM   #29
DF96 is offline DF96  England
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Originally Posted by bimo
But if you can make 90V/uS amp with low THD, and stable, why limit it for lower slew rate?
In electronics, as in all engineering, increasing one parameter always carries the risk of reducing another one either because physics says so or because the attention of the designer has been diverted. Hence there is a need for caution in aiming higher than necessary, especially after a margin for error has already been included.

For example, what if raising the slew rate means raising quiescent current but that means more noise? How do you balance noise against slew rate? Or raise slew rate by reducing the relevant capacitance but that leads to instability with some loads. How do you balance load tolerance against slew rate?
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Old 21st June 2017, 11:26 AM   #30
mchambin is online now mchambin  France
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Overkill is an immediate cost increase, too.
And prone to eliminate easy solutions, easy components, if not making things impossible.
In a word, overkill is bad engineering practice.
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Last edited by mchambin; 21st June 2017 at 11:32 AM.
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