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Slew Rate
Slew Rate
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Old 20th June 2017, 11:05 PM   #1
mchambin is offline mchambin  France
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Default Slew Rate

What is acceptable ?
For instance 20 V / uS gives no distortion on a 100 Watt rms sine signal in 8 ohm at 1KHz....BUT, at 20 Khz, distortion comes in at 10 Watt and becomes bad over 10W.
This is theoretical results about pure sine signals. What about real audio ? What slew rate is needed for real audio quality ?
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Old 20th June 2017, 11:11 PM   #2
hitsware is offline hitsware
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A rule of thumb is 1/2 volt/us for each peak volt .
So for (say) +&- 15 volt rails, ~ 7 volt/us
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Old 20th June 2017, 11:33 PM   #3
Mark Johnson is offline Mark Johnson  United States
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Slew Rate
Bob Cordell's power amplifier book makes a recommendation for the bare minimum slew rate that an amplifier should meet or exceed. It's in Section 6.1 of the book.

I, personally, happen to think Bob's method of analysis is excellent, but his final recommended number is not conservative enough; I think it is too low. I, personally, think that 1 volt/usec per volt of peak signal swing is a good target to aim for. {this is 2X what member hitsware has suggested}.

So an amplifier capable of delivering 100 Watts into an 8 ohm load, needs a slew rate of about 40 volts per microsecond to achieve the target I recommend.

Math: 100 watts = power = (Vrms * Vrms / 8 ohms) ; thus Vrms = 28.3 V

Vpeak = sqrt(2)*Vrms = 40 volts.

Minimum Acceptable Slew Rate = 1 V/us/volt * 40 volts = 40 V/us
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Old 21st June 2017, 12:17 AM   #4
mchambin is offline mchambin  France
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100W rms sine signal in 8 ohm is a 40V peak amplitude.
For such amplifier output, by this rule of thumb the Slew Rate should be 20V/ uS.

100Watt, 8 ohm -----> 20Volt/microSecond.
Right ?
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Old 21st June 2017, 12:20 AM   #5
Mark Johnson is offline Mark Johnson  United States
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Slew Rate
Wrong.

1 volt/usec/volt * 40 volts = 40 volts/usec

{this is 2X what member hitsware has suggested}.
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Old 21st June 2017, 12:28 AM   #6
john curl is offline john curl  United States
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Walt Jung and I think 50V/us min for 100W out is better, there is more to highest quality audio than 20KHz bandwidth.
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Last edited by john curl; 21st June 2017 at 12:32 AM.
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Old 21st June 2017, 12:36 AM   #7
DPH is offline DPH  United States
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Plenty of thoughts on this, Mark's rec ends up being a power bandwidth (which is really the figure of merit rather than slew rate, but they're entirely relatable) of about 150 kHz, hitsware is about 75 kHz.

Ostensibly you *really* only need a power bandwidth equivalent to signal bandwidth, but obviously over-provisioning is worthwhile. You'll get generally anywhere from 3x - 10x audio bandwidth.
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Old 21st June 2017, 12:42 AM   #8
Mark Johnson is offline Mark Johnson  United States
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Slew Rate
Cleanly reproduce the 9th or 11th harmonic of 20 kHz might be a goal
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Old 21st June 2017, 01:26 AM   #9
goodlionaudio is offline goodlionaudio  United States
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Default less can be more...

The further you open the window the more garbage comes in...

If you consider CD to be an acceptable media for quality sound production then 4V/us is adequate (80 volt step in 22.6 us). Most single ended tube amplifiers fall in this category and they are ranked highly in terms of sound quality.
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Old 21st June 2017, 01:38 AM   #10
goodlionaudio is offline goodlionaudio  United States
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Default less can be more...

The further you open the window the more garbage comes in...

If you consider CD to be an acceptable media for quality sound production then 4V/us is adequate (80 volt step in 22.6 us). Most single ended tube amplifiers fall in this category and they are ranked highly in terms of sound quality.
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