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Slew Rate
Slew Rate
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Old 21st June 2017, 01:53 AM   #11
PRR is offline PRR  United States
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> a power bandwidth ...of about 150 kHz ...about 75 kHz

Speech/music will NOT be full-power past 6KHz; your ears would bleed (until your tweeters croaked).

But! When actually slewing, NFB is dead-zero. Long before that, NFB is weak. It would be reasonable to provision for 10X-20X headroom between the signal and the slew limit, so you still have 10X-20X of NFB in the 6KHz range. So 60KHz-120KHz (or 75K-150K) power bandwidth is probably a good target for high-NFB amps.

> single ended tube amplifiers

These also typically have more linear power devices and little NFB in normal operation. Pushing to "near" slew limit may not cause as much distortion as high-NFB systems.

Also the "6KHz" number comes from heavy rock and disco. If your tastes are more mellow, you may have few to no strong transients over 2KHz. So "it depends".
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Old 21st June 2017, 02:32 AM   #12
john curl is offline john curl  United States
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Good--- you would have been correct in you assumption 50 years ago, but 40 years ago Walt Jung and Matti Otala independently proved that 50V/us for a 100W amp was more correct. They both wrote long papers on the subject.
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Old 21st June 2017, 02:51 AM   #13
bimo is offline bimo  Indonesia
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You should compare by yourself 100W amplifier with different slew rate. If you not know the different which one is 20V/uS and 40V/uS, you do not need higher slew rate than 20V/uS.

It is easy to design a 100W amplifier with 40V/uS slew rate. I design a 100W amplifier with 90V/uS using blameless topology (according Douglas Self). It is stable.
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Old 21st June 2017, 03:09 AM   #14
goodlionaudio is offline goodlionaudio  United States
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Default JC, more is better...

Yes, I am aware there is a big difference between theory and practice. Personally, I like to go for the more is better approach all the time but then again feel humbled by how good something can sound by not paying attention to these theories. That's why I consider audio reproduction to be an art form; a meeting place of theory and practice. We must keep our minds and ears open to everyting under the sun; we're definitely not there yet.
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Old 21st June 2017, 04:06 AM   #15
john curl is offline john curl  United States
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When it comes to tubes, we find that the slew rate number can be much lower. It apparently comes from a different mechanism than OP-AMP based TIM that is resident in both op amps and solid state power amplifiers. Also the 'open loop bandwidth' in tube designs is almost always much higher that op amp based amp designs. This might be even more important than simple slew rate measurements.
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Old 21st June 2017, 05:52 AM   #16
Kay Pirinha is offline Kay Pirinha  Germany
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Quote:
Originally Posted by Mark Johnson View Post
Cleanly reproduce the 9th or 11th harmonic of 20 kHz might be a goal
And you're sure you're able to hear these harmonics?

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Old 21st June 2017, 06:40 AM   #17
jcx is offline jcx  United States
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and something must be broken in your open loop gain, or you're clipping or current limiting to have 9th or higher harmonics above even -40 dBc
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Old 21st June 2017, 07:19 AM   #18
traderbam is offline traderbam  Europe
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Quote:
Originally Posted by mchambin View Post
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 ?
The maximum voltage slew rate of an undistorted 20kHz sine wave with 40V amplitude is just over 5V/us

This is the minimum a 100W amplifier must do.

There are a couple of reasons why one might need to make an amplifiers slew rate limit much higher:
1. The amp is required to amplify frequencies higher than 20kHz.
2. 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.
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Old 21st June 2017, 07:30 AM   #19
mchambin is offline mchambin  France
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The math is:
40 x 2pi x 20 000.
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Old 21st June 2017, 07:56 AM   #20
Kay Pirinha is offline Kay Pirinha  Germany
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So you get a number of 5.026.548. What does it tell to us?

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