Slew Rate

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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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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
 
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.
 
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.
 
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.
 

PRR

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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".
 
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.
 
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.
 
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.
 
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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