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Slew Rate
Slew Rate
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Old 3rd January 2018, 12:10 PM   #201
DF96 is offline DF96  England
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Quote:
Originally Posted by rhythmsandy
but wouldn`t that slow down the current slew?
Yes - and do absolutely no harm at all.

Quote:
Originally Posted by MrMagic
I admit I haven't read this huge thread, but here's my opinion about slew rate:
That is not a promising start to a post.

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If your music amplifier is capable to output a full power, undistorted signal at the full audio spectrum (20-20Khz), (incl. a 20Khz sine wave at max amplitude), then slew rate as a parameter is completely redundant to characterize that amplifier and even misleading. Period.
It all depends on what you mean by "undistorted". You cannot mean 'no distortion at all' as that is not possible in this universe. If you mean 'quite low distortion' then it may be helpful to know what is the source of that distortion (e.g. limited slew rate, smooth nonlinearity, crossover glitches). Of course, any circuit parameter can be used in a misleading sense (and most of them often are treated in this way) but that does not mean that the parameter is redundant.

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Even if you care about frequencies that you can't hear, THD along with frequency response is enough and far more informative.
THD tells us something, but not much. Anyway, thank you for confirming to THD-haters that there are genuine THD fans about; they will use this evidence against those of us who have a sober view of THD, by confusing us with genuine THD fans like yourself.
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Old 3rd January 2018, 12:17 PM   #202
MrMagic is offline MrMagic  Greece
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Originally Posted by lcsaszar View Post
True if you feed the amplifier with band limited signal, which is not always the case.
Why, what will happen if you feed an amp characterized at 20-20Khz and ultra-low THD up to the 10nth harmonic, with a signal that extends to 30Khz?
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Old 3rd January 2018, 12:35 PM   #203
MrMagic is offline MrMagic  Greece
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Quote:
Originally Posted by DF96 View Post
That is not a promising start to a post.
Yet it's a honest one.


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Originally Posted by DF96 View Post
It all depends on what you mean by "undistorted". You cannot mean 'no distortion at all' as that is not possible in this universe.
Of course I mean ultra-low distortion.

Quote:
Originally Posted by DF96 View Post
If you mean 'quite low distortion' then it may be helpful to know what is the source of that distortion (e.g. limited slew rate, smooth nonlinearity, crossover glitches). Of course, any circuit parameter can be used in a misleading sense (and most of them often are treated in this way) but that does not mean that the parameter is redundant.
Since the amp has already ultra-low distortion according to the THD parameter, you do NOT care about ...the sources of distortion anymore.
Slew rate might be useful during development, NOT to characterize the amp for the customer/builder.


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Originally Posted by DF96 View Post
THD tells us something, but not much.
THD along with frequency response, output power, power bandwidth and load specs, is all a customer/builder needs, that makes slew rate redundant.

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Originally Posted by DF96 View Post
Anyway, thank you for confirming to THD-haters that there are genuine THD fans about; they will use this evidence against those of us who have a sober view of THD, by confusing us with genuine THD fans like yourself.
I don't have any insecurities with "THD-haters" and I'm a fan of perfection, not just THD, but I'm all ears to enlighten me about something I'm missing. "it doesn't tell us much" is too vague to be considered an argument.

Last edited by MrMagic; 3rd January 2018 at 12:38 PM.
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Old 3rd January 2018, 01:02 PM   #204
DF96 is offline DF96  England
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THD (the clue is in the name) tells you the total harmonic distortion. It does not tell you what the distortion orders are, yet you need to know these. It also does not necessarily tell you everything about IMD, yet this can be more important than harmonic distortion. A "fan of perfection" will need much more than a THD figure to tell him whether an amp has acceptable distortion performance.
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Old 3rd January 2018, 01:22 PM   #205
traderbam is offline traderbam  Europe
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Originally Posted by DF96 View Post
THD (the clue is in the name) tells you the total harmonic distortion. It does not tell you what the distortion orders are, yet you need to know these. It also does not necessarily tell you everything about IMD, yet this can be more important than harmonic distortion. A "fan of perfection" will need much more than a THD figure to tell him whether an amp has acceptable distortion performance.
Yes.
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Old 3rd January 2018, 01:32 PM   #206
traderbam is offline traderbam  Europe
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Amplifier slew rate is what I call a hygiene factor. That is, it needs to meet a minimum standard but is otherwise not relevant in and of itself.

IMO it is analogous to the top speed of a car. It may be a nice sales gimmick to boast a car that can do 150mph but this doesn't tell you anything about how nice it is to drive at normal speeds.

Most home listening is done with digital sources that produce virtually nothing above 20kHz. At 100W average 8-ohms that's 5V/us (and that is extremely loud). Some margin is sensible but where does 50V/us or 100V/us come from? Where is the explanation?

In my experience it is quite easy to exceed the minimum slew rate requirement in a circuit so it is not a sonic concern.

It may be that in some circuit designs, parts of the circuit become very non-linear as they approach the design's slew rate limit. In this case, for this design, it may be necessary to design for, say, 10x the slew rate needed to keep the circuit linear enough. But this is not a requirement of slew rate. This is a side-effect of other factors in the design.
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Last edited by traderbam; 3rd January 2018 at 01:38 PM.
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Old 3rd January 2018, 01:44 PM   #207
scottjoplin is offline scottjoplin  Wales
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Originally Posted by traderbam View Post
IMO it is analogous to the top speed of a car.
Not the 0 to 60 (0 to 100) time when you put the pedal to the metal?
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Old 3rd January 2018, 02:21 PM   #208
traderbam is offline traderbam  Europe
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Originally Posted by scottjoplin View Post
Not the 0 to 60 (0 to 100) time when you put the pedal to the metal?
Happy New Year, Matt.
My analogy likens the rate of change of voltage to the car's speed.
Your analogy likens the rate of change of voltage to the car's acceleration.

A car's acceleration is about power response. It is to do with the engine torque curve and mass of the car. In a power amplifier I'd liken the torque to the current delivery and the mass to the load capacitance. The resistance of the load is like friction.

The voltage slew rate of an amplifier is ultimately defined by a current limit somewhere. The music signal has a maximum dV/dt requirement. It may be that it is beneficial in a certain design to have excess current capability that also happens to allow the amp to far exceed the music signals dV/dt, but that is not actually a slew rate requirement.

The reason I am going on and on about this is because I have seen some designs where high slew rates have been achieved at the expense of far more important sonic parameters. It's a bit like THD - there are plenty of speakers with 1% THD that sound wonderful and plenty of amps with 0.001% THD that sound unimpressive.

Basically, I am saying don't get fixated with the pursuit of excessive slew rate...look for what actually affects the sound quality.
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Old 3rd January 2018, 02:57 PM   #209
MrMagic is offline MrMagic  Greece
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Originally Posted by DF96 View Post
THD (the clue is in the name) tells you the total harmonic distortion. It does not tell you what the distortion orders are, yet you need to know these. It also does not necessarily tell you everything about IMD, yet this can be more important than harmonic distortion. A "fan of perfection" will need much more than a THD figure to tell him whether an amp has acceptable distortion performance.
If THD is medium, then yes, you need to check the causes of the existing distortion, but if it is "ultra-low" as was the requirement in my first post eg around or less than -120db, that practically means that there is NO distortion for you to care, at all.
THD is a linearity measurement and so is IMD. For example, I'm developing a high voltage SS ES amp for some time now and i have never measured IMD. THD was always my guide, along with frequency response (and noise), but I was measuring THD at 20Khz, not just 1Khz, on an amp with a bandwidth that would exceed 1Mhz (if the mosfets could cope with the extremely high capacitive currents).
So I just did that now and IMD harmonics are even lower: -128db is the highest harmonic in FFT @20Khz fundamental and -132db is the highest harmonic in IMD measurement with 20Khz and 19Khz tones.
And power bandwidth replaces slew rate and is far more intuitive and meaningful. If your amp has the power bandwidth you desire along with ultra-low THD specs, it also has the proper slew rate.

So if you properly measure the other important parameters, you don't need slew-rate which is just a dependent subset meant for other purposes.


As for perfection, I mentioned 5 parameters on the previous post, not just THD and there are more, like stability, reliability, efficiency, transistor/mosfet protection, noise, etc.
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Old 3rd January 2018, 05:13 PM   #210
mchambin is offline mchambin  France
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I am glad to see this thread going back on topic.
Post #1 is:
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 ?
I am glad to see in the last answers remarks on the technical realities of slew rate, well apart from bogus arguments.
So I read as much I could stand in this long thread to gather informations and make my own opinion.
IMHO Slew rate is redundant, umoungous slew rate values are non sense.
An amplifier needs no more slew rate as necessary to not induce distortion.
Correct design provisions enough head room about voltage and current to avoid distortion from clipping.
As well, correct design must provision enough margin about slew rate ( voltage/uS and current/uS ) to avoid distortion
We do not design for 100V head room for 10V signals, designing for 50V/uS is a similar non sense when 5V/uS is enough.
Another interesting consideration is: How much slew rates do we have in real audio sources. All audio souces are band limited, there cannot be umongous slew rates in true audio.
Another interesting point, is the relevance of THD. Yes it is not perfect, but if we must point at the best measure ? do we have any better ?
What we ask from an ampifier is an output which is a best fit with the amplified input.
Mathematicians, to measure ( compare ) how two curves fit, came with the mean square root method. This is precisely what THD is.
THD 20Khz and IMD 20Khz / 19Khz are worthwhile measures. Slew rate is nothing more than a side issue.
High slew rates are met with large amplitude signals AND large frequencies ( SR = 2 Pi F Vpk ).
About a 100 Watt in 8 ohm amplifier ( Vpk is 40V ) There exist no signal at full power 100 Watt beyond, say 10 Khz, your tweeters would explode and your ear drums bleed.
I admit, we need a bandwith of up to 80 Khz, but not at full power from 10Khz to 80 Khz.
IMO, I think slew rate required to avoid distortion at 10 Khz full power is enough, and that's all I need for an amplifier.
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