Amplifier Slew Rate What is it?

A

admin

Audioholics Robot
Staff member
When reviewing the spec sheet of an amplifier, one potentially unfamiliar term you may run into is slew rate. There are a lot of gobbledygook explanations floating around the web which seem to misunderstand the basic premise, so we at Audioholics are here to clear the air. In short, slew rate has little to do with how an amplifier produces dynamics so much as its ability to effectively maintain output into higher frequencies. It is the rate of how quickly an amplifier can respond to a rapid change of input level. This is measured as a change in voltage with respect to time as can be seen in the main image of this article. We discuss how to calculate slew rate based on amplifier bandwidth and power and also discuss real world implications of the spec.


Discuss "Amplifier Slew Rate What is it?" here. Read the article.
 
slipperybidness

slipperybidness

Audioholic Warlord
You’ll notice that as you go up in frequency, the amplitude of the sine wave changes at a greater rate than at lower frequencies

I think you may have made an error here, simply mis-spoke, or maybe just a little confusion to clear up???

Amplitude is usually defined as the Peak Value. In that usage, the amplitude (peak level) isn't changing. Rather, it is the slope that gets steeper at higher frequencies. Meaning that you hit your peak value faster, larger value of angular momentum in the phasor model.

Perhaps you meant to say that the instantaneous value is changing at a faster rate? That make sense.

May be a bit of splitting hairs, but want to be sure we are all on the same page.
 
gene

gene

Audioholics Master Chief
Administrator
You’ll notice that as you go up in frequency, the amplitude of the sine wave changes at a greater rate than at lower frequencies

I think you may have made an error here, simply mis-spoke, or maybe just a little confusion to clear up???

Amplitude is usually defined as the Peak Value. In that usage, the amplitude (peak level) isn't changing. Rather, it is the slope that gets steeper at higher frequencies. Meaning that you hit your peak value faster, larger value of angular momentum in the phasor model.

Perhaps you meant to say that the instantaneous value is changing at a faster rate? That make sense.

May be a bit of splitting hairs, but want to be sure we are all on the same page.
You know I must have read it a few times and missed that. Thanks I will tweak the verbiage.

I changed it to:

You’ll notice that as you go up in frequency, the value of the sine wave changes at a greater rate than at lower frequencies: this is what slew rate is all about. Simply, slew rate tells us how high in frequency an amplifier can play for a given voltage level.
 
SaviorMachine

SaviorMachine

Enthusiast
Very informative

Thanks so much! I'd never even heard of these terms, newbie that I am.
 
3db

3db

Audioholic Overlord
Its a spec that doesn't get posted very often. I would like to know what the relationship is between the slew rate and an amplifier's ringing to an impulse response.
 
M Code

M Code

Audioholic General
A slew rate spec by itself has little meaning...
To understand its importance, one has to examine other specs including open loop bandwidth, and amount of negative feedback used. A good sounding, stable amplifier requires careful attention to each component particularly the voltage/current of the output devices..

An amplifier with a high slew rate spec but with high negative feedback and limited open loop bandwidth will not have the dynamics and punch especially when driven hard.. In the old analog source days, these specs were less crucial but now with the available high dynamic range, lossless digital streams they now become very pertinent..

Just my $0.02... ;)
 
3db

3db

Audioholic Overlord
I haven't talked bout this topic in many many years so bare with me. Maybe I'm confusing what you are saying mcode with open loop gain bandwidth. I've studied amplifiers and its been my experience that the gain of an amp stage is inversely proportional to its bandwidth capability. Meaning an amp with open loop gain is by far more restricted on bandwidth than an amp with unity gain. I figure that an amp stage with lower gain would have much higher slew rates compared to that of stages where gain is high because its not hitting up against the open loop bandwidth curve inherent in all amps.
 
M

mp3streetparty

Audioholic Intern
In the early 90' when Mosfet amps came out a Slew Rate of 50 V/uS was a good figure from memory.

My understanding of it is that it relates to the response to the Amps response to harmonic frequency's in a signal.

For example a 10 mHz bandwidth analog cathode ray tube is pushing to display 4 mHz square wave signal because there is lots and lots of odd harmonics in a square wave.

So for audio the higher the Amplifiers bandwidth the better.

The Harman Kardon AVR 1565 boasts a slew rate of 40V/usec as far as I know has nothing to do with the power output it all about how well it can respond to odd and even harmonic frequency's of the signal.
 
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slipperybidness

slipperybidness

Audioholic Warlord
You know I must have read it a few times and missed that. Thanks I will tweak the verbiage.

I changed it to:

You’ll notice that as you go up in frequency, the value of the sine wave changes at a greater rate than at lower frequencies: this is what slew rate is all about. Simply, slew rate tells us how high in frequency an amplifier can play for a given voltage level.
That does make the presentation of the info a little more clear and concise.

I want to add that this is one of the things that I really appreciate about this site. You didn't take offense or get defensive when I pointed this out, but rather thanked me for pointing it out.

It takes a lot of confidence and a real desire to be as accurate as possible to respond in such a manner. I suppose one way to look at a correction isn't criticism, but rather peer-review. In the general scientific community, peer-review is a critical part of the publishing process.
 
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Steve81

Steve81

Audioholics Five-0
You didn't take offense or get defensive when I pointed this out, but rather thanked me for pointing it out.
Gene presents a cool, collected front, but I'm sure the author had to cry himself to sleep for the last couple nights.

On the up side, if your correction is the worst thing that happens here, I've seen much worse forum receptions to an article or two in the last few months.
 
gene

gene

Audioholics Master Chief
Administrator
That does make the presentation of the info a little more clear and concise.

I want to add that this is one of the things that I really appreciate about this site. You didn't take offense or get defensive when I pointed this out, but rather thanked me for pointing it out.

It takes a lot of confidence and a real desire to be as accurate as possible to respond in such a manner. I suppose one way to look at a correction isn't criticism, but rather peer-review. In the general scientific community, peer-review is a critical part of the publishing process.
One thing you will always find with Audioholics is I don't consider any of us journalists. Journalists typically have too big of an ego to allow their work to be peer reviewed and corrected/edited when needed. On the other hand, I an much of my staff are degreed engineers. It's in our nature to want our work peer reviewed and scrutinized for accuracy. Technically the term "changing amplitude" as originally stated in the article could slide as being considered acceptable but why not clarify it if it causes confusion among the readers?

We always appreciate constructive feedback to our articles. thanks again.
 
C

cvi4u

Audiophyte
I thought that slew rate in an amplifier is an indication of the amplifiers' ability to follow signal not to create one. If signal on the input is changing at 1V/us, and A=1, amplifier with 1V/us slew rate will be severely lagging in response thus creating distortion and time delay. Time delay will change with the change in slew rate of the input signal. I might be completely wrong so can someone who better understands the issue shine some light on it.
 
Steve81

Steve81

Audioholics Five-0
I thought that slew rate in an amplifier is an indication of the amplifiers' ability to follow signal not to create one.
Not sure I follow you here. You see how an amplifier is tracking the input signal by looking at its output.

Time delay will change with the change in slew rate of the input signal.
There's no time delay to speak of; the amplifier is effectively responding to the (variable) input in real time. For example, if an amplifier is fed a high frequency transient peak that it doesn't have sufficient slew rate to reproduce, the peak will be dulled but the amplifier would keep on trucking AFIAK. Things like ringing are separate from looking purely at slew rate.
 
slipperybidness

slipperybidness

Audioholic Warlord
I thought that slew rate in an amplifier is an indication of the amplifiers' ability to follow signal not to create one. If signal on the input is changing at 1V/us, and A=1, amplifier with 1V/us slew rate will be severely lagging in response thus creating distortion and time delay. Time delay will change with the change in slew rate of the input signal. I might be completely wrong so can someone who better understands the issue shine some light on it.
A couple good items from the wiki on slew rate:
Slew rate - Wikipedia, the free encyclopedia
The slew rate of an electronic circuit is defined as the maximum rate of change of the output voltage
Slew rate helps us to identify what is the maximum input frequency and amplitude applicable to the amplifier such that the output is not distorted
 
RichB

RichB

Audioholic Field Marshall
What specification best describes the speed of an amplifier or its ability to deliver current?

For example, when I was looking at the Emotiva amps, I interested in the XPA-2 or the XPR series.
The XPA was described as more "linear". I took that to mean that the XPA was able to better deliver power as required.

- Rich
 
Steve81

Steve81

Audioholics Five-0
What specification best describes the speed of an amplifier or its ability to deliver current?
I'm not sure how you would qualify "speed" with respect to an amplifier beyond slew rate. To figure current delivery, you can look at rated output into low impedance loads. For example, an amplifier that is specified to deliver 100 watts into 4 ohms is swinging 20 volts and delivering 5 amperes of current. For more information on that (formulas and such):
Understanding Ohm's Law, Impedance And Electrical Phase 101 | Audioholics

For example, when I was looking at the Emotiva amps, I interested in the XPA-2 or the XPR series.
The XPA was described as more "linear". I took that to mean that the XPA was able to better deliver power as required.
I'm assuming they're referring to the differences between a class A/B amplifier (XPA) and class H (XPR) which we've talked about before. However, Emotiva would be better able to clarify versus my guesses.
 
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C

cyclebrain

Audiophyte
What is the difference between slew rate and bandwidth?
Given a fixed voltage swing, only frequency will have an effect on rate of change.
 
jinjuku

jinjuku

Moderator
Bandwidth is the total frequency the amp can swing through from crest to trough. Slew is how quickly it can do so. Amps are basically frequency agile power supplies.
 
Steve81

Steve81

Audioholics Five-0
What is the difference between slew rate and bandwidth?
Given a fixed voltage swing, only frequency will have an effect on rate of change.
One other way of wording it:
Amplifier Sound - What Are The Influences?

Slew Rate # : Closely related to power bandwidth, the slew rate is the maximum rate of change (measured in Volts per microsecond) of the amplifier output. The higher the amplifier power, the higher the slew rate must be to obtain the same power bandwidth.
 

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