What is Slew Rate

Jeroleen · 7th February 2005

What does it mean, what does it do and how does it effect what we hear?

EngineEars · 7th February 2005

Here is a thread that I started. I would love to learn more is there is more info to be had.

tINY · 7th February 2005

Slew rate is the change in voltage over time. If you hook an amplifier up to a switch and switch it from -10 to +10v, instantly, the amplifier will not change its output instantly. It "slews" at a certain rate: usually this is expressed in Volts per micro-second.

The biggest thing that you can hear is the slew rate effect on the highest frequency that an amplifier will produce at a given gain.

The "colored" tube pre-amps are often designed this way. There may be feedback controling the bandwith of the preamp, but the first stage will not have enough slew rate to keep up with the signal at the gain it is set to. This causes mild to severe distortion and is usually considered pleasing especially when the rising and falling slew rate aren't the same.....

-tINY

Hiwatt · 8th February 2005

Check out the Vipre by Groove Tubes it has a variable slew rate control. Definatly a cool piece of gear.

Ethan Winer · 8th February 2005

Jeroleen,

> What does it mean, what does it do and how does it effect what we hear? <

tINY has the right answer, and I'll add one point: The reason that hitting the slew rate limit of an amplifier distorts music is because it turns every waveform into a triangle wave.

--Ethan

**Tim Farrant** · 9th February 2005

I posted some additional thoughts on slew rate and risetime here...

http://www.3daudioinc.com/cgi-bin/ul...c&f=1&t=001134

Also this from Horowitz and Hill - The Art of Electronics....

"The slew rate of an amplifier will limit the amplitude of a sinewave output at some critical frequency (the frequency at which the full supply swing requires the maximum slew rate of the amplifier). A sinewave of frequency f (hertz) and amplitude A (volts) requires a minimum slew rate S of 2(pi)Af volts per second."

Therefore, if you wish to reproduce a 20kHz sinewave without distortion at +20dBu (22 volts peak-to-peak), the device must have minimum slewrate of 2,764,601.5 volts per second = approx 2.7V per micro second.

Does this make sense to you Jeroleen?

Cheers
Tim

Jeroleen · 9th February 2005

Not the math but the concept does. That is interesting information. Thank you.

Seems to me the main reason why musicians need to know this kind of information to some degree nowadays is because we are all, to some degree, acting as our own engineers.

That means we must maintain some type of recording system which also means we are fodder for the advertising hype mill.

Without at least a basic understanding, the manufacturers can sell us with concepts such as tube "warmth" as the antidote for digital harshness.

From the people on this site I have learned that the sound we associate with "warmth" is probably more attributable to transformers than tubes.

If I am not reading this incorrectly, it seems that slew rate is also a factor in the hackneyed pursuit of "warm" tones.

**Tim Farrant** · 9th February 2005

Jeroleen,

I would say that slew rate only really effects the top end of the audio band where the speed of the amplifier has an effect on the ability of the device to reproduce transients.

The "warmth" you speak of is more to do with distortion artifacts. Transformers (depending upon their construction and the type of metal used in the core) can generate harmonic distortions at low frequencies. The effect to the ear is like hearing soft overtones to the fundamental note (given a pure tone). Obviously with a complex waveform such as "music" then the overtones become very complex too.

Tubes can also generate quite high harmonic distortions, depending on the circuit they are in so the combination of tubes and transformers can be VERY warming indeed.

Well that's my slant on it anyway.

Tim

jbuntz · 10th February 2005

Something interesting that Tim told me (correct me if I misquote) is that when slew rate is limited by current and distorts, it can make sounds brighter since it's actually a distortion (usually mainly 3rd harmonic if I'm not mistaken) and generates harmonics on top of some of the higher frequencies.

By contrast, transformers and negative feedback seem to have more of a softening affect of transients or impulses.

Ethan Winer · 10th February 2005

JB,

> when slew rate is limited by current and distorts, it can make sounds brighter since it's actually a distortion <

Sure. It may seem counter-intuitive because the waveforms look so different, but slew rate limiting and simple clipping can sound pretty similar.

> transformers and negative feedback seem to have more of a softening affect of transients or impulses. <

Transformers can reduce highs, but negative feedback is entirely different and it always reduces distortion and other anomalies.

--Ethan

tINY · 10th February 2005

Just to tweak your head:

The slew rate isn't always constant when measured at different voltage levels. Tubes and FETs tend to have slower slew near the bottom of their operational range than near the top. This can induce even order harmonics (which is said to be more "musical")

-tINY

Jeroleen · 12th February 2005

While we are on the subject correct me if I have this wrong. Negative Feedback. Is this the output of a device being re-routed through itself or something like that.

Why is this done and what would happen without it?

Ethan Winer · 12th February 2005

Jeroleen,

> Negative Feedback. Is this the output of a device being re-routed through itself or something like that. Why is this done and what would happen without it? <

Negative feedback is used to make the output of an amplifier circuit more closely resemble the input. That is, it improves the frequency response and lowers distortion. For example, an oven with a thermostat is an application of negative feedback. When the heat rises too high the thermostat turns off, and when the temperature drops too low it switches back on. The net effect is to maintain a constant temperature, which is a linear condition. Likewise with a toilet, where the valve and float work together to maintain a constant water level in the tank.

--Ethan

**Tim Farrant** · 14th February 2005

Quote:

Originally posted by jbuntz
Something interesting that Tim told me (correct me if I misquote) is that when slew rate is limited by current and distorts, it can make sounds brighter since it's actually a distortion (usually mainly 3rd harmonic if I'm not mistaken) and generates harmonics on top of some of the higher frequencies.

Indeed, do not assume that because a device has a limited slewrate it will sound soft. On the contrary, if you subject a "slow" amplifier to a fast transient, it may well distort it by exaggerating the edge or ringing and introducing further artifacts/harmonics etc, so a "click" might turn into a "clank"

Tim..

tINY · 14th February 2005

Quote:

Originally posted by Ethan Winer
Jeroleen,
Likewise with a toilet, where the valve and float work together to maintain a constant water level in the tank.

So, you can lower the slew rate by partially closing the valve that feeds the bottom of the tank....

-tINY

7th February 2005	#1
Jeroleen Gear nut Join Date: Oct 2004 Posts: 84 Thread Starter	What is Slew Rate What does it mean, what does it do and how does it effect what we hear?

8th February 2005	#5
Ethan Winer Gear Guru Join Date: Oct 2002 Location: New Milford, CT, USA Posts: 12,048	Re: What is Slew Rate Jeroleen, > What does it mean, what does it do and how does it effect what we hear? < tINY has the right answer, and I'll add one point: The reason that hitting the slew rate limit of an amplifier distorts music is because it turns every waveform into a triangle wave. --Ethan __________________ Ethan's audio book is coming!

9th February 2005	#6
Tim Farrant Moderator Join Date: Jun 2002 Location: New Zealand Posts: 2,309	I posted some additional thoughts on slew rate and risetime here... http://www.3daudioinc.com/cgi-bin/ul...c&f=1&t=001134 Also this from Horowitz and Hill - The Art of Electronics.... "The slew rate of an amplifier will limit the amplitude of a sinewave output at some critical frequency (the frequency at which the full supply swing requires the maximum slew rate of the amplifier). A sinewave of frequency f (hertz) and amplitude A (volts) requires a minimum slew rate S of 2(pi)Af volts per second." Therefore, if you wish to reproduce a 20kHz sinewave without distortion at +20dBu (22 volts peak-to-peak), the device must have minimum slewrate of 2,764,601.5 volts per second = approx 2.7V per micro second. Does this make sense to you Jeroleen? Cheers Tim __________________ "Come on, make my day" Dirty Harry (Clint Eastwood) Visit Buzz Audio

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7th February 2005	#2
EngineEars Gear addict Join Date: Jul 2004 Location: LA Posts: 460	Here is a thread that I started. I would love to learn more is there is more info to be had.

7th February 2005	#3
tINY Gear Guru Join Date: Jul 2004 Location: Orygun Posts: 10,206	Slew rate is the change in voltage over time. If you hook an amplifier up to a switch and switch it from -10 to +10v, instantly, the amplifier will not change its output instantly. It "slews" at a certain rate: usually this is expressed in Volts per micro-second. The biggest thing that you can hear is the slew rate effect on the highest frequency that an amplifier will produce at a given gain. The "colored" tube pre-amps are often designed this way. There may be feedback controling the bandwith of the preamp, but the first stage will not have enough slew rate to keep up with the signal at the gain it is set to. This causes mild to severe distortion and is usually considered pleasing especially when the rising and falling slew rate aren't the same..... -tINY

8th February 2005	#4
Hiwatt Gear addict Join Date: May 2004 Location: Canada Posts: 389	Check out the Vipre by Groove Tubes it has a variable slew rate control. Definatly a cool piece of gear.

10th February 2005	#9
jbuntz Lives for gear Join Date: Jul 2003 Posts: 1,716	Something interesting that Tim told me (correct me if I misquote) is that when slew rate is limited by current and distorts, it can make sounds brighter since it's actually a distortion (usually mainly 3rd harmonic if I'm not mistaken) and generates harmonics on top of some of the higher frequencies. By contrast, transformers and negative feedback seem to have more of a softening affect of transients or impulses.

12th February 2005	#12
Jeroleen Gear nut Join Date: Oct 2004 Posts: 84 Thread Starter	While we are on the subject correct me if I have this wrong. Negative Feedback. Is this the output of a device being re-routed through itself or something like that. Why is this done and what would happen without it?