What Is Amplifier Slew Rate & Does It Affect Performance?

If you've stumbled upon this article, you've likely been confused about the slew rate specification on an amplifier specifications sheet and want to know more about this rare and odd data point.

What is amplifier slew rate, and how does it affect performance? Amplifier slew rate is a measurement of how quickly an amplifier is able to respond to a change in input level, typically measured in volts per microsecond. Though this spec seems related to dynamic responsiveness, it actually has more to do with the amplifier's high-frequency response.

In this article, we'll talk about what slew rate is and what it isn't, examples of slew rate specifications in various amplifiers, and how slew rate affects the performance of an amplifier.

What Is Slew Rate?

Slew rate, in electronics terminology, is defined as the change of voltage, current, or any other electrical quantity, per unit of time.

With amplifiers (and other signal processors), the slew rate generally refers to the alteration (distortion) that happens to the signal as it is processed. The changes in voltage (both slow and fast) at the input are somewhat lagged at the output.

The presence of this distortion, on the surface, seems to affect the dynamics (particularly the transients) of the audio waveform, which it certainly does. However, in practice, slew rate has more of an effect on high-frequency clarity.

Slew rate is most easily understood with a simple illustration of an input square wave and an output wave that is affected by slew rate:

In the above illustration, we see that the output has been amplified but that there is waveform distortion.

The amplifier does not perfectly recreate an amplified version of the input. There is a bit of lag. This lag, defined by the change in voltage per change in time, is the amplifier's slew rate.

Amplifier slew rate is nearly always given in SI units V/µs (volts per microsecond). The measurement is practically always a minimum slew rate value unless otherwise noted.

As suggested and as we can see in the above illustration, the slew rate of an amplifier affects the signal's dynamics. The transients of the square wave, for example, are softened with a ramp up and ramp down.

However, the real issue of slew rate is in the high frequencies, where the voltage changes very rapidly, and the amplifier must be capable of keeping up with the input signal's high-frequency short-wavelength waveforms.

Dynamics & High-Frequency Response

The slew rate of an amplifier essentially tells us how well the amplifier will react to a given input signal.

Let's begin with what slew rate is not. It is not a specification that has to do with the dynamic prowess of the amplifier.

Slew Rate Is Not A Measurement Of Dynamics

Thinking about this slowly, it's fair to assume that this will affect the transients and overall dynamics of the signal.

Let's say we have a sharp percussive recording (a snare drum, for example). Suppose the amplifier (or another processor) cannot react to the peak transient quickly enough. In that case, the resulting signal will sound over-compressed and lifeless rather than dynamic as it truly should.

In the above illustration, we have a transient input signal (of a snare drum hit). The output signal has a black dotted line representing what the amplified signal should be. However, with the amplifier only able to produce a limited voltage change in a given amount of time, the actual output signal gets severely distorted with a great loss in dynamics.

This is not how slew rate works!

Note that typical slew rates are measured in several volts per microsecond. That's several volts difference in a signal's waveform per 0.000,001 seconds.

We cannot and do not hear transients or dynamics at the rate of speed that slew rate is measured at.

Slew Rate Has To Do With Frequency Response

Slew rate is the rate (in volts per microsecond) at which an amplifier can respond to a change in output.

Let's now look at several sine waves of equal amplitude at various frequencies:

As we can see, if each sine wave has the same amplitude, it is the waveform with the highest frequency (shortest wavelength) that has the greatest voltage change per unit time.

Therefore, as frequency increases, the slew rate has more and more effect until a threshold is reached where the slew rate (change in voltage per unit time) cannot keep up with the waveform's frequency.

So then, slew rate will affect the high-frequency response, causing severe degradation and distortion to the signal above a certain point.

Practical Amplifier Slew Rates

It's been established that, above some frequency point, the slew rate of an amplifier will play a major role in distorting the signal.

The equation to find the minimum slew rate of an amplifier is as follows:

\text{Slew Rate}=2π•\text{Frequency}_\text{max}•\text{Voltage}_\text{peak}

So what would be a bare minimum amplifier slew rate?

Let's begin with the max frequency. Human hearing tops out at 20,000 Hz, so that would be a good point to start at. Of course, a little headroom is always nice to account for errors, but we'll use 20 kHz for this calculation.

What about peak voltage? Well, this is much more difficult to put an exact number on. Amplifiers are often defined by their output power but not by their output voltage.

To make things simple, let's simplify and say our amplifier has a speaker output power rating of 1000W, and it's connected to an 8Ω speaker.

We'll disregard the resistance of the cable and the inevitable change in impedance across the speaker's impedance.

Power can be defined as

P=\frac{V^2}{R}

And R can be swapped for impedance.

Solving for V gets us:

V=\sqrt{PR}=\sqrt{1000Ω•8Ω}=\sqrt{8000}=89.4\text{ volts}

V = √(P • R) = √(1000Ω • 8Ω) = √8000 = 89.4 volts

Note that higher slew rates are required to driver higher load (speaker) impedances.

This is actually an incredibly high voltage that would rarely, if ever, be demanded from a power amplifier. It should give us a very conservative estimate of a minimum slew rate for an amplifier to produce equal amplitude across 20 Hz – 20,000 Hz.

\text{Slew Rate}=2π•\text{Frequency}_\text{max}•\text{Voltage}_\text{peak}

\text{Slew Rate}=2π•20000Hz • 89.4V=112000000V/s\text{ or } 11.2 V/µs

More common peak voltages would be around 50 V or less, which would yield a minimum slew rate of 6.3 V/µs to achieve full output up to 20 kHz.

Most amplifiers (even the cheap ones) should have a slew rate above 6.3 V/µs.

The seemingly high slew rates of most amplifiers are simply good engineering. Having a slew rate that yields a maximum frequency well above the audible range will pretty much eliminate any potential errors and unwanted distortion whatsoever.

After all, there are plenty of other distortion-producing factors to be worried about. Slew rate should not be one of them!

Many amplifiers put some sort of low-pass filter on the audio signal to eliminate the largely unnecessary high-end frequencies above the audible range of human hearing. If we can't hear them, why amplify them in the first place?

Although this practice is certainly up for debate, reducing or removing the ultrasound frequencies from the signal further protects us from distortion due to slew rate.

On top of that, practical audio signals for our listening enjoyment do not demand high output at high frequencies. The brilliance range (arguably 6 kHz to 20 kHz) actually requires less output than the lower frequencies to be heard in a well-balanced audio mix.

Recap Of Amplifier Slew Rate

So what should we make of all this?

Essentially, any audio amplifier worth its price will have a slew rate that allows it to effectively produce all audible audio frequencies at whatever its maximum voltage output is designed to be.

Remember that the slew rate specification, though it may seem like it has to do with dynamics, actually has more to do with high-frequency distortion. That being said, it's not an overly critical specification to know (or for a manufacturer to list on a specs sheet) so long as it allows the amplifier to output great audio!

Amplifier Slew Rate Examples

The Anthem STR is a solid-state stereo integrated amplifier with a built-in DAC and a Slew Rate of 30 V/μs.

The Benchmark AHB2 is a solid-state 2-channel power amplifier with a slew rate of 16 V/µs.

The Crown Audio XLi 2500 is a popular stereo power amplifier with a slew rate of >10 V/µs.

What About Tube Amplifiers?

Interestingly, we do not see slew rate figures on tube amplifier specifications sheets. Why is this so?

Vacuum tubes tend to work just as well at DC as at high frequencies (even well past the audio range and into radio frequencies. This would mean that typical tubes have incredibly high slew rates (to maintain any coherence between the input and output signals at the very high radio frequencies.

Voltage levels also tend to be much higher in tube electronics than in op-amps. This furthers the need for very high slew rates.

Vacuum tube “slew rates” are so high that they are never considered when defining a tube audio amplifier.

The limiting factor for the vacuum tube slew rate is the high source impedance and the inter-electrode/input capacitances of the tube. These factors cause a low-pass filter in the tube that can have similar effects to a poor slew rate (i.e., distortion and filtering in the high-end).

To learn more about the differences between tube amplifiers their solid-state counterparts, check out my article Solid-State Vs. Tube Amplifiers (Pre, Power & Guitar Amps).

Related Questions

What is the common-mode rejection ratio of an amplifier? The common-mode rejection ratio (CMRR) is the ratio of the powers of the differential gain over the common-mode gain. Balanced audio uses two copies of the same signal on two conductors (positive polarity and negative polarity). The differential amplifier in a balanced audio input uses CMRR to effectively eliminate any noise/interference common to both negative and positive conductors. At the same time, it sums the differences (the two copies of the audio signal) into a stronger audio signal. The common-mode ratio is often measured in decibels (dB).

What specs are important to understand when choosing an amplifier? Important amplifier specifications to consider when choosing an amp include:

• Rated output impedance
• Power rating (if the manufacturer's listed spec is legitimate)
• Number of channels
• Inputs
• Circuit topology class
• Other features

To learn all the amplifier specifications, check out my article Complete Guide To Power Amplifier Specifications & Data.

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