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 to have to do with 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 amplifier 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.
The slew rate of an amplifier, as was suggested and as we can see in the above illustration, does affect 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). If the amplifier (or other processor) cannot react to the peak transient quickly enough, 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 measures 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 response 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 have the greatest voltage change per unit time.
Therefore, as frequency increases, the slew rate has more and more of an effect until a threshold is reached where the slew rate (change in voltage per unit time) cannot keep up with the frequency of the waveform.
So then, slew rate will affect high-frequency response, causes 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:
Slew Rate = 2 π • Frequencymax • Voltagepeak
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 error but for this calculation we’ll use 20 kHz.
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 = V2 / R and R can be swapped for impedance.
Solving for V gets us:
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 what a minimum slew rate would be for an amplifier to produce equal amplitude across 20 Hz – 20,000 Hz.
Slew Rate = 2 π • Frequencymax • Voltagepeak
Slew Rate = 2 • π • 20,000 Hz • 89.4 V = 112,000,000 V/s 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?
To learn more about low-pass filters, check out my article Audio EQ: What Is A Low-Pass Filter & How Do LPFs Work?
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 (link to check the price at Crutchfield) is a solid-state stereo integrated amplifier with a built-in DAC and a Slew Rate of 30 V/μs.
The Benchmark AHB2 (link to check the price at B&H Photo/Video) is a solid-state 2-channel power amplifier that has a slew rate of 16 V/µs.
Benchmark is featured in My New Microphone’s Top Best Headphone Amplifier Brands In The World.
The Crown Audio XLi 2500 (link to check the price on Amazon) is a popular stereo power amplifier with a slew rate of >10 V/µs.
Crown Audio is featured in My New Microphone’s Top Best Power Amplifier Brands In The World.
What About Tube Amplifiers?
It’s interesting that 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 (in order 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 (ie: 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).
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 (one in positive polarity and the other in negative polarity). The differential amplifier in a balanced audio input uses CMRR to effectively rid of any noise/interference common to both negative and positive conductors while 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
- Circuit topology class
- Other features
To learn all of the amplifier specifications, check out my article Complete Guide To Power Amplifier Specifications & Data.