Slew Rate

Technical note
Number 1

November 1990

Slew rate
The slew rate (from now on referred to as SR) figure of a power amplifier has nothing to do with the sound
quality of the power amplifier, as it doesn't tell how much distortion the square wave produces during its
transition.
It began with a phone call. The caller said that he had compared two 500 W (into 4 ohms) power amplifiers in anactive system. "One of them 'sounds faster' in the bass region", he said. He claimed that the reason for this
"faster" bass sound had to be the higher slew rate (60 V/µs) of that amplifier compared with the slower one (30
V/µs)! "Of course", I replied, "there can be some differences in the sound between amplifiers, but I can prove
that it's not the slew rate figure which makes the difference".As we can calculate by the formulas shown below, the maximum slew rate available in the customer's amplifier
was only 0.08 V/µs in the bass region. (The system was actively divided with a crossover frequency at 200 Hz).
Many indications shows that the difference in sound between power amplifiers has more to do with the short
circuit protection circuit design, and the behaviour of the amplifiersduring overload and clip situations.

What is Slew Rate? How to measure it
The most straight off method to measure slew rate is to put a square wave at the input of the amplifier, and
increase the voltage up to the clip-level. You then look at the square wave response in an oscilloscope and
measure the voltage difference and the time segment of the step transition. The slew rate is thencalculated by
dividing the voltage difference by the time segment (see fig. 2).

Square wave response

Fig.1 Square wave at the input

Fig.2 Square wave response at the output

Fig.3 Unlinear stepresponse

The slew rate figure tells nothing about the distortion during the step transition (see fig. 3), and the slew rate has
therefore nothing to do with the audio quality. The slew ratefigure belongs to the switch-mode and digital
technology.
The internal slew rate figure of the amplifier is naturally important to minimise the TIM- and DIM-distortion
types. The internal frequency dependent headroom in the circuits is however just as important in this matter. The
internal slew rate figures of the circuit is something one deals with during the circuit designing, and itdifferentiate often a lot from the overall slew rate figure.
Another way to determine the SR figure is to measure how high frequency an amplifier can reproduce at full
power. This is made by putting a sine wave on the input and then measure the power (and distortion) at the
output while increasing the frequency. (This method is similar to the one by which you determine the power
bandwidth.) The benefitof this method, compared with the square wave method, is that it's possible to measure
the distortion.

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Technical note
Number 1

November 1990

Calculations
To determine how large the SR must be toreproduce a sine wave frequency, we must derive the formula for a
sine wave:

π=
π

π

π

Û=U

√

√
π

Û

√
√

Sine wave: U = Upeak * sin ωt

U= instantaneous voltage
Upeak = maximum voltage swing available

Derivate: dU = Upeak ω cos ωt
dt

ω = 2 πf
f = frequency

The max dU is then cos ωt = 1
dt

√

dU (max) = Upeak ω = Upeak 2πf
dt

dU is the SR
dtThe voltage U can be calculated from the output power:
P = U2
z

z = loudspeaker impedance

Upeak = √ P z
The U for a sine wave is U= √ 2 P z

By substituting U the SR. can be calculated:

SR = 2·π·f ·√ 2·P·z
In the above we can see that the SR is simply a product of the voltage (max power) and the maximum frequency.
The maximum frequency can be set to 20 kHz, as we can't hear higher...