Essentially, an audio amplifier is a normal voltage amplifier optimised for the amplification of audio signals. The limited frequency response of the ear sets the bandwidth limits: 20 Hz - 20 kHz, although most people are not able to hear 20kHz. Most power is concentrated in the mid frequencies, and occasionally in the low frequencies. Generally, the amplitude probability density function of audio signals is gaussian. This means that the ratio between maximum and average power is large: 10 … 20 dB. In average, it is 15 dB, which is 12dB below the power of a rail-to-rail sinewave. The primary specifications of an audio power amplifier include maximum power, frequency response, noise, and distortion.
Rated Output Power
The ear has a very large dynamic range. To give an example: the ratio between the acoustic power of a rock concert and the sound of breathing can be as large as 10^11. This makes large demands on the dynamic range of the audio amplifier.
Maximum output power is almost always quoted for a load of 8 Ω and is often quoted for a load of 4 Ω as well. A given voltage applied to a 4 Ω load will cause twice the amount of current to flow, and hence twice the amount of power to be delivered. Ideally, the output voltage of the power amplifier is independent of the load, both for small signals and large signals. This implies that the maximum power into a 4 Ω load would be twice that into an 8 Ω load. In practice, this is seldom the case, due to power supply sag and limitations on maximum available output current.
The correct terminology for power rating is continuous average sine wave power, as in 100 W continuous average sine wave power. However, many often take the liberty of using the term W RMS. Although technically incorrect, this wording simply is referring to the fact that the power would have been measured by employing a sine wave whose RMS AC voltage was measured on a long-term basis. There are other ways of rating power that are sometimes used because they provide larger numbers for the marketing folks, but we will ignore them here. When you hear terms like peak power just realize that these are not the same as the more rigorous continuous average power rating.
Frequency Response
The frequency response of a power amplifier must extend over the full audio band from 20 Hz to 20 kHz within a reasonable tolerance. Modern amplifiers usually far exceed this range, with frequency response from 5 Hz to 200 kHz not the least bit uncommon. The frequency response for such an amplifier is illustrated with the solid curve on Picture 1. While the tolerance assigned to the frequency response of loudspeakers is often ± 3 dB, the tolerance associated with power amplifiers is usually + 0 dB, – 3 dB, or tighter. Specifying where an amplifier is down by 3 dB from the nominal 0 dB reference is the conventional way of specifying the bandwidth of a system. This is often referred to as the 3-dB bandwidth. The frequency response for a less capable amplifier is shown with the dashed curve on Picture 1. This amplifier has a 3-dB bandwidth from 10 Hz to 80 kHz. Its response is down 1 dB at 20 Hz and 0.5 dB at 20 kHz.
Picture 1: Amplifier frequency response
Noise
It is important that power amplifiers produce low noise, since the noise they make is always there, independent of the volume control setting and the listening level. This is particularly so when the amplifiers are used with high-efficiency loudspeakers. The noise is usually specified as being so many decibels down from either the maximum output power or with respect to 1 W. The former number will be larger by 20 dB for a 100 W amplifier, so it is often the one that manufacturers like to cite. The noise referenced to 1 W into 8 Ω (or, equivalently, 2.83 V RMS) is the one more often measured by reviewers.
The noise specification may be unweighted or weighted. Unweighted noise for an audio power amplifier will typically be specified over a full 20-kHz bandwidth (or more). Weighted noise specifications take into account the ear’s sensitivity to noise in different parts of the frequency spectrum. The most common one used is A weighting, illustrated on Picture 2. Notice that the weighting curve is up about +1.2 dB at 2 kHz and down 3 dB at approximately 500 Hz and 10 kHz.
Picture 2: A weighting frequency response
The A-weighted noise specification for an amplifier will usually be quite a bit better than the unweighted noise because the weighted measurement tends to attenuate noise contributions at higher frequencies and hum contributions at lower frequencies. A very good amplifier might have an unweighted signal to noise ratio (S/N) of 90 dB with respect to a 1-W output into 8 Ω, while that same amplifier might have an A-weighted S/N of 105 dB with respect to 1 W. A fair amplifier might sport 65 dB and 80 dB S/N figures, respectively. The A-weighted number will usually be 10–20 dB better than the unweighted number.
Distortion
The most common distortion specification is total harmonic distortion (THD). It will usually be specified at one or two frequencies or over a range of frequencies. It will be typically specified at a given power level with the amplifier driving a specified load impedance. A good 100-W amplifier might have a 1 kHz THD (referred to as THD-1) of 0.005% at 100 W into 8 Ω. That same amplifier might have a 20-kHz THD (THD-20) of 0.02% up to 100 W into 8 Ω. Although 1 kHz THD is at a frequency in the middle of the audible frequency range where hearing sensitivity is high, it is not very difficult to achieve low THD figures at 1 kHz. Good THD-20 performance is much more difficult to achieve and is generally a better indicator of amplifier performance.
In practice, the harmonic distortion specification will be described as THD + N, where the N refers to noise. This reflects the way in which THD is most often measured. When measuring THD-1, a 1 kHz fundamental sine wave is applied to the amplifier input. The 1 kHz fundamental appearing in the output signal is then notched out by a very sharp filter. Everything else, both distortion harmonics and noise, is measured, giving rise to the THD + N specification. At higher power testing levels, the true THD will often dominate the noise, but at lower power levels the measurement may often reflect the noise rather than the actual THD being measured. Graphs that show rising THD + N at lower power levels can be misleading. The rising level may actually be noise rather than distortion. This is because a fixed noise voltage becomes a larger percentage of the level of the fundamental as the fundamental decreases in amplitude at lower power levels.
The Federal Trade Commission (FTC) long ago tried to wrap things up in a single statement that would largely capture power, distortion, and bandwidth together. It would read something like “100-W continuous average power from 20 Hz to 20 kHz with less than 0.02% total harmonic distortion.” This was a reasonably comprehensive and honest way to describe the most basic capability of an amplifier. It is unfortunate that it has fallen into disuse by many manufacturers. Part of the reason was that it also required that the amplifier could be run at 1/3 rated power into 8 Ω for an extended period of time without overheating. Operating at 1/3 rated power is close to the point where most amplifiers dissipate the most heat, and it was expensive for many amplifier manufacturers to provide enough heat sinking to meet this requirement.
Total Harmonic Distortion (THD)
When a sinusoidal signal is applied to a non-linear amplifier, the output contains the base frequency plus higher order components that are multiples of the base frequency. The Total Harmonic Distortion is the ratio between the power in the harmonics and the power in the base frequency. This can be measured on a spectrum analyser. Most distortion analysers, however, subtract the base signal from the amplifier’s output and calculate the ratio between the total RMS value of the remainder and the base signal. This is called THD+N: Total Harmonic Distortion + Noise. Normally, the noise will be low compared to the distortion, but the noise of a noisy amplifier or the switching residues in a class D amplifier can give garbled THD figures. For a THD+N measurement, the bandwidth must be specified. For class D measurements, a sharp filter with a 20kHz corner frequency is necessary to prevent switching residues - that are inaudible - to show up in the distortion measurements.
InterModulation distortion (IM)
When two sinusoids are summed and applied to a non-linear amplifier, the output contains the base frequencies, multiples of the base frequencies and the difference of (multiples of) the base frequencies. Suppose a 15 kHz sinusoid is applied to an audio system that has a 20 kHz bandwidth, and the THD+N needs to be measured. All the harmonics are outside the bandwidth and will be attenuated, resulting in too low a THD+N reading. The same situation occurs when the distortion analyser has a 20 kHz bandwidth. In these cases, an IM measurement can be a solution.
The first standard was defined by the SMPTE (Society of Motion Picture and Television Engineers). A 60Hz tone and a 7kHz tone in a 4:1 amplitude ratio are applied to the non-linear amplifier. The 60Hz appears as sidebands of the 7kHz tone. The intermodulation distortion is the ratio between the power in the sidebands and the high frequency tone. Another common standard is defined by the CCITT (Comité Consultatif Internationale de Télégraphie et Téléphonie), and uses two tones of equal strength at 14kHz and 15kHz. This generates low frequency products and products around the two input frequencies, depending on the type (odd or even) of distortion.
Interface InterModulation distortion (IIM)
In this test, the second tone of an IM measurement set-up is not connected to the input, but to the output (in series with the load impedance).
Transient InterModulation distortion (TIM)
When a squarewave is applied to an amplifier with feedback, its input stage has to handle a large difference signal, probably pushing it into a region that is less linear than its quiescent point. When a sinusoid is added to the squarewave, the nonlinearity induced by the edges of the squarewave will distort the sinusoid, giving rise to TIM, also called transient distortion or slope distortion. There are many ways of testing TIM and it remains unclear how much it adds to the existing measurement methods. If the maximum input signal frequency during normal operation of an amplifier is limited to 20 kHz, a 20 kHz full power sinusoid is the worst case situation. When that generates little distortion, TIM will not occur.
Cross-over distortion
Cross-over distortion is generated at the moment the output current changes sign. At that moment, the output current gets supplied by another output transistor. The process of taking over generates distortion, visible as spikes in the residual signal of a THD measurement. This kind of distortion is notorious for its unpleasant sound (a small percentage error is quickly noticeable). Because it’s usually present around zero amplitude, the impact on small signals can be relatively large.
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