AUDIO AMPLIFIER DISTORTIONS
Distortions introduced to the audio output signal are the main cause of the degradation of the audio signal transmitted to of the power amplifier output. However, any enthusiast of good sound, if they’ve had a bit of experience with audio equipment, has certainly noticed that the THD parameter declared by manufacturers has little to do with actual sound quality. In this article we will discuss these reasons and explain the significance of the distortion-related parameters given in catalogue sheets.
Distortion and noise
The main job of a power amplifier is to deliver sufficient current to the speaker to power the speaker membranes, which in turn stimulates air vibrations in the listening room. Considering the achievable volume and the mass to be moved, one can conclude that the amplifier should provide really large amounts of current. And indeed, the current amplification of electroacoustic power amplifiers can reach tens of thousands. The electronic system inside has a difficult job to do, so sometimes it feeds too much or too little current to the output. These amplifier errors can be classified in two types:
noise – errors that do not depend on the signal processed,
distortions – errors that depend on the scale or rate of input signal changes.
Noise depends mainly on the quality of the components that make up the amplifier. If the noise is relatively low, then it does not significantly affect the quality of the music reception. Today only turntable amplifiers still struggle with significant levels of output noise; in other systems of good quality noise levels are negligible. Distortions, on the other hand, remain significant, as does their impact on sound quality.
Total Harmonic Distortion
The total harmonic distortion (THD) level is a parameter describing in a very technical way the static linear characteristics of the amplifier. This parameter is determined by feeding a single tone (usually a 1 kHz sinusoid) to the output and observing how much of an error the amplifier caused compared to output signal strength. Harmonic distortions are a numeric expression of the errors in the amplifier’s functioning, which are introduced as the sinusoidal voltage fed to the input continuously fluctuates. The difficulty is that the measurement is done in a situation that does not really tax the amplifier system very much. The signal fed to the input is perfectly smooth and predictable, so in such situations the system usually makes no errors. If the amplifier is built correctly and the power supply allows, it should not introduce distortions greater than 0.1%. However, it also happens that amplifiers with larger distortions play great anyway. This is because minor non-linearities in the transient characteristics of the amplifier are not crucial in fact - they only cause a certain change in tone colour, comparable to a situation when the artist would play on a different, not necessarily inferior instrument.
Tone colour is specifically determined by harmonic components. They result from a complex resonance process that musical instruments perform. Often, creators go to great lengths to enrich the sound of their instruments by adding the right harmonics. Therefore, the THD parameter is highly technical and should not be seen as the key deciding factor when making a purchase. However, it is expected from high-end devices that they should keep these distortions very low, so that the audio system reproduce the colour of the originally recorded tone. At the same time, it should be remembered that the harmonic distortions caused by speakers are usually dozens or even thousands of times greater. The effect of the amplifier on this type of distortion is therefore negligible.
Intermodulation distortion (IMD) also is a certain type of measure of static non-linearity of the amplifier system. The thing is, non-linear systems tend to mix signals. Mixing is a technical term for the phenomenon where errors in the form of new tone signals with frequencies that are a variety of combinations (sums or differences) of tone frequencies originally fed to the input are added to the output of a non-linear system. Mixed signals are not harmonics (integer multiples), which only modify tone colour, but tones that don't exactly fit the key of the music, so they are perceived as false and disturbing the background.
IMD is measured by feeding the amplifier input with two tones with frequencies that are not their integer multiples, and measuring how strong these additional "mixed-in" tones are in relation to the signal at the output. Distortions of this type are more important for the listening quality of the signal, but due to the very good linearity of modern systems, these distortions are usually not dominant. It should be added that as with THD, speaker columns cause much more trouble in this respect.
Transient Intermodulation Distortion
Transient intermodulation (TIM) distortion is a measure of error belonging to a wide group of errors introduced into the audio signal as a result of dynamic non-linearity of the amplifier. TIM distortion is a measure of errors caused when the amplifier – due to limited current feed rate to the output – changes its properties dramatically without conducting weak signals at all. A dynamically overloaded amplifier briefly becomes a very non-linear system, introducing very strong distortions into some components. Importantly, the current potential of the power supply unit is not crucial TIM distortions. They are created as a result of selecting the wrong voltage amplifications in the amplifier feedback control circuit.
Selecting the right parameters for the equation that describes the operation of the amplifier control system is a serious research challenge, and doing it in practice is very difficult. Distortions of this type do not occur in non-feedback systems, but such systems are characterised by high non-linearity and are sensitive to the character of the load. TIM distortions have a very large impact on the actual/listening sound quality, and they can be generated both by analog and digital electronics.
Phase distortions arise in non-linear-phase systems, i.e. those that process sound components of different frequencies at different times (with different delays). When processed in a non-linearly phased system, a multiple-component signal has a completely different shape. Phase-distorted sound changes its colour slightly, but does not change the harmonics content. However, phase distortions have a serious impact on the reception of space and the positioning of musical instruments in multi-channel systems (e.g. stereo systems). Phase distortions appear stronger in the dynamic elements of a song. In addition to the power amplifier, the source of phase distortions can also be loudspeakers and various types of tone colour equalizers (analog tone adjustment) and IIR digital filters (e.g. shelf).
The Information Void of Technical Specifications
The distortion types described here do not fully reflect the mechanisms of distortion formation in power amplifiers – these are only parameters that try to characterise the quality of the device in a very imperfect way. It should be emphasised that most manufacturers only provide the THD parameter, which has the smallest real impact on the quality of the output signal and in fact does not prove the technical excellence of the device. On the other hand, manufacturers do not usually provide TIM values due to the fact that the standard method of measuring this type of distortion does not allow them to be accurately characterised with a numerical value. Therefore, there is no single commonly accepted, standardised and objective method of measuring dynamic distortions, and as a result technical specifications give little idea about the actual output sound quality of the device. From the consumer's point of view, this is a serious problem. As a consequence, it is necessary to follow your own hearing when choosing an amplifier.