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What is Power Amplifier Distortion and Its Calculation?

  • Contents

Introduction

The audio power amplifier is a device used to drive the speaker to produce sound, thereby reproducing the sound. It is used in all electronic products that produce sound, which amplifies the smaller audio signal, increases its power, and then outputs it. Pre-amplification mainly used for small signals, and amplifies the voltage of the input audio small signals by using a non-inverting amplifier circuit to obtain the input required by the latter stage. The latter stage mainly amplifies the power so that it can drive the resistor to obtain the required audio.

Making an Audio Power Amplifier

Catalog

Introduction

Ⅰ What is Power Amplifier Distortion?

Ⅱ Types of Audio Power Amp Distortion

2.1 Harmonic Distortion (THD)

2.2 Intermodulation Distortion (IMD)

2.3 Transient Distortion 

2.4 AC Interface Distortion

Ⅲ Conclusion

Ⅳ FAQ


Ⅰ What is Power Amplifier Distortion?

Distortion is the phenomenon that the input signal and the output signal change in the amplitude proportional relationship, phase relationship and waveform shape. The distortion of audio power amplifiers is divided into electrical distortion and acoustic distortion. The former is caused by the circuit, and the latter is caused by the speaker of the sound reproduction device. The types of electrical distortion are: harmonic distortion, intermodulation distortion, and transient distortion. The acoustic distortion is mainly the distortion of the AC interface. According to the nature, there are nonlinear distortion and linear distortion.
Linear distortion refers to the change in the amplitude and phase relationship between signal frequency components, and only the amplitude and phase distortion of the waveform appear. The characteristic of it is that no new frequency components are generated.

Linear Distortion

Figure 1. Linear Distortion

Non-linear distortion means that the signal waveform has been distorted and new frequency components have been generated. 

 

Ⅱ Types of Audio Power Amp Distortion

The main points of distortion produced by audio power amplifiers are as follows:

2.1 Harmonic Distortion (THD)

🔸What is THD?
This distortion is caused by non-linear components in the circuit. After the signal passes through these components, new frequency components (harmonics) are generated, which affect the original signal. The characteristic of this distortion is the input signal waveform inconsistent with the shape of the output signal. That is, the waveform is distorted. The following is a specific analysis.
Harmonic distortion refers to the more harmonic components of the output signal than the input signal. It is caused by a system that is not completely linear. The sum of all additional harmonic levels is called total harmonic distortion, which is related to frequency. Generally speaking, at a frequency of 1000Hz, the distortion is the smallest. So many products confirm the distortion at this frequency as a indicator.
Harmonic distortion is actually the distortion of the sound when the speaker reproduces the sound due to the resonance phenomenon that occurs during the working process of the speaker. Although only the fundamental frequency signal in the speaker is the original sound signal, due to the inevitable resonance phenomenon (the second, third and even multiple harmonics are generated on the basis of the original sound wave), there is no longer only the fundamental frequency signal in the sound signal, but also includes harmonics and their frequency multiplier components. These frequency multiplied signals will cause distortion when the speaker is playing. For ordinary speakers, a certain harmonic signal component is allowed, but it must be a prerequisite that it does not have a large impact on the sound fundamental frequency signal output.
The total harmonic distortion usually expressed as a percentage. Generally speaking, the smaller the value, the better. Generally speaking, the THD value of high-quality equipment is very low (less than 0.002%), but there are exceptions. Many electronic tube devices have very high THD, but transistor devices must have low THD because their excess harmonics can make the sound uncomfortable.

🔸How to Calculate THD?

THD

In the above formula, G represents the effective value of the harmonic component. It will be replaced by I when expressing current and U when expressing voltage as required. The value of H is given in each standard related to the limit. According to the above definition, THD does not include interharmonics, and there is a fixed upper limit of harmonics.

THD

In the above formula, Q is the total effective value, and Q1 is the fundamental effective value, which can represent the voltage or the current. According to the above definition, THD includes interharmonics and DC components.

🔸How to Reduce THD?
The main ways to reduce harmonic distortion are: 
1) Apply an appropriate amount of negative feedback. 
2) Choose amplifiers with high characteristic frequency, low noise figure and good linearity. 
3) Increase the power reserve of the power supply and improve the filtering performance of it.

 

2.2 Intermodulation Distortion (IMD)

🔸What is IMD?
This distortion is usually produced by active devices in the circuit (such as transistors and tubes). Two or more signals of different frequencies pass through an amplifier or speaker to produce beats and form new frequency components. The magnitude of the distortion is related to the output power. Since these newly generated frequency components have no similarity with the original signal, the less intermodulation distortion is also easy to be noticed by the human ear.

🔸How to Calculate IMD?
The excitation signal used in the measurement technique of IMD is more than a single simple sinusoidal signal. In the fields of professional audio, broadcasting and consumer audio, two sine waves are used as excitation signals to measure it. When any two sine signals with frequencies of F1 and F2 are applied to the nonlinear device, the original two sine waves will be generated plus countless IMD terms, that is, countless combined frequency components, as shown in the following formula: mF1±nF2
Where m and n are any positive integers. The order of any particular IMD value is the sum of m and n. The order of some terms are listed below:
F1-F2 2nd order (even order)
F1+F2 2nd order (even order)
2F1-F2 3rd order (odd order)
F1-2F2 3rd order (odd order)
2F1+F2 3rd order (odd order)
3F1-F2 4th order (even order)
3F1+2F2 5th order (odd order)
…….
The above "odd" and "even" refer to whether m+n is odd or even.
The measurement method of IMD is actually the measurement of harmonic voltage components, and its formula is:
Where, F1 is high frequency, F2 is low frequency.

🔸How to Reduce IMD?
Methods to reduce intermodulation distortion: 
1) Use electronic frequency division to limit the working bandwidth of the amplifier circuit or loudspeaker, thereby reducing the generation of beats. 
2) Choose a tube or circuit structure with good linearity.

 

2.3 Transient Distortion 

Transient distortion is an important indicator of modern acoustics. It reflects the ability of the power amplifier circuit to keep track of transient signals, so it is also called transient response. This kind of distortion makes music lack level or transparency, and there are two forms of expression:

🔸A. Transient Intermodulation Distortion (TIM)
When inputting a pulsed transient signal, the output terminal cannot get the proper output voltage immediately due to the capacitance in the circuit, and the negative feedback circuit cannot get a timely response. The amplifier is in an open loop state at this moment, making the output instantaneous. Clipping occurs due to overload. This clipping distortion is called transient intermodulation distortion, and it is more serious on transistor machines.
Transient intermodulation distortion is a dynamic indicator of the power amplifier, which is mainly caused by the deep negative feedback inside the power amplifier. It will affect the sound quality of the stone machine and cause "transistor noise" and "metal noise".
The main methods to reduce this distortion are as follows: 
1) The TIM can be eliminated by controlling the gain within the delay time of the negative feedback amplifier. 
2) Multistage negative feedback can be used, so it is not easy to cause TIM due to fast feedback time and short path.
3) Take well anti-interference measures.

🔸B. Distortion Caused by Too Low Conversion Rate
As mentioned above, high-level input pulses cause the amplifier to clip and then cause transient intermodulation distortion. Will low-level input pulses cause distortion? It depends on the response time of the amplifier. Because the response time of the amplifier is too long, the change of the output signal of the amplifier cannot keep up with the rapid change of the input signal. The transient distortion is caused by the low conversion rate. It reflects the response speed of the amplifier to the signal. This low-distortion amplifier has very good sound quality resolution, layering and positioning.

 

2.4 AC Interface Distortion

The distortion of the AC interface is caused by the back EMF of the speaker (the electric potential generated by cutting the magnetic lines of force when the speaker sounds vibration) feedback to the circuit.
The improving methods are:
1) Reduce the output impedance of the circuit.
2) Choose a suitable speaker to make the damping coefficient more reasonable.
3) Reduce the internal resistance of the power supply.

 

Ⅲ Conclusion

For different types of audio power amplifiers, due to the differences the circuit itself, their sound is different. Transistor power amplifiers are affected by odd harmonic distortion and transient intermodulation distortion, and the sound will be cold, straightforward, burr or metallic. The sound of the tube amplifier is warm, thick, and stretched. So far, the electronic tube is still the active amplifier device with the best linearity and the smallest native distortion, and the transistor cannot be compared. But in fact, the total harmonic distortion (THD) of the tube power amplifier is several orders of magnitude larger than that of the transistor power amplifier. This is because it is difficult to add sufficient negative feedback to improve the linearity of the tube power amplifier. Most of these non-linear distortions are even harmonics that are pleasing to the ears, but they make people feel pleasant to the ears. The transistor power amplifier uses a large loop deep negative feedback to significantly improve the linearity. Due to the stability requirements, a compensation capacitor is introduced as the main pole. However, it may cause a problem of poor transient response. With the advancement of technology, the sound quality of integrated chip audio power amplifiers is getting closer and closer, even surpassing the above two types of power amplifiers, and it also has obvious advantages in terms of volume, cost, and scope of application.

 

Ⅳ FAQ

1. What is power amp distortion?
The short answer is that power amp distortion derives from overloading/distorting an amplifier's power section. After the preamp portion of an amplifier is 'done' with the signal, it is then passed on to the phase inverter and out to the amp's power tubes.

 

2. Why is my amp distortion?
You can create distortion by merely increasing the volume of your guitar and setting the input gain high enough on your amplifier. This combination of volume and preamp gain will create distortion as explained above, the gain exceeding the voltage capacity, causing the sound waves to clip.

 

3. Is amp distortion better than pedal distortion?
Distortion pedals are considered more versatile with more parameters to shape your ideal sound. Their downfall is that some cheap pedals can sound unnatural. In contrast, the majority valve amp distortion provides a more, smoother, and overall, more well-rounded distortion.

 

4. How do I adjust the distortion on my amp?
Turn knobs marked "gain" or "overdrive" all the way up. Turn other volume knobs down to get the loudness you want. Once you've proved you can get heavy distortion, dial things back to find the tone you want. Some amps won't distort at low volumes.

 

5. Do amps have built in distortion?
The answer to the question do all guitar amps have distortion is no; not all guitar amps have distortion. In fact, there are different categories you could place guitar amps in depending on how they generate their distortion.

 

6. Does preamp reduce distortion?
Preamp distortion generally offers higher gain than power amp distortion and is more compressed and smoother, with higher sustain at lower volume levels. ... In general, vintage-style amps sound best when driven into power amp distortion — depending on the amp, its preamp may not even be able to distort.

 

7. Do tube amps need distortion pedals?
Most tube amps won't need a distortion pedal, digital or otherwise. When an amp has both a dirty (gain) and a clean channel, this allows you to dial your distorted tones along with whatever clean sound you might want to use.

 

8. Can you use a distortion pedal without an amp?
Most modern multi-effects pedals have a jack where you can plug in any speakers or headphones. This means you can play an electric guitar without an amp by using a multi-effects pedal. Simply plug your guitar into a suitable multi-effects pedal and plug headphones or speakers into the pedal.

 

9. What amp is good for distortion?
Top 5 Amps for Creating Clean and Distortion Sounds
1) Orange Micro Terror
2) Blackstar HT1R
3) Peavey Vypyr VIP 1
4) Fender Champion 20
5) Marshall MG50CFX

 

10. Why is my audio distorted?
The most common reason for distortion is an input overload like the microphone overload mentioned above. Mic'ing an instrument, or even a vocal, is more than sticking a microphone right up to the sound source. ... A distorted sound can be resolved by placing a greater distance between the sound source and the microphone.

Lydia

Lydia is a highly experienced technical writer with over 8 years of specialized knowledge in the semiconductor electronics industry. She possesses deep industry insights and a visionary creative approach, enabling her to translate complex technical concepts into easily digestible content.

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