Bad sound quality？You amp and speaker always distorted? Let me share you some tricks!

Distortion is the change phenomenons of input signal and output signal amplitude proportional relationship, phase relationship and waveform shape. The distortion of audio power amplifier is divided into electrical distortion and acoustic distortion. Electrical distortion is caused by the circuit, and acoustic distortion is caused by the sound reproduction device speaker. The types of electrical distortion are: harmonic distortion, intermodulation distortion and transient distortion. Acoustic distortion is mainly refer AC interface distortion. According to the nature, there are nonlinear distortion and linear distortion.

Linear distortion refers to the change of amplitude and phase relationship between signal frequency components, it only appear the waves of amplitude and phase distortion. The characteristic of this distortion is that no new frequency components are generated. The nonlinear distortion refers to the distortion of the signal waveform and the distortion of new frequency components. The main points of the distortion produced by the audio music hifi amplifier are as follows:

01 Harmonic Distortion

This distortion is caused by nonlinear components in the circuit. After the signal passes through these components, new frequency components (harmonics) are generated, and these new frequency components interfere with the original signal. The characters of this distortion is the waveform of the output signal is inconsistent with the waveform shape of the output signal, which means the waveform is distorted.

You can follow below ways to reduce harmonic distortion:

1. Apply the moderate negative feedback.

2. Select amplifiers with high characteristic frequency, low noise figure and good linearity.

3. Improve the power reserve and filtering performance of the power supply.

02 Inter-modulation Distortion

Two or more signals with different frequencies will generate beats and form new frequency components after passing through amplifier or speaker, this kind of distortion is usually produced by active devices in the circuit (such as transistors, tubes). The magnitude of the distortion is related to the output power,however, since these newly generated frequency components have no similarity to the original signal, actually there are not many inter-modulation distortion can be hear by the human ears.

Methods to reduce inter-modulation distortion:

1. Use electronic frequency division method, to limit the working bandwidth of the amplifier circuit or speaker, therefore to reduce the generation of beats.

2. Choose tubes or circuit structures with good linearity.

03 Transient Distortion

Transient distortion is an important indicator of modern acoustics. It reflects the ability of the power amplifier circuit to keep track of the transient jump signal, so it is also called transient response. This distortion will make music lack of layering or transparency, mainly perform in two ways:

A, transient inter-modulation distortion

When input 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, neither. At this moment, the amplifier is in an open-loop state, the overload of output instantaneous makes clipping, and this clipping distortion is called transient inter-modulation distortion. This distortion is more serious on stone machines.

Transient inter-modulation distortion is a dynamic index of power amplifier, which is mainly caused by deep negative feedback inside the power amplifier. It is the main culprit that affects the sound quality of the stone machine and causes "transistor sound" and "metal sound". The main methods to reduce this distortion are:

1. Select good components, and adjust the operating point to maximize the open-loop gain and open-loop frequency response of the amplifier.

2. Strengthen the negative feedback of each amplifier, and cancel the negative feedback of the large loop.

B. Distortion caused by low conversion rate

As mentioned above, the high-level input pulse causes the amplifier clipping and transient inter-modulation distortion. Then does a low-level input pulse cause distortion? This depends on the response time of the amplifier. Due to long response time of the amplifier, the change of the output signal of the amplifier cannot keep up with the rapid change of the input signal, then generate transient distortion which is called slew rate low distortion. It reflects the response speed of the amplifier to signal. This kind of amplifier with low distortion has good sound quality resolution, layering and positioning.

04 AC interface distortion 

Ac interface distortion is caused by the feedback of the speaker's back electromotive force to the circuit.(back electromotive force means when the speaker vibrates, the force generated by cutting the magnetic lines)

Improvement methods as below:

1. Reduce the output impedance of the circuit.

2. Select the appropriate loudspeaker to make the damping coefficient more reasonable.

3. Reduce the internal resistance of the power supply.