First, harmonic distortion
This distortion is caused by nonlinear components in the circuit. After passing through these components, new frequency components (harmonics) are generated. These new frequency components interfere with the original signal. This distortion is characterized by the input signal. The waveform is inconsistent with the shape of the output signal waveform, that is, the waveform is distorted.
The main ways to reduce harmonic distortion are:
1. Apply an appropriate amount of negative feedback.
2. Select an amplifying device with high characteristic frequency, small noise figure and good linearity.
3. Improve the power reserve of the power supply and improve the filtering performance of the power supply.
Second, intermodulation distortion
Two or more different frequency signals pass through the amplifier or the speaker to create a beat and form a new frequency component, which is usually generated by active devices (such as transistors, tubes) in the circuit. The amount of distortion is related to the output power. Since the newly generated frequency components have no similarity to the original signal, less intermodulation distortion is also easily detected by the human ear.
Ways to reduce intermodulation distortion:
1. The electronic frequency division method is adopted to limit the working bandwidth of the amplifying circuit or the speaker, thereby reducing the generation of the beat.
2. Choose a linear tube or circuit structure.
Third, 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 transition signals, so it is also called transient response. This distortion makes music lack of layering or transparency and has two manifestations:
A. Transient intermodulation distortion.
When a pulsed transient signal is input, the output terminal cannot immediately obtain the desired output voltage due to the capacitance in the circuit, so that the negative feedback circuit cannot obtain a timely response, and the amplifier is in an open loop state at this moment, so that the output is instantaneous. Clipping occurs due to overload. This clipping distortion is called transient intermodulation distortion, and this distortion is more serious on the stone machine.
Transient intermodulation distortion is a dynamic indicator of the power amplifier, mainly caused by deep negative feedback inside the power amplifier. It is the chief culprit that affects the sound quality of stone machines and leads to "transistor sound" and "metal sound".
The main ways to reduce this distortion are:
1. Select a good device and adjust the working point to maximize the open loop gain and open loop frequency of the amplifier.
2. Strengthen the negative feedback of each amplification stage and cancel the negative feedback of the large loop.
B. Distortion caused by too low conversion rate.
As mentioned above, a high-level input pulse causes the amplifier to clip and cause transient intermodulation distortion. So does the low-level input pulse cause distortion? This depends on the response time of the amplifier. Because the response time of the amplifier is too long, the output signal of the amplifier cannot keep up with the transient distortion caused by the rapid change of the input signal, which is called the conversion rate is too low distortion. It reflects the response speed of the amplifier to the signal. This small distortion amplifier has good resolution, layering and positioning.
Fourth, the AC interface distortion
The AC interface distortion is caused by the back electromotive force of the speaker (the potential generated by the cutting magnetic field when the speaker is vibrating) is fed back to the circuit.
The improvement methods are as follows: 1. Reduce the output impedance of the circuit. 2. Select the appropriate speaker to make the damping coefficient more reasonable. 3. Reduce the internal resistance of the power supply.
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