The basic circuit of the digital power amplifier is the existing Class D amplifier (called Class D amplifier in China). In the past, due to price and technical reasons, this amplifying circuit was only used in laboratories or high-priced test instruments. In recent years, technological development has enabled the components of digital power amplifiers to be integrated into a single chip, while manufacturing costs are also declining. Facts have proved that the efficiency of Class D amplifiers can reach more than 80% or even higher. Although no more ideal switching element has been found so far, some power loss will inevitably occur. If the device used is not good, the loss will be greater. But in any case, its amplification efficiency is still more than 70%, which is only 25% efficiency compared to the A and AB types, which is a few levels of improvement.
The digital power amplifier is an audio power amplifier with the characteristics of low distortion, low noise and large dynamic range. It is incomparable to traditional power amplifiers in terms of sound quality, resolution, background quietness, and low frequency shock.
The digital power amplifier is similar to the DC-DC switching inverter circuit. The input audio analog signal is modulated by the PWM circuit to form a pulse chain whose duty cycle is proportional to the input signal. After being amplified by the switch circuit, the high-frequency component is filtered by the low-pass filter to restore the amplified input signal Waveform, played by the speaker. As we all know, the pulse wave output from the above-mentioned field effect tube H-bridge circuit is inconvenient to directly drive the speaker to sound. In order to reproduce the amplified audio signal, the output waveform must be restored to the original sine wave. In the past few years, the design of Class D amplifiers was mostly solved by low-pass filters. Since the audio frequency band ranges from 20Hz to 20000Hz, and the carrier frequency is usually more than 5 times it, the process of filtering the carrier frequency is quite simple, that is, connect a low-pass filter with a cut-off frequency of about 25kHz in front of the speaker. When using a subwoofer amplifier, the cut-off frequency of the low-pass can be reduced to about 5kHz because it is dealing with low frequencies. The filter can use Chebyshev, Butterworth or Bessel circuits according to performance requirements. The design requirements of the filter are high, and if it is not done well, it will cause radio frequency interference. In order to reduce power consumption, passive components are generally used.
Today, the editor brought three Korean Neofidelity (Korea NF) digital audio power amplifier chips represented by Gongcai.com:
-NTP8918, is a digital Class-D audio power amplifier chip with built-in DSP, which is different from traditional Class A or Class AB power amplifiers in that the chip integrates two major modules: digital audio signal processing module (referred to as DSP) and full digital PWM modulation Module (referred to as POWER), so it achieves a conversion efficiency of up to 90%, and can perform differentiated sound adjustments to meet high-fidelity sound output. The entire series of Korean NF products are IIS data interfaces, and the sampling rate can be met from 8K to 192KHz (specifically sub-models). In terms of power, the NTP8918 can provide up to 2x15W output (BTL mode) or 30W single-channel output (PBTL mode). In the case of reasonable structural design, the radiator can be omitted, which is very cost-effective. In terms of DSP performance, NTP8918 has up to 32 freely adjustable EQ points and efficient and precise DRC power control; in addition, South Korean NF has integrated their unique technical patents on NTP8918, such as NS noise reduction algorithm, APEQ function adjustment, and virtual 3D, etc., greatly enhance and optimize the sound output effect. Even better, all the functions of NTP8918 can be controlled by IIC, which is very convenient in design and use, and has a wider range of applications, which has won a good reputation in the market.
-NTP8835, the power output is increased to 2x30W, and the ASRC (asynchronous sampling rate conversion) function is added, so that the sampling frequency of the input IIS can be freely changed from 8kHz to 192kHz to cope with the design of different circuit schemes. NTP8835 is in addition to the conventional In addition to 2.0CH (BTL) and 0.1CH (PBTL) designs, 2.1CH (2xSE+1xBTL) output designs can also be realized;
- NTP8849, the flagship chip of South Korea NF, adds a closed-loop function to the previous chip. It is an all-digital Class-D closed-loop power amplifier with powerful performance; it can suppress multi-order harmonics and deal with power failures. Stable output under stable conditions, it is used in many high-end models of well-known brands.
South Korea's NF power amplifier enjoys a good reputation at home and abroad due to its advantages in power consumption and size.
With the rapid development of consumer products such as DVD home theaters, mini-audio systems, set-top boxes, personal computers, LCD TVs, flat-panel displays, and mobile phones, especially the emergence of new audio source specifications with high sampling frequencies such as SACD and DVD Audio, and The evolution of audio systems from stereo to multi-channel surround systems has accelerated the development of digital power amplifiers. Now there is a new term "pure digital power amplifier" for HIFI enthusiasts. It directly transmits digital signals, then the PCM digital signal is up-converted and the clock is reset, and then undergoes PCM->PWM conversion. Direct PWM performs the digital signal Amplify, the PWM power output and the LPF circuit composed of LC form a 1BIT DAC directly to drive the speaker. The advantage is that the digital signal will not bring phase delay, phase distortion, crossover distortion, etc. during the transmission process. The advantage is that the sound will be more transparent, the positioning is more accurate, and the sound will be closer to reality.
In the field of digital power, South Korea's NF is one of the leaders.