This article refers to the address: http:// Without this DC blocking capacitor, large DC currents will flow into the headphones without any obstruction, causing unnecessary power loss and possibly even damaging the headphones and headphone amplifiers. However, the increased DC blocking capacitor requires a large enough capacity, which in turn increases the size and cost of the circuit. Moreover, the impedance of the DC blocking capacitor and the headphone constitute an RC high-pass filter. The -3dB cutoff frequency point is determined by: It is worth noting that this high-pass filter will attenuate the low-frequency signal, degrading the low-frequency response of the amplifier circuit, causing distortion of the audio signal. Large-capacity output DC blocking capacitors can reduce the attenuation effect of low-frequency signals, but the disadvantage is that the capacitors are too large and expensive. Figure 2 shows the low-frequency attenuation characteristics of DC-blocking capacitors with different capacitance values. It can be noticed that when the headphone load is 16Ω and the output blocking capacitor is 100uF, its -3dB cutoff frequency point is 100Hz, which falls within the audio range, causing the attenuation of the low frequency signal. The change in capacitance caused by a change in the capacitance voltage is called the voltage coefficient of the capacitor. At the -3dB attenuation frequency point, the capacitance is mainly reactive, and the voltage coefficient appears as frequency-dependent distortion. Figure 3 shows the distortion caused by the DC blocking capacitor. It can be seen that THD+N increases sharply at low frequencies below 100 Hz. MAXIM's patented audio amplifier direct drive technology overcomes these shortcomings. At the heart of direct drive technology is the use of a charge pump circuit to generate an internal negative voltage that causes the headphone amplifier's DC bias point to be at the ground level of the power supply. From the circuit point of view, the output terminal no longer needs a DC blocking capacitor with a capacity of up to 220uF, but only increases the two small capacitors (1uF) required by the charge pump circuit. Therefore, the size and cost of the circuit are greatly reduced. Moreover, the improved output dynamic range is almost twice that of the conventional circuit, and the frequency response is improved by removing the DC blocking capacitor. Figure 4 is a simplified schematic of the MAX4410 with direct drive technology. Figure 5 shows a typical circuit for a headphone amplifier using the MAX4410. Compared with traditional circuits, it has the following advantages: Large-capacity output DC blocking capacitors (typically 100μF-470μF) are no longer needed, while eliminating THD distortion caused by the voltage coefficient effect of the capacitor. The -3dB cutoff point is determined by the input resistance and capacitance. The cutoff point obtained from the input resistance and capacitance values ​​in the circuit of Figure 5 is around 1.6 Hz. In the traditional AC-coupled output circuit, the 16Ω headphone requires an output blocking capacitor of 6200μF to achieve a low-frequency -3dB cutoff point of 1.6Hz. This is almost impossible in practical applications. Moreover, the low frequency response of the Figure 5 circuit is no longer related to the load. Switch/switching noise cancellation circuit In conventional single-supply headphone amplifier circuits, DC-blocking capacitors are the primary source of switching/switching noise. Because at the time of power-on, the DC-blocking capacitor is charged to the DC bias voltage; when it is powered off, it is discharged from the DC bias voltage to the ground level. During this transient, audible noise is produced on the headphones. MAXIM direct drive technology removes the DC blocking capacitor and removes the largest source of switching/switching noise. At the same time, MAXIM's audio amplifiers feature an additional noise reduction control circuit that removes switching/switching noise from the input section during power switching. In most applications, the MAX4410's pre-driver will have a DC offset, typically half the VCC. During power-up, the input coupling capacitor is charged to the DC bias voltage through the R F of the MAX4410, causing voltage fluctuations in the input capacitor to form a switch/switching noise that is perceptible to the human ear. The input filters R IN and C IN associated with the pre-driver form a time constant. The MAX4410 delays the rising time of the control pin by power-on/SHDN, which is approximately 4 to 5 times the input RC circuit time constant (200 to 300 ms). ), remove this switching noise associated with the input. As shown in Figure 6, during the power switching process, the audio components in the spectrum of the output signal are minimized. Hdd Duplicator,Hard Disk Duplicator,USB Duplicator HDD Protector Box, Express Card Co., Ltd. , http://www.chhddstorage.com
f -3dB =1/(2Ï€R L C OUT )
R L is the headphone load impedance and C OUT is the output DC blocking capacitor.