Small headphone amplifier circuit composed of MAX4110

Many modern new devices are equipped with headphone connectors. However, due to the highly integrated and miniaturized design, there is not much space left for the headphone amplifier, which requires the corresponding amplifier to be very small and able to solve the many problems caused by low power supply voltage. Otherwise, if special technical measures are not taken, not only will the amplifier be unable to be installed, but the output power and margin of the amplifier designed in this way will also be very limited.

This problem can be solved smoothly by using Maxim's MAX4410 op amp chip. Not only is it small in size, but the chip also has a power supply pump designed inside to solve the various problems caused by low power supply. This supply pump only requires the addition of two external ceramic surface mount capacitors C6 and C7 to work well. Therefore, it is very small. The power supply of the output stage is completely symmetrical with respect to the ground, so there is no residual debiased voltage, thus avoiding the problem of having to use large output capacitors to block the DC voltage from reaching the headphones.

Each channel of the two stereo can be turned off independently by disconnecting jumpers JP1 and 2. During normal operation, JP1 and JP2 are connected to the power supply. If both channels are turned off at the same time, the supply pump will be automatically cut off. At this time, the current consumption is only about 6uA, which is very power-saving.

The MAX4410 chip has dual functions of thermal protection and short-circuit protection, and can automatically switch to standby mode when the power supply voltage is too low. In addition, there is a noise elimination circuit to prevent the output end from making a popping sound when the power is turned on and off. The reasonable working power supply voltage of the circuit is 1.8~3.6V, and the output power is 80mW/16Ω per channel. The power consumption current is at least 200mA during normal operation. In actual operation, this power supply is also used by other circuits, so the reserve current should be at least 300mA.

The amplifier is configured in inverting op amp mode, and the gain is determined by R3/R1 or R4/R2. The input impedance depends on R1 and R2. C1 and C2 are mainly used to block possible residual DC components from the two input terminals. The MAX4410 data sheet recommends that they use small tantalum capacitors, but this is not suitable here. Plastic film capacitors are used, although they are a little larger. But the performance is much better. RF decoupling can be solved by connecting 100pF capacitors in parallel with R3 and R4 respectively. The bandwidth of the amplifier designed in this way can exceed 150 kHz, and the typical distortion is 0.0003%. When installing the amplifier, pay special attention to the selection of decoupling capacitors and DC blocking capacitors, as well as the location of these capacitors on the printed circuit board and the overall component layout. In addition, the 14-pin TSSOP package (with a pin spacing of only 0.65mm) and the surface mount components of the 0402 package are very difficult to solder, requiring special attention during the production process.