Class A headphone amplifier circuit diagram using op amp

　　Using the Class A amplifier introduced in this article, you can get better high-fidelity effects. Class A headphone amplifier (TL072OP with BC142, 143)

　　In daily life, when other family members are watching TV or sleeping and cannot be disturbed, you cannot enjoy the beautiful music. In this case, you need to consider the amplifier introduced in this article. Although the circuit is simple, its high-fidelity effect exceeds the standard power amplifier in the store. With the tuner and answering machine movement, you can install a high-quality sound system at a relatively low price.

　　Design ideas: Before introducing the circuit in detail, let's talk about the design ideas and explain how to achieve Class A performance, while most stereo power amplifiers use Class B.

　　We know that the output stage of a general amplifier consists of two branches: one branch processes the negative polarity (negative half cycle) output signal, and the other branch processes the positive polarity (positive half cycle) output signal. This can minimize the quiescent current, but the voltage and current must have a large amplitude to drive the speaker.

　　Unfortunately, these amplifiers all have a certain amount of distortion, called crossover distortion. It occurs at or near the zero crossing point of the output signal. If the two branches of the circuit are not perfectly matched, crossover distortion will often occur, which makes the effect of listening to the program through headphones very poor.

　　Connecting the headphone socket to the main amplifier output creates another problem: some resistance needs to be added in series between the headphone and the output stage to reduce the signal level. This changes the negative feedback characteristic, which usually results in a high peak in the bass response, with a rapid roll-off below the peak frequency.

　　In short, in order to produce an amplifier that can successfully drive headphones and obtain the best sound quality, the above two problems need to be solved.

　　To successfully design any audio device, the first step is to understand what device you are designing and determine its circuit. Ten years ago, the impedance of most headphones was 8Ω, but now most of them use polyester film diaphragms, and the impedance is usually 32Ω.

　　If you go to a few high-fidelity supply stores, you will find that most products (including those with an impedance of 8Ω) have a sensitivity of 87dB-96dB/mW and a maximum input power of nearly 100mW. A test after adding this amplifier to 10 pairs of headphones found that most headphones can output sufficient volume with an input power of 10MW.

　　Class A amplifier: For output power, the amplifier also needs low distortion, low noise, and wide bandwidth to meet the requirements of high-fidelity equipment. To avoid crossover distortion, the output stage must be a Class A amplifier.

　　Now let's talk about the ordinary small signal Class A transistor amplifier stage. To ensure the maximum dynamic range of the signal, the collector voltage should be half of the effective power supply voltage. The current consumption of this stage is determined by the value of the load resistance connected to the collector. In a power amplifier, the load is usually an 8Ω speaker.

　　Now let's go back to the headphones. Providing 10MW power to a pair of 32Ω headphones requires a 560MW R.MS voltage dynamic range and a 25MA current consumption, so it can be achieved by using small signal transistors in the output stage. But in fact, the maximum output power of the amplifier is just over 100MW, and the headphones have already begun to distort before the amplifier outputs the highest level.

　　In order to make the output stage work properly, an operational amplifier was finally selected as the driver (its output stage is Class A). The total harmonic distortion (THD) and bandwidth test of this headphone Class A amplifier are as follows:

　　THD＜0.005%（1KHZ）

　　Frequency response: 2.5HZ-100KHZ (-3dB)

　　Signal-to-noise ratio: -90dB

　　Maximum output: 120mW/32Ω

　　Circuit principle:

　　See Figure 1 for the circuit diagram. The numbering of the components on the right side of the earphone is the numbering of the components on the left side plus 100. For example, R1 on the right side is written as R101, C2 is written as C102, and so on.

　　The amplifier here is essentially a boost amplifier. Its operational amplifier and output stage are both Class A. In order to achieve direct coupling, the circuit uses a dual power supply.

　　First, the input signal is coupled to the volume control potentiometer VR1 through the DC blocking capacitor C1. The resistance of VR1 is relatively large, so that the sound is at the -3DB point in the bass area (i.e. 2HZ).

　　For most signal sources, if its signal does not generate any additional DC voltage on the capacitor, then the capacitor can be omitted. Of course, for safety reasons, it is best to retain the capacitor, otherwise the DC offset at the input will cause a larger offset at the output.

　　If a DC offset occurs, a small value will increase current consumption at the output stage and cause distortion; a large value will damage the headphones.

　　The volume control VR1 determines the input impedance of the amplifier to be 47KΩ, because the input stage of IC1 is a junction field effect transistor (JFET), which has an input resistance of about 10-12MΩ.

　　There are a large number of operational amplifiers on the market that can be widely used in audio circuits, but after a lot of experiments, only TL072 has the best performance, reasonable price, low noise, response rate of 13V/mS, and high current absorption capability.

　　Despite these features, these components rarely operate under optimal conditions. For example, the output current of an operational amplifier is 2mA, and it only works in Class A and B, with a load less than 10KΩ. The solution is to connect a resistor of appropriate resistance between the output terminal and the negative pole of the power supply, thus forcing it to be adjusted to Class A.

　　The operational amplifier IC1 in the figure is a common-mode input amplifier, connecting the input signal from the sliding contact of the variable potentiometer VR1 to its same-name terminal (+). Resistors R3 and R4 have two functions: the first function has been mentioned before, which is to force the operational amplifier to operate in Class A mode; the second function is to provide bias for the output stage composed of TR1 and TR2.

　　The complementary transistors TR1 and TR2 adopt emitter follower operation mode, so that their input impedance seen from the base is high, while the output impedance of the output is low.

　　In circuit design, resistors R5 and R6 are very important because they are connected in series with the emitters of TR1 and TR2 to generate local negative feedback, making the output stage work in a linear state.

　　The voltage drop on R5, R6 and R3 is also very important. It makes the output stage enter the Class A working state. The negative feedback is fed back from the connection point of resistors R5/R6 through resistor R2 to the inverting input terminal (pin 2) of IC1. The voltage gain of the amplifier is determined by the ratio of resistors R2 and R1 (10 times). Capacitor C2 is used to block DC, so that its AC negative feedback coefficient is R5/R6, and the DC negative feedback is 100%. The output terminal of the amplifier is directly connected to the headphones.

　　After introducing the amplifier circuit, attention should be turned to the power supply. The transformer has two 6V secondary coils, which can provide AC power to the bridge rectifier. After rectification, it is filtered by electrolytic capacitors C3 and C4. This is a very basic voltage stabilization circuit. Of course, in order to achieve better results, you can also choose a better power supply method, I believe you can find it in other circuits.