K214/J77 Single-ended Headphone Amplifier

K214/J77 Single-ended Headphone Amplifier

The single-ended amplifier circuit is a model of exchanging efficiency for sound quality. The circuit introduced below is a single-ended headphone amplifier made of FET and MOS tubes. It is suitable for driving medium and low impedance headphones with not too low sensitivity.

1. Circuit principle: As can be seen from the figure below, the entire amplifier can be divided into 4 parts:

1. Buffer circuit composed of TR1 (K246) and TR2 (J103): used to isolate the signal interference between the front and rear stages and improve the clarity of the sound. This circuit is a Class A complementary source output device with high input impedance and low output impedance. It is used in many circuits of Japanese audiophiles

2. Use a twin TR3 and TR4 (NPD5566) to form a differential amplifier circuit: The advantage of using twin field effect tubes is that the tubes have good symmetry, which omits the heavy and complicated pairing work. When the field effect tube works at a high current, it can withstand large dynamic input signals and always maintain the Class A working state, and will not introduce high-frequency noise due to the increase in working current. The static current of this stage is about 9MA per tube. In order to cooperate with the high current working state of NPD5566, the load of the differential amplifier stage uses an active load, which is composed of Q1 (A1145) and Q2 (A1145) to form a mirror current source.

3. The output stage uses K214/J77 with mellow and delicate sound quality as the output stage, and the circuit adopts the drain output configuration.

4. The circuit of TR5 and TR6 (NPD5566) not only provides a constant current source load for the TR3 and TR4 differential circuits, but also provides a bias voltage for Q4, so that Q4 has a fixed operating current and provides a constant current source load for the output amplifier Q3.

2. Working Principle:

The input signal is coupled to the buffer stage through the input capacitor, and after being isolated by the buffer stage, it is input to the differential amplifier circuit. The amplified signal drives Q3 to output to the load.

3. Installation and debugging:

During the installation process, as long as the components are measured and welded correctly before installation, there will be basically no problems. The key lies in the debugging process. The usual debugging process has two steps:

1. Adjust the precision adjustable resistor R3 to make the output potential closest to 0V. In the prototype made by the author, the output midpoint can be controlled within 10MV.

2. Adjust the resistor R11 so that the quiescent current of the output stage is between 80-140MA. This current can be converted by measuring the voltage across the resistor R13 or R19, that is, the voltage across the resistor is: 0.26V-0.46V. Practice has shown that the smaller the current, the purer the sound quality, and the larger the current, the heavier the sound quality. It can be adjusted according to personal preference.