Low noise hybrid headphone amplifier

Having a good pair of headphones that connect directly to the source is the easiest way to listen to music. However, even when the maximum signal output amplitude of the standard audio source is 0.755V, the power is only: U2/R=0.755V/32Ω=18mW, which cannot achieve satisfactory results. Therefore, an amplifier is required between the audio source and the headphones.

The hybrid headphone amplifier produced by the author (see the attached diagram for the circuit) uses a vacuum tube 12AU7 cathode follower and a Darlington tube BC517 emitter follower to form a high input and low output impedance amplification circuit. The high input impedance characteristic of V1 prevents the input impedance of T3 from being affected by changes in the preamp volume potentiometer or signal source, keeping the audio output frequency response stable. T3, T1, and T2 form a constant current source Class A single-ended power amplifier circuit, and R5 sets the operating current of the constant current source to 27mA. Because the screen electrode of V1 works at a low voltage of 12v and the screen current is weak, T3 uses a high-current gain Darlington tube. C2 is the radio frequency filter capacitor. The R2 and R4 voltage divider circuit biases the V1 gate at 1/2 the power supply voltage. C1 and R1 form a signal input circuit, where R1 is the gate input voltage matching resistor. Note that the capacitance of C1 cannot be too small. T5, R7, and C5 form a capacitance multiplication filter circuit, which has the advantages of low circuit impedance, low energy consumption, smooth and stable filter output, and provides a DC working voltage with a noise-suppressing background to the V1 screen electrode and T3 collector. In addition, when the power is turned on, the power supply charges C5 through R7, and T5 is turned on in a delayed manner, causing the V1 filament to gradually heat up. T4 is used to block the interference signal generated by the filament circuit from entering the audio amplification channel. The on-resistance of T4 is only 0.16Ω.