Class A-Based Headphone Amplifier

Class A Based Headphone Amplifier


The design of the circuit is based on the operation of a class A amplifier for the purpose of driving a pair of headphones using a mini amplifier.

Terminology Class A – refers to an output stage where the bias current is greater than the maximum output current thus, all output transistors are always conducting current due to its linearity and low distortionBC184/BC214 – a complementary silicon planar epitaxial transistor used in AF small signal drivers and amplifiers as well as for low noise preamplifier applications due to its feature of good linearity of DC current gainBD135 – NPN power transistor used for driver stages in hi-fi amplifiers and television circuits because of its low voltage at 80 V maximum and high current at 1.5 A maximumBD136 – a Silicon PNP power transistor used for driver stages in high-fidelity television circuits and amplifiers with features such as high current and packaged with TO-126 Circuit Explanation

The headphone amplifier was a simple circuit to design. This is due to the fact that neither the load characteristics nor the power requirements in the output is very difficult to achieve because the headphones are typically having a 50 ohm and higher load impedance up to 600 ohms. For the normal output, it will only require 1 V to 2 V RMS at maximum. A class A stage of amplifier is preferred because only low power is required on this circuit. Oftentimes, class A amplifiers comprise of a constant current transistor connected from output to negative power supply and a driven transistor connected from output to positive power supply. The constant bias current flows directly from the positive supply to the negative supply in the absence of input signal. This will result with no output current but with several powers consumed.

In this circuit, the transistors Q5 and Q6 should be rated at 100 mA each, to be able to operate as class A amplifier efficiently. A power output of 1.5 Watts is produced from the 15 V power supply. A small heatsink is required though for each transistor. The audio input signal is regulated by a 10K ohm logarithmic potentiometer RV1 while the output DC offset voltage for 0 V is regulated by the trimmer TR1. Two circuits will be needed however, if it is intended for stereo application.

Part List

R1-4= 1.2Kohm
R2-3= 3.9Kohm
R6-10= 10Kohm
R7-9= 2.2Kohm
R8= 150 ohm
R11-13= 6R8 ohm
R12= 4.7 ohm
RV1= 10Kohm  Log. pot.
TR1= 10Kohm Trimmer
C1= 4.7uF 63V MKT
C2-7= 100uF 25V
C3-5= 100nF 100V  MKT
C4-6= 100uF 16V
C8-10= 470uF 25V
C9-11= 100nF 100V ceramic
Q1-3= BC184
Q2-4= BC214
Q5= BD136 or BD538
Q6= BD135 or BD537
All resistors are 1/4W 1%


A headphone amplifier is a type of audio amplifier and a miniaturized power amplifier designed specifically to drive the small speakers inside the headphones correctly instead of loudspeakers. They are commercially available and embedded in electronic devices such as portable music players, televisions, and integrated amplifiers. They also allow the use of high-impedance headphones from low power audio sources. The dynamic range extension and timbral & tonal detail resolution is improved substantially while musical clarity is maximized.

In desktop or workstation setting, headphone amplifiers can also be used as a preamplifier stage for self-powered desktop speakers and as a central controller switch between different analog and digital sound sources. USB connections are also available in other dedicated headphone amplifiers, used along with other digital inputs like coaxial and optical, that are directly from a computer’s digital USB feed.

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