The stereo headphone amplifier is just a small stereo power amplifier. It provides a sufficient output to operate a pair of standard headphones having an impedance of 32 ohms approximately. The unit is equipped with a stereo volume-control potentiometer which allows the sound to be adjusted to a comfortable listening level. The unit was initially designed to be embedded in a homemade audio mixer for adding headphone monitoring capability. The circuit is very small and light so it may be mounted inside an audio mixer by using the volume-potentiometer bush fixing alone.
The circuit comprises two sections and a small number of components common to both sections. The first section is based on U1 and it is associated with the Left audio channel. The second section is based on U2 which is responsible for the Right audio channel.
Intergraded circuits U1 and U2 are LM386 amplifiers. LM386 can provide up to 325mW into an 8Ω load. Standard headphones usually have greater impedance than this, so the available output will be reduced. However, this does not matter because only a very small output is sufficient to fully load the headphones. Since the components used on both audio sections are identical, only a description of a single channel is needed:
The input signal for the left channel is fed to pin3 of U1. Pin 3 is the non-inverting input of the amplifier. The LM386 is a power amplifier designed for use in low voltage consumer applications. The gain is internally set to 20 (26db) to keep external parts count low. Since the gain is very high, a line –level signal would be too high for the amplifier and must first be reduced. This is carried out using a potential divider arrangement.
The incoming audio signal passes through capacitor C2, preset potentiometer R1 and rotary volume control R2 to the ground. When R1 is adjusted to its maximum resistance (1ΜΩ) and R2 is also set to maximum, the signal voltage will be divided by 100 approximately. With preset potentiometer R1 adjusted to less than maximum resistance, a smaller amount of attenuation is provided. Whatever the adjustment of the R1 would be, with R2 at its minimum setting, the voltage at the sliding contact (wiper) of R2 will be zero.
For any input signal level, the preset potentiometer R1 must be adjusted so that, when volume (R2) is at maximum, there is minimal distortion combined with sufficient volume. R4, which is the preset potentiometer for the Right audio channel, must also be adjusted so that there is a correct balance (equality) in the volume between the left and the right channels.
The audio signal of the left channel is carried by R2’s wiper and is fed to U1 threw the C4 capacitor. C4 isolates the AC audio signal from a small bias (offset voltage) which is present at U1’s input pin (pin 3). The bias is set automatically from internal components of U1 and allows both the positive and negative parts of the input signal waveform to be amplified by swinging above and below this voltage.
The amplified signal of the Left channel appears at the output pin (pin 5) of U1 and AC-coupled to the left-hand headphone threw C3. Capacitor C5 , which is connected in series with R3, is used to stabilize the amplifier and prevent any oscillation that might otherwise occur.
The circuit is powered from a 6-9V power source threw D1. D1 is used to provide reserved-polarity protection. C1 capacitor is used for power supply filtering. When using batteries, C1 serves also another useful purpose: It holds a reserve of charge which will provide any instantaneous current demands. This is extremely useful when batteries are nearing the end of their life.
The construction of the Stereo Headphone Amplifier is based on a single-sided printed circuit board. Details, about this specific board are provided at the download section. Some care must be taken when placing the electrolytic capacitors. The electrolytic capacitors must be placed with the correct polarity according to the schematic.
For R2 stereo potentiometer, it is better to use a logarithmic type because it provides an improved physiological response in volume change. The circuit board is very small and light so it may be mounted inside an audio mixer by using the volume-potentiometer bush fixing alone.
In the provided PCB there is place for every component, except than the phone jack socket for the headphones. This socket must be connected to the board by using some wires.
The power supply unit may consist of a standard low-current power supply unit or of a 6V or 9V battery. If the unit is used as stand-alone unit, four AA size alkaline cells would be satisfactory.
Printed Circuit Board Layout and Components Placement guide