This is a headphones amplifier based exclusively in common, low price discrete components. It is very simple, requires no adjustment and can drive headphones with an impedance of 8 ohms up to 1K. The circuit includes a volume control potentiometer.
The electronic circuit of the transistor headphones amplifier is shown in Figure 1. The amplifier is stereo, so it uses two identical circuits, one for the right (R) and one for the left (L) audio channel. We will only describe the circuit on the left (L) audio channel, since the right (R) one is identical.
The amplifier is actually an operational amplifier designed with discrete components. The output stage is designed to deliver enough power at loads ranging from 8Ω to 1KΩ. The power is in the order of some hundred milliwatts.
Each amplifier consists of three stages. For the left channel (L), the first stage, as in all operating amplifiers, is a differential amplifier consisting of transistors T1, T2 and T3. The second stage is for driving and biasing the final stage. The final stage is a power amplifier made from a complementary NPN-PNP transistor pair (T6 and T7).
The differential amplifier stage provides a high common-mode rejection ratio (CMRR), which is extremely important for noise suppression. The differential amplifier is made from the T1 and T2 pair while T3 is used as a constant current source for biasing. The inverting input of the differential amplifier is the gate of T2 while the non-inverting input is in the gate of T1.
The input audio signal is applied to the non-inverting input while some feedback from the amplifier’s output is applied to the inverting input. Resistors R5 and R6 function as a voltage divider that transfers a percentage of the signal from the amplifier output to the inverting input of the differential pair. This way we get negative feedback. Typically, the total gain of the amplifier is determined by the feedback loop and is equal to 1 + R5 / R6 = 11. The C3 capacitor, located parallel to R5 limits the gain at high frequencies for purposes of noise reduction.
The output of the differential amplifier is from T1’s drain. From there, the signal is passed at the base of T4. T4 drives the final stage. T4 drives directly the base of T6 while we also use an additional transistor, the T5, to drive the T7. T4 and T5 are in series and have the same collector current.
The output stage of the amplifier consists of the complementary transistor-pair T6-T7. The T6 operates during the positive half-cycle of the signal while the T7 during the negative. To suppress any crossover distortion in the complementary transistor pair, we use the LD1 LED. This is a common red colored LED that ensures a voltage drop around 2.1V to eliminate the cross-over. The normal way is to use an 1.2V voltage drop of a series silicon diodes pair but in our design we prefer to use a LED.
The base of T5 has the same reference voltage as the base of T3. The reference voltage is generated by two diodes connected in series (D1 and D2) and is therefore around 1.2 V. The reference voltage applied to the T5 and T3 bases also sets the voltage at the emitters of T5 and T3 at about 600 mV (the base voltage minus the voltage drop VBE). Keeping this constant potential at R10 and R3, makes the T5 and T3 transistors to operate at an almost constant current, interdependently from the AC signal (ie they act as current sources).
The P1 is a logarithmic stereo potensiometer, used to adjust the level of the audio signal applied to the input of the amplifier and therefore the volume at the outputs. The input of the headphone amplifier can be a signal from some tens of mV to about some Volts. The headphones are connected to the output of the amplifier via the J1 plug, which is a common 6.35mm female stereo plug.
The circuit requires a 2x12V symmetrical power supply. Resistors R21 and R22, together with C6 and C12, act as additional filters for the supply voltage.
Making the transistor headphones amplifier
For easy assembly of the circuit, we have made the appropriate printed circuit board. The printed circuit board has copper on one side only and is shown in Figure 2. All components should be placed on the printed circuit board according to the assembly guide of Figure 3.
All resistors we use in the circuit are of 1/4W and 5% tolerance type, apart from R7, R8, R9, R17, R18, R19 and R21, R22 which are of 1/2W, 5% type.
The electrolytic capacitors we use are of 16V (this is not crucial as long as they withstand power supply voltage ratings). All the other capacitors are of polyester or ceramic, low voltage type.
When completing the assembly, be careful not to forget to place the three wire bridges on the board in the points shown in the assembly guide.
Two supply voltages of +12 and -12V, respectively, are required to supply the circuit. That is, you need a dual-symmetric power supply.