The design shown below is a stereo class A headphone amplifier audio is fed in at J4 for both left and right channels. The audio channels then goto SW1 and are either bypassed directly to the output or switched into the audio amplifier.
Both left and right channels are identical, the signal is initially routed to a dual ganged pot that acts as the volume control.
Audio is then AC Coupled via C1 and biased by R1 & R2 so Q1 is acting as a common emitter amplifier with a voltage gain of roughly 5 set by the ratio of R3 and R4.
Q2 which is a as an emitter follower to drive the headphones via C2.
Negative feedback is provided via resistor R12, the purpose of R14 is to discharge the output capacitor if the headphones are not present.
The power supply either external or 9V battery is switched into node VSWO and goes through the ripple remover formed around Q5.
Please see the EEVBlog video on this circuit.
This project is on a dual sided PCB and the Bill of Materials is shown below
QTY PART-REFS VALUE --- --------- ----- Resistors --------- 2 R1,R5 60k 2 R2,R6 15k 3 R3,R9,R24 1K 2 R4,R10 4.7k 1 R7 3.3k 2 R8,R11 300 2 R12,R13 47k 2 R14,R15 1M 8 R16-R23 0R 1 R25 10k Capacitors ---------- 2 C1,C4 1uF 2 C2,C5 2200uF 1 C3 10uF Transistors ----------- 3 Q1,Q3,Q5 BC847B 2 Q2,Q4 BC817 Diodes ------ 1 D1 LED 2 D2,D3 1N4148 Miscellaneous ------------- 2 J1,J4 SIL-100-03 2 J2,J3 SIL-100-02 1 P1 PDB182-B 1 SW1 3PDT