Radio Headphone Amplifier II

At start up Q1 conducts until the voltage at C3 is about 0.8 V below the supply rail. 

If the headphones are connected then since the collector current of Q1 is going through the base of Q2, Q2 greatly assists Q1 in pulling up the voltage at C3.  

The saturation voltage of Q2 is about 0.2 V.  That means there is about 0.8 - 0.2 V = 0.6 V voltage swing available at the collector of Q2 which is more than enough for headphones.

After start up the maximum volume is basically determined by R1. The value shown will cause clipping at moderate volume which is good for hearing protection. If you like louder you can reduce R1.

There are still 2 high volume situations. Start up and inserting the headphones into the jack.

Placing say a 120 ohm resistor in parallel with the headphone jack should reduce the insertion volume a lot.

A soft start power supply can remove the start up thump.

The input impedance of the headphone amplifier without R3 is very high. However high input impedance is not always wanted with transistor amplifiers. That is because transistors are current based devices and a high input impedance means less current signal for a given input signal voltage.  

Including R3 gives a big boost in gain for many applications.  However there is a price to pay in stability, mainly because the audio gain gets very high. 

A value of 33k for R3 is fine in most cases, if you reduce R3 to say 12k you can start to have trouble with supply rail decoupling or RF leakage from an RF stage.

R4 is an optional click noise reduction resistor when someone inserts headphones while the amplifier is on.