Class A Headphone Amplifier

This circuit is derived from the Portable Headphone Amplifier featuring an NPN/PNP compound pair emitter follower output stage. An improved output driving capability is gained by making this a push-pull Class-A arrangement. Output power can reach 427mW RMS into a 32 Ohm load at a fixed standing current of 100mA. The single voltage gain stage allows the easy implementation of a shunt-feedback circuitry giving excellent frequency stability.

Circuit diagram :

Class-A Headphone Amplifier Circuit diagram

The above mentioned shunt-feedback configuration also allows the easy addition of frequency dependent networks in order to obtain an useful, unobtrusive, switchable Tilt control (optional). When SW1 is set in the first position a gentle, shelving bass lift and treble cut is obtained. The central position of SW1 allows a flat frequency response, whereas the third position of this switch enables a shelving treble lift and bass cut.

Note:

• Before setting quiescent current rotate the volume control P1 to the minimum, Trimmer R6 to zero resistance and Trimmer R3 to about the middle of its travel.
• Connect a suitable headphone set or, better, a 33 Ohm 1/2W resistor to the amplifier output.
• Connect a Multimeter, set to measure about 10Vdc fsd, across the positive end of C5 and the negative ground.
• Switch on the supply and rotate R3 in order to read about 7.7-7.8V on the Multimeter display.
• Switch off the supply, disconnect the Multimeter and reconnect it, set to measure at least 200mA fsd, in series to the positive supply of the amplifier.
• Switch on the supply and rotate R6 slowly until a reading of about 100mA is displayed.
• Check again the voltage at the positive end of C5 and readjust R3 if necessary.
• Wait about 15 minutes, watch if the current is varying and readjust if necessary.

Parts List :

P1 : 22K Dual gang Log Potentiometer 
R1 : 15K 
R2 : 220K
R3 : 100K
R4 : 33K 
R5 : 68K 
R6 : 50K 
R7 : 10K 
R8,R9 : 47K 
R10,R11 : 2R2 
R12 : 4K7
R13 : 4R7
R14 : 1K2
R15,R18 : 330K
R16 : 680K
R17,R19 : 220K
R20,R21 : 22K
C1,C2,C3,C4 : 10µF/25V 
C5,C7 : 220µF/25V
C6,C11 : 100nF
C8 : 2200µF/25V
C9,C12 : 1nF
C10 : 470pF
C13 : 15nF
D1 : LED
D2,D3 : 1N4002 
Q1,Q2 : BC550C 
Q3 : BC560C 
Q4 : BD136 
Q5 : BD135 
IC1 : 7815
T1 : 15CT/5VA Mains transformer
SW1 : 4 poles 3 ways rotary Switch 
SW2 : SPST slide or toggle Switch