Class one Headphone Amplifier

This Class one Headphone Amplifier 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.
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Circuit diagram :
Class-A Headphone Amplifier-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.
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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 :
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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

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