Thursday, August 21, 2014

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.

     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.
     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.
    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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