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Pure Class-A Headphone Amplifier

2015-01-05 08:15  
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400mW RMS into 32 Ohm load, Single-rail Supply - Optional Tilt Control

This design is derived from the Portable Headphone Amplifierfeaturing 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.

Tilt control:

The 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.

Circuit diagram:
Pure Class-A headphone Amplifier Circuit Schematic Diagram Class-A Headphone Amplifier Circuit Diagram


P1 = 22K Dual gang Log Potentiometer (ready for Stereo)

R1 = 15K 1/4W Resistor
R2 = 220K 1/4W Resistor
R3 = 100K 1/2W Trimmer Cermet
R4 = 33K 1/4W Resistor
R5 = 68K 1/4W Resistor
R6 = 50K 1/2W Trimmer Cermet
R7 = 10K 1/4W Resistor
R8 = 47K 1/4W Resistors
R9 = 47K 1/4W Resistors
R10 = 2R2 1/4W Resistors
R11 = 2R2 1/4W Resistors
R12 = 4K7 1/4W Resistor
R13 = 4R7 1/2W Resistor
R14 = 1K2 1/4W Resistor
R15 = 330K 1/4W Resistors (Optional)
R16 = 680K 1/4W Resistor (Optional)
R17 = 220K 1/4W Resistors (Optional)
R18 = 330K 1/4W Resistors (Optional)
R19 = 220K 1/4W Resistors (Optional)
R20 = 22K 1/4W Resistors (Optional)
R21 = 22K 1/4W Resistors (Optional)

C1 = 10μF 25V Electrolytic Capacitors
C2 = 10μF 25V Electrolytic Capacitors
C3 = 10μF 25V Electrolytic Capacitors
C4 = 10μF 25V Electrolytic Capacitors
C5 = 220μF 25V Electrolytic Capacitors
C6 = 100nF 63V Polyester Capacitors
C7 = 220μF 25V Electrolytic Capacitors
C8 = 2200μF 25V Electrolytic Capacitor
C9 = 1nF 63V Polyester Capacitors (Optional)
C10 = 470pF 63V Polystyrene or Ceramic Capacitor (Optional)
C13 = 15nF 63V Polyester Capacitor (Optional)
C11 = 1nF 63V Polyester Capacitors (Optional)
C12 = 1nF 63V Polyester Capacitors (Optional)

D1 = 5mm. or 3mm. LED
D2 = 1N4002 100V 1A Diodes
D3 = 1N4002 100V 1A Diodes

Q1 = BC550C 45V 100mA Low noise High gain NPN Transistors
Q2 = BC550C 45V 100mA Low noise High gain NPN Transistors
Q3 = BC560C 45V 100mA Low noise High gain PNP Transistor
Q4 = BD136 45V 1.5A PNP Transistor
Q5 = BD135 45V 1.5A NPN Transistor

IC1 = 7815 15V 1A Positive voltage regulator IC
T1 = 220V Primary, 15 15V Secondary-5VA Mains transformer

SW1 = 4 poles 3 ways rotary Switch (ready for Stereo)
SW2 = SPST slide or toggle Switch

J1 = RCA audio input socket
J2 = 6mm. or 3mm. Stereo Jack socket
PL1 = Male Mains plug

Q4, Q5 and IC1 must be fitted with a small U-shaped heatsink.For a Stereo version of this circuit, all parts must be doubled except P1, IC1, R14, D1, D2, D3, C8, T1, SW1, SW2, J2 and PL1.If the Tilt Control is not needed, omit SW1, all resistors from R15 onwards and all capacitors from C9 onwards. Connect the rightmost terminal of R1 to the Base of Q1.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.Those lucky enough to reach an oscilloscope and a 1KHz sine wave generator, can drive the amplifier to the maximum output power and adjust R3 in order to obtain a symmetrical clipping of the sine wave displayed.

Technical data:
Output power (1KHz sinewave):32 Ohm: 427mW RMS64 Ohm: 262mW RMS100 Ohm: 176mW RMS300 Ohm: 64mW RMS600 Ohm: 35mW RMS2000 Ohm: 10mW RMS
140mV input for 1V RMS output into 32 Ohm load (31mW)500mV input for 3.5V RMS output into 32 Ohm load (380mW)Total harmonic distortion into 32 Ohm load @ 1KHz:1V RMS 0.005% 3V RMS 0.015% 3.65V RMS (onset of clipping) 0.018%Total harmonic distortion into 32 Ohm load @ 10KHz:1V RMS 0.02% 3V RMS 0.055% 3.65V RMS (onset of clipping) 0.1%Unconditionally stable on capacitive loadsSource: www.redcircuits.com

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