Class-A Headphone Amplifier Circuit

400mW RMS into 32 Ohm load, Single-rail Supply - Optional Tilt Control
This design 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.

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:

Class-A Headphone Amplifier Circuit

Class-A Headphone Amplifier Circuit Diagram

Parts:
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


Notes:
  • 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 RMS
  • 64 Ohm: 262mW RMS
  • 100 Ohm: 176mW RMS
  • 300 Ohm: 64mW RMS
  • 600 Ohm: 35mW RMS
  • 2000 Ohm: 10mW RMS


Sensitivity:
  • 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 loads


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