Pure Class-A Headphone Amplifier

400mW RMS into 32 Ohm load

Single-rail Supply - Optional Tilt Control

Circuit diagram:

Class-A Headphone Amplifier


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,R9__________47K  1/4W Resistors
R10,R11_________2R2 1/4W Resistors
R12_____________4K7 1/4W Resistor
R13_____________4R7 1/2W Resistor
R14_____________1K2 1/4W Resistor
R15,R18_______330K  1/4W Resistors (Optional)
R16___________680K  1/4W Resistor (Optional)
R17,R19_______220K  1/4W Resistors (Optional)
R20,R21________22K  1/4W Resistors (Optional)

C1,C2,C3,C4____10µF  25V Electrolytic Capacitors
C5,C7_________220µF  25V Electrolytic Capacitors
C6,C11________100nF  63V Polyester Capacitors
C8___________2200µF  25V Electrolytic Capacitor
C9,C12__________1nF  63V Polyester Capacitors (Optional)
C10___________470pF  63V Polystyrene or Ceramic Capacitor (Optional)
C13____________15nF  63V Polyester Capacitor (Optional)

D1_____________5mm. or 3mm. LED
D2,D3________1N4002 100V 1A Diodes

Q1,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 (30V center-tapped)
                    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


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 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. See the graph below:

Tilt Control Frequency response


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
140mV input for 1V RMS output into 32 Ohm load (31mW)
500mV input for 3.5V RMS output into 32 Ohm load (380mW)
Frequency response @ 2V RMS:
See the above graph
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