Pure Class-A Headphone Amplifier

400mW RMS into 32 Ohm load

Single-rail Supply - Optional Tilt Control

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,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

Comments:

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:

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)

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

Having Questions?