Yet Another Headphone Amplifier

Pure Class-A Single-ended circuitry

500mW into 32 Ohms load - Very low distortion

Circuit diagram:

Single ended Class-A Headphone Amplifier


P1_____________50K  Log. Potentiometer (or 47K)
                    (twin concentric-spindle dual gang for stereo)

R1,R2__________47K  1/4W Resistors
R3____________100K  1/4W Resistor
R4______________8K2 1/4W Resistor
R5____________470R  1/4W Resistor
R6______________2K7 1/4W Resistor
R7______________1K  1/4W Resistor
R8____________560R  1/4W Resistor
R9____________100R  1/4W Resistor
R10____________10R  1/4W Resistor
R11____________22R  1/2W Resistor

C1____________470nF  63V Polyester Capacitor
C2_____________47µF  25V Electrolytic Capacitor
C3,C6,C7______220µF  25V Electrolytic Capacitors
C4____________100µF  25V Electrolytic Capacitor
C5_____________47pF  63V Polystyrene or Ceramic Capacitor

Q1___________BC560C  45V 100mA Low noise High gain PNP Transistor
Q2___________BC337   45V 800mA NPN Transistor
Q3,Q4________BD437   45V 4A NPN Transistors


The extreme simplicity of the 7W Class-A Audio Amplifier circuit, leads invariably to think that it could be naturally converted into a Headphone Amplifier. So, this was immediately done and as a result, a 500mW into 32 Ohms load Headphone Amplifier was obtained, featuring ultra low THD, about 100mA current drawing per channel and small heatsinks for the output transistors

Power Supply Circuit diagram:

Power Supply for the Single ended Class-A Headphone Amplifier

Power Supply Parts:

R1_______________2K7 1/2W Resistor
R2_______________6K8 1/4W Resistor
R3______________18K  1/4W Resistor
R4_____________270R  1/4W Resistor

C1____________3300µF  35V or 50V Electrolytic Capacitor
C2______________10µF  25V Electrolytic Capacitor

D1_____Diode bridge  100 to 400V, 1A
D2______________LED  Any type and color

IC1___________LM317  3-Terminal Adjustable Regulator 1.5A

SW1_____________SPST Mains switch

T1______________230V Primary, 24V Secondary,
                     7-10VA or 290 to 400mA, Mains transformer

PL1_____________Male Mains plug with cord


Technical data:

Max output power:
545mW RMS into 32 Ohms (1KHz sine wave)
Input sensitivity:
250mV RMS for 30mW output, 1V RMS for full output
Frequency response @ 100mW RMS:
Flat from 20Hz to 20KHz
Total Harmonic Distortion @ 1KHz and 10KHz:
0.5mW 0.005% 1mW 0.004% 15mW 0.002% 100mW 0.002% 136mW 0.0045% 250mW 0.013% 500mW 0.04%