Circuit diagram:
Parts:
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
Comments:
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 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
Notes:
- The regulated power supply circuit is straightforward and employs an LM317 3-Terminal Adjustable Regulator IC. Parts values suggested are suited to power a stereo version of this design.
- Total current drawing of each amplifier is about 100mA and should not require adjustment.
- Use small, U-shaped heatsinks for Q3 and Q4. A similar heatsink could be used also for IC1, though in most cases it is not necessary.
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%
