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 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
- 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.
- 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%