Circuit diagram:
Parts:
P1_____________50K Log. Potentiometer (or 47K)
(twin concentric-spindle dual gang for stereo)
R1____________100K 1/2W Trimmer Cermet
R2____________100K 1/4W Resistor
R3______________8K2 1/4W Resistor
R4,R8_________100R 1/4W Resistors
R5______________2K7 1/4W Resistor
R6______________1K 1/4W Resistor
R7____________680R 1/4W Resistor
R9______________1R 1/2W Resistor
C1____________470nF 63V Polyester Capacitor
C2_____________47µF 25V Electrolytic Capacitor
C3,C6_________470µF 25V Electrolytic Capacitors
C4____________220µF 25V Electrolytic Capacitor
C5_____________47pF 63V Polystyrene or Ceramic Capacitor
C7___________1000µF 25V Electrolytic Capacitor
Q1___________BC560C 45V 100mA Low noise High gain PNP Transistor
Q2___________BC337 45V 800mA NPN Transistor
Q3___________BD437 45V 4A NPN Transistor
Q4___________BD675A 45V 4A NPN Darlington Transistor
SPKR___________One or more speakers wired in series or in parallel
Total resulting impedance: 8 Ohm
Minimum power handling: 10W
Comments:
More than eleven years after its publication on this website and an ever increasing success, the 3 - 5W Class-A Audio Amplifier has undergone some small modifications (the addition of R1, Q2, R6 and the substitution of Q4 with a Darlington transistor) capable of producing a major improvement.
In fact, the open-loop gain was increased, distortion figures at all power levels were dramatically reduced (e.g. up to 30 times @ 500mW), current drawing was reduced to less than 500mA, maximum output power was pushed up to 7.2W RMS into 8 Ohm load and input sensitivity was fixed to about 250mV RMS: in this way a CD player or Tuner can be directly connected to the amplifier input without the need of a preamplifier.
The reduced current consumption allowed the use of an ultra-simple but very efficient regulated power supply employing a common LM317 IC, capable of driving, with a heatsink of modest size, two amplifiers.
The power supply voltage of the amplifier must be set exactly to 24V by means of Trimmer R2, visible in the Power Supply Circuit diagram shown below.
In the amplifier, Trimmer R1 was added to obtain, with reasonable certainty, the maximum output power possible (7.2W RMS). By means of this component the voltage at the positive lead of C6 must be set exactly at one half the supply voltage, i.e. 12V (with P1 turned fully anticlockwise).
Power Supply Circuit diagram:
Power Supply Parts:
R1_______________2K7 1/2W Resistor
R2_______________5K 1/2W Trimmer Cermet
R3_____________220R 1/4W Resistor
C1____________4700µF 35V or 50V Electrolytic Capacitor
D1_____Diode bridge 100 to 400V, 2 to 4A
D2______________LED Any type and color
IC1___________LM317 3-Terminal Adjustable Regulator 1.5A
SW1_____________SPST Mains switch
T1______________230V Primary, 24V Secondary,
30-32VA or 1.25 to 1.33A, Mains transformer
PL1_____________Male Mains plug with cord
Comments:
The regulated power supply circuit is straightforward. Parts values suggested are suited to power a stereo version of this design.
Notes:
- With both amplifiers connected to the regulated power supply, adjust Trimmer R1 to obtain exactly 24V at the output pin of IC1.
- Adjust R1 in each amplifier to obtain exactly 12V across C6 positive lead and negative ground.
- Total current drawing of each amplifier is about 450-500mA and should not require adjustment.
- Each output transistor (Q3 and Q4) must be mounted on a finned heatsink of 120x50x25mm. minimum dimensions.
Technical data:
- Output power:
- 7.2 Watt RMS into 8 Ohm (1KHz sine wave)
- Input sensitivity:
- 270mV RMS for full output
- Frequency response @ 1W RMS:
- Flat from 50Hz to 20KHz; -2dB @ 20Hz
- Total Harmonic Distortion @ 1KHz and 10KHz:
- 500mW 0.01% 1W 0.02% 2W 0.035% 3W 0.08% 5W 0.12% 7W 0.15%
- Unconditionally stable on capacitive loads
