7 Watt Class-A Audio Amplifier

Behaves like a one-valve operated amplifier

Improved version of the 3-5W Class-A amp

Circuit diagram:

7 Watt Class-A Amplifier


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


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 for the 7 Watt Class-A Audio Amplifier

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


The regulated power supply circuit is straightforward. Parts values suggested are suited to power a stereo version of this design.


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