Transistorized Hi-Fi Amplifier (40 Watt)




This 40W Hi-Fi Amplifier described in details are capable to deliver truly 40W r. m. s. power. Silicon transistors have been used in all stages. Power transistors 2N 3055 or BD 182 has been used in the power output stage. The output power transistors should be Matched Pair and the output stage employs in a quasi-complimentary single ended Class-B push-pull configuration. The amplifier consists of the following stages, (1) Pre-amplifier; (2) Pre-driver; (3) Driver & (4) Power Output.

The full ‘Circuit Schematic’ with ‘Bill of Materials’ is available at the attached LINK. The amplifier is described here is only one channel. Please just double the circuit in case of Stereo application.

The following circuits may be inducted in case of stereo application, the sub-assemblies required for making a stereo system it needs, 2-sets of ‘Main Amplifier’, 1-set of ‘Stereo Balance Control Circuit’, 1-set of ‘Stereo Balance Meter’, 2-sets of ‘Pre-Amplifier’ and 1-set of Stabilized ‘Power Supply Unit’ that shall be capable of handling 3-Amp at 60V DC and 2-sets of protection circuit.

TECHNICAL SPEC: Nominal Power Output = 40W r.m.s. (8 ohms Load)
Input Sensitivity = 440mv
Input Impedance = greater than 150 K Ohms
Frequency response = 20Hz to 60 KHz
Nominal Power Supply = 60V DC
Full Load Current (Mono Version) = 1.2 Amp
Total harmonic Distortion (Po = 40W) = Less than 0.8%

DESCRIPTION: In the circuit, one p-n-p transistor BC157B (TR1) has been used in the preamplifier stage. This high gain transistor enables large amount of Dc and AC feedback by using R3, R4 and R6. The operating current of this transistor is about 0.5ma. Due to this high amount of feedback the input impedance of this stage is about 150 kilo Ohms.

The Class –A pre-driver stage n-p-n transistor BD 139 (TR2) has been used. The approximate collector current shall be 8ma so Heat Sink shall not be required for this stage.

Two n-p-n & p-n-p complimentary matched pair transistors TR4 (BD139) and TR5 (BD 140) have been used in driver stage. The quiescent current of this stage shall be approximately 10ma. At full output, maximum current which TR4 has to deliver about 210ma and TR5 shall consume somewhat lower current. Resistors R14 & R15 have been included to limit short-circuit and overdrive conditions. Proper heat sinker shall be required for the driver transistors. The size of the Heat Sinker shall be 2X2 cm2. Inch aluminum and of 2mm thick for each transistor.

The peak output current for Po = 40 Watts into 8 Ohms load shall be about 3Amp. The peak output voltage for 40 W shall be 25.5V. The upper circuit losses shall be about 4V and lower circuit losses shall be about 5V. For the genuine 40 watts output power the supply voltage must be equal to the sum of the total voltage losses and twice the peak output voltage
excursion. Therefore, it becomes 4V + 5V + 2(25.5V) = 60 Volts. So the mid-point voltage shall be 30V. As the output transistors dissipates high output power and consumes high current the heat sinker for the output transistors shall be 8 sq. cm2 made of 2mm thick aluminum metal for each transistor.

It is highly recommended to use stabilized power supply. The Circuit diagram is published at the attached link.

It is better to use BD 182 as power transistors. But the transistor BD 182 is more costly, therefore to economize the cost 2N 3055 may be used as power transistors. The quiescent current of the power transistors shall be about 40 to 45 ma which can be adjusted by variable resistance R11.

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Bill of material and Circuit Schematic

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