Transistorized Hi-Fi Amplifier (25 Watt)

In this 25W Hi-Fi Amplifier described in details are capable to deliver truly 25Wr. 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, such as, (2-sets of ‘Main Amplifier’, one set of ‘Stereo Balance Control Circuit’, one set of ‘Stereo Balance Meter’, two sets of ‘Pre-Amplifier’ and one suitable ‘Power Supply Unit’ that shall be capable of handling 2.5 Amp at 48V DC.

TECHNICAL SPEC: Nominal Power Output = 25W r.m.s. (8 ohms Load)
Input Sensitivity = 350mv
Input Impedance = greater than 150 K Ohms
Frequency response = 20Hz to 40 KHz
Nominal Power Supply = 48V DC
Full Load Current (Mono Version) = 0.85Amp
Total harmonic Distortion (Po = 25W) = Less than 0.7%

DESCRIPTION: In the circuit, one p-n-p transistor BC158B (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.

One n-p-n transistor BD 137 (TR2) has been used in the pre-driver stage. If it would be possible to use stabilized 48V DC supply then BC 147B may use in place of BD 137.

Two n-p-n & p-n-p complimentary matched pair transistors TR4 (BD137) and TR5 (BD 138) have been used in driver stage. At full output, maximum current which TR4 has to deliver about 135ma and TR5 shall consume somewhat lower current. Resistors R14 & R15 have been included to limit short-circuit and overdrive conditions. The zero-output (quiescent) current for the driver transistors shall be about 10 ma and in case of the maximum power (Full load) the driver transistors shall dissipates about 450 mw. Therefore, no heat sinker shall be required for the driver transistors.

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

Electronic Circuits – Simplified