Hi-fi Audio Amplifier Circuit Design and implementation

The Need
Designing Audio amplifier from the scratch using discrete components  is interesting, as it allow users to design amplifiers that can satisfy their various requirements. With audio amplifiers, low level sounds from mobile devices around us can be made more louder and lively as well.

The practical Power amplifier design
Based on the specifications of the Hi-fi Audio amplifier, the values of the system components were calculated and selected accordingly  to give rise to the schematic shown in figure 1.0.

Fig. 1.0: The practical hi-fi amplifier design circuit

D1 is a green LED, and should be a standard type. Don't use a high brightness LED, or change the colour. This is not for appearance (although the green LED looks pretty neat on the board), but for the voltage drop. Different coloured LEDs have a slightly different voltage drop.

VR1 is used to set the quiescent current, and normally this will be about 50-100mA. The amp will work happily at lower current, at less than around 40mA. Transistor Q4 and the output drivers (Q5 and Q6) will normally not require a heat sink. Q7 and Q8 seriously need a proper heat sink, else they will burn out!

Hi-fi Audio amplifier power supply design

The power requirements of the amplifier are calculated using equations below.

Vo = √ (2*Rload*Pload); Output Voltage  ----------------------Eqn.1

Io = Vo / Rload; Output Current  ---------------------------------Eqn.2

Vcc = Vo + Vccsafety; Voltage Supply to Hi-fi audio amplifier -----Eqn.3

Preliminary Calculations for Power Requirements

The specification of the Amplifier design was chosen to be: Rload = 8Ω; load impedance Pload = 40W; watts into load

Note: Any specification can be chosen, but the modification on the circuit will affect the power supply used and the output transistors (MJ15015, MJ15016).Make sure that the output transistor’s Maximum I_C (Collector Current) is greater than the calculated I_O. You can get the detailed datasheet of all the components at Alldatasheet.com.

Vccsafety = 5V;  Vcc safety region chosen to avoid transistor saturation

Basic Calculations:

Vo = √ ( 2 * 8 * 40 ) = 25.3V

Io = 25.3 / 8 = 3.2A

Vcc = 25.3 + 5 = 30.3 ≈ 30V

Design Considerations
Operation into 4 ohm loads is not recommended with the 30V supplies. Peak dissipation will exceed 40W in each output transistor, leaving no safety margin with typical inductive loads.

For 2 channels at 40 watts, the total power requirement is approximately 7 amps current drawn from each supply under full load conditions.  The peak voltage requirements are about 30 volts for both Vcc and Vee.

A possible power transformer capable of meeting these requirements is a 22-0-22 or 44V RMS AC center-tapped secondary that can supply 10 amps.  Once rectified, the DC voltage on both positive and negative supplies will be approximately 22*√ (2) = 31.11 volts, which of course will drop somewhat under load. The Power supply design is shown in figure 1.1. 

                               Fig. 1.1: The power supply circuit design  

It is very important, that in all cases a slow blow fuse is required because of the inrush current of the transformer. A 1.5A slow-blow type was selected for the mains fuse. A packaged rectifier bridge rated for 10A at 50V was chosen, or equivalent. Wiring needs to be heavy gauge, and the DC must be taken from the capacitors - not from the bridge rectifier.

End note

The audio amplifier produced a high fidelity audio that appeals to any listening audience.

For those that may prefer an Integrated Circuit Audio Power amplifier that will produce approximately the same power like the one discussed in this work , I will recommend TDA7265. The TDA7265 class AB audio amplifier offers application circuits for 25W + 25W split supply or a 50W bridge supply to 8Ω speakers.