40W audio amplifier

4.5 5 4 Product

This audio amplifier is a classic design and it is exclusively based on transistors. There are not any integrated circuits. The amplifier is able to deliver up to 40W rms at 8 ohm. Its output power may reach about 80W during peaks.

The amplifier runs from a 55 to 65V (the recommended voltage is 60V) power source and its current consumption is about 1A during peaks. Its idle current (with zero input) is around 70mA. The input resistance of the amplifier is about 40KΩ and the total harmonic distortion (THT) does not exceed 0.15% at full power. The amplifier reaches its maximum when driven from 1.3V rms at its input. It has a quite broad bandwidth, from 20Hz to 100KHz.

The circuit schematic of the 40W audio amplifier is presented in Figure 1. The first stage of the amplifier is based on T1. T1, functions as a classic common emitter amplifier. Some negative feedback from R16 stabilizes its gain. Resistors R1 and R2 are used to properly bias the base of T1, and their values are high enough, to ensure a high input resistance. T1 is a classic BC107B transistor. Some other equivalent types, such as BC147, BC171, BC207 or BCY59 can be used instead.

Electronic circuit schematic
Figure 1. The electronic circuit of the 40W audio amplifier

From the collector of T1, the signal is driven through C3 to the base of T2. T2 is a BC141 and operates also as a common emitter amplifier. However, it is DC coupled to the next stages and its bias point can be adjusted from the R8 trimmer. This adjustment also affects the operation point of the final stage.

The R13 trimmer is also used to adjust the idle current of the final stage. T3 is used to produce an appropriate potential difference of about 1V between the bases of T4 and T5. This eliminates any crossover distortion that would be otherwise present in the push-pull final stage.

The output stage is based on transistors T4, T5, T6 and T7. Actually, the load current passes almost exclusively through T6 and T7. T6 and T7 are the complementary PNP and NPN transistors TIP33A and TIP34A, respectively. MJE2955 (PNP) and MJE3055 (NPN) can also be used instead. T4 and T5 act as current drivers for T6 and T7. T4 is an NPN type BD137 or BD139, while T5 is a BD138 or a BD140 of type PNP.

How to assemble the amplifier

The 40W audio amplifier can be easily assembled in the printed circuit provided below. All components should be placed on the circuit board according to the assembly guide of Figure 2.

All components are mounted on the board, apart from T3, T6 and T7. These transistors should be placed on a heatsink, in order for the dissipated heat to be re-moved.  Actually, T3 does not heat up, but it must be placed on the same heatsink, along with the T6 and T7 to achieve some temperature compensation.

How to assemble the amplifier
Figure2. Assembly instructions for the 40W audio amplifier

Any temperature increase in the final stage leads to an increase in the current which leads to a further increase in temperature. This phenomenon can cause overheating and a possible transistor burn-out. Thus, some kind of temperature compensation is required. This is up to T3. During temperature increase, the current through T3 is increasing and results in a reduction of the bias voltages in the bases of T4 and T5. This results to the current reduction in the final stage and acts as an effective temperature compensation mechanism.

Therefore, it is critical to place the T3 on the heatsink, along with the T6 and T7, and it is even better to place it near the T6 or T7. T3 has a plastic case, so it should be placed on the heatsink either with some suitable glue or with the use of any metallic screw plate.

T6 and T7 must also be electrically insulated from the heatsink and from each other but at the same time, appropriate thermal contact must be ensured. For this purpose, some mica insulating sheets between the body of each transistor and the sink and also plastic insulators on the screws should be used. Good electric insulation should be verified with an ohmmeter. It is also recommended to use a small heatsink also for the T2, as it can be seen in the photo of the prototype.

All resistors in the circuit are of 1/2W type, except for R19 and R20 which must withstand 3W. Both R19 and R20 have the same value of 0.33 ohm. Alternative, a value of up to 0.47 ohm is acceptable for R19 and R20 in exchange with some slight reduction in the maximum output power of the amplifier.

Initial settings

In order for the circuit to operate properly, two initial settings are required. It is advisable to make these settings by using a load in place of the loudspeaker.

Before the circuit is even powered up, you should o connect a voltmeter between the positive terminal of the C9 and the earth (negative terminal of the power supply). After applying the power supply, the trimmer R8 should be adjusted until the voltmeter shows exactly half the voltage of the power supply. That is, if we have a 60V power supply, the voltmeter should display 30V.

Next, we need to connect a milliamp meter in series with the power cord and adjust the trimmer R13 until we reach an idle current of around 70mA. If the setting of R13 affects the previous setting of R8, we will have to repeat the setting of R8 to get again a voltage equal to half the supply voltage at the common connection point of R19, R20 and C9.

After the initial settings, the voltages at different points in the circuit should be approximately equal to those shown in Figure 1 and the amplifier will be ready for normal operation. The indicative values in Figure 1 refer to a 60V supply voltage.


The printed circuit board artwork for the 40W audio amplifier


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