Here is a circuit project of a reliable audio amplifier that does not distort even when working at its maximum power. It can drive up to 30W RMS in an 8 Οhm speaker and up to 45W RMS in a 40 Ohm speaker. Its power is not high, but it’s considered adequate for a typical home audio system.
The circuit is quite simple and it is based on common low price components. The total harmonic distortion (THD) at full power does not exceed 0.05%. The amplifier has flat frequency response range from 10Hz to 40 KHz. The idle current consumption is about 80mA and the maximum current does not exceed 1.5A at full power.
The audio amplifier circuit
The 30W audio amplifier circuit is shown in figure 1. The whole circuit is actually an operational amplifier designed with discrete components. The main difference with typical operational amplifiers is that the output stage of our amplifier is designed to deliver high current and hence high power at a load of 8 or 4 Ohm.
The 30W amplifier circuit consists of three stages. The first stage, as with all operator amplifiers, is a differential amplifier consisting of transistors T1, T2 and T3. The second stage is for driving and biasing the final stage and consists of transistors T4, T5 and T6. The final stage is a power amplifier made from a complementary NPN-PNP transistor pair (T7 and T8).
The differential amplifier provides a high common-mode rejection ratio (CMRR), which is extremely important for noise suppression. The common-mode rejection ratio is around 70db. The inverting input of the differential amplifier is at the base of T2 while the non-inverting input is at the base of T1. The differential amplifier is made actually from the T1 and T2 pair while T3 is used as a constant current source for biasing.
The input audio signal is applied to the non-inverting input while some feedback from the amplifier’s output is applied to the inverting input. Resistors R6 and R7 function as a voltage divider that transfers a percentage of the signal from the amplifier output to the inverting input of the differential pair. This way we get negative feedback. Typically, the total gain of the amplifier is determined by the feedback loop and is equal to 1 + R6 / R7 = 40.
C5 capacitor located parallel to R6 limits amplification at high frequencies. The upper cutoff frequency of the circuit is typically at 40KHz. C4 determines the response to low frequencies and the low cutoff frequency is around 20Hz. Thus, the amplifier has a typical band-pass response from 20 Hz to 40 KHz.
The output of the differential amplifier is from T1’s collector. From there, the signal is passed at the base of T4. The AC signal at the base of T4 controls the collector current of T4. T7 and T8 are in series with T4, so their collector current is also controlled from the same signal. T4 is used to drive the base of T7, while T5 drives the base of T8. T6 is used to adjust the bias symmetry of the final stage and at the same time provides some temperature compensation.
During any temperature increase, the current of typical transistor tends to increase and accelerates even more the heating of the transistor. This is a serious problem in all power circuits and typically some kind of thermal compensation is required. In our circuit, we decided to place the T6, T7 and T8 at the same heat sink. Thus, if the temperature increases, the current flowing through T6 increases, but this increase leads to a decrease in the emitter-base voltage of T7 and T8 and therefore reduces the current to its former value.
The output stage of the amplifier consists of the complementary transistor-pair T7 - T8. T7 operates during the positive half-cycle of the signal while T8 during the negative. To adjust T7 and T8 to operate at an ideal middle DC operating point, we use the R14 potentiometer. For setting the circuit for its best performance, R14 adjustment is required to be done with care.
The L1 coil is used to compensate for any parasitic capacitance in the speaker (mainly from the crossover). C10 and R18 elements are used to filter-out high frequencies. R12 with C2 are used to filter the supply voltage of the differential stage and C2 is also used as a bypass capacitor for AC (free-throw capacitor). C8 and C9 are used to eliminate unwanted high frequencies and noise from the T7 and T8 bases. C6 and C7 are used for stabilization and C11, C12, C13 and C14 are used to decouple and further filter the supply voltage.
Construction and operation details
To simplify the assembly of the circuit, we have made the appropriate printed circuit board. The printed circuit has copper only on one side and is shown in Figure 2:
All components should be soldered to this printed circuit board according to the assembly guide of Figure 3.
All resistors used in the circuit are of 5% tolerance and of 1/4W type, apart from R16, R17, R18 and R19 that are of 5, 5, 1 and 1 / 2W type, respectively. These power resistors are also appropriately marked in the schematic.
The electrolytic capacitors we use are of 35 or 50V (this is not crucial as long as they withstand power supply voltage ratings). The other capacitors are of polyester or ceramic, low voltage type. The L1 coil should be wrapped around the R9 resistor and consists of 12 turns made from 1mm thick wire.
Transistors T6, T7 and T8 should be placed on the same heat sink for temperature compensation. However, the metal body of the transistors must not have any electric contact with the metal heat sink. For this purpose you should use insulating mica (heat-conducting electric-insulator) and you must also use properly electrical insulation even on the screws that hold the transistors on the heat sink. These details are graphically illustrated in Figure 4.
When completing the assembly, be careful not to forget to place the two wire bridges on the board in the points shown in the assembly guide.
Two supply voltages of +30 and -30V, respectively, are required for the supply of the circuit. That is, a double - symmetric power supply unit is needed. In practice there is no need for any stabilized power supply. A centre tapped secondary winding transformer, a rectifier bridge and some filtering capacitors are the only parts you need to make the power supply unit as shown in Figure 5.
The transformer should have a centre tapped secondary winding and should be able to supply up to 2A of current. The rectifying bridge should be 100V/5A, and requires some cooling. So, it’s best to place it on a heat sink or on the metal enclosure of the amplifier (if you use any metal enclosure).
For the amplifier to work properly, only one setting is required. You should set the R14 potentiometer to achieve a resting current of around 70mA in the positive power cable. That is, you should connect a milliamper meter in series with the positive power cable line and without applying any audio signal at the input (or with a grounded input) you should set R14 until the milliamper shows 70mA. Alternatively, if you have an oscilloscope and a frequency generator, you may adjust R14 to completely eliminate any remaining crossover distortion.
Calibrating the circuit with an oscilloscope should be done in real operating conditions with the speaker or with an appropriate load connected to the output (I assume you will prefer the load so, as will not "disturb" your neighbors).
The amplifier circuit is mono. This means that to make a stereo amplifier you should use two identical circuits, one for the right and one for the left audio channel, respectively. In addition, you will need two power supply units.
Your amplifier can be placed in a suitable enclosure along with the power supply. On the front of the box you can place the appropriate plugs for the sound signals as well as a switch or even a VU meter. This way you’ll have an excellent and a nice looking amplifier.
The printed circuit board of 30W audio amplifier in pdf file