Audio amplifier - 32W

The circuit presented here is an audio amplifier built with operational amplifiers and two Darlington – type power transistors. It can be powered from a simple, non-stabilized power supply unit. Power supply voltage can be from 20 to 35V DC. Although it's a simple and low-cost amplifier, its performance is very good and can be easily built even from newbies.

Click on the image above to enlarge the schematic.

The circuit consists of two sections. The first section is a preamplifier based on op-amps, and the second section is the final amplifier, built from BDX53 and BDX54, complementary Darlington-type transistors.

R13 trimmer resistor in the preamplifier stage, is used for gain adjustment. Adjusting R13 for minimum resistance, results in a gain of about 1 (0db), while adjusting it at its maximum value, results at a voltage gain of about 22 (8db). Using minimum gain, strong audio signals of about 8-10 Vp-p can be applied at the input without resulting in distortion. Using the maximum gain, the amplifier can be driven from an audio signal of about 0.5-1Vp-p in amplitude (Line-level signal).

The input signal is first amplified by  IC1B and then is being applied at the non-inverting inputs of  IC1A and IC2B, through capacitors C5 and C6. The IC2A op-amp is used to drive the BDX53, which only enhances at the positive transitions of the signal, and IC2B drives BDX54 that is used to amplify the negative transitions of signal. Trimmer resistor R11 is used to adjust the bias currents of  T1 and T2. The output of the amplifier is formed at the connection point of R9-and R10, and is fed to the speaker through capacitor C11, which suppresses DC. 

IC1A is used as a voltage regulator. Taking reference voltage from a Zener diode (D1), IC1A produces a stabilized voltage which is applied to the voltage divider R5-R6, and biases the non-inverting input of IC1B. The same reference voltage is also applied to the voltage divider R4-R11-R12 and biases the non-inverting inputs of IC2A and IC2B. For correct biasing, R3 should be adjusted so that the DC voltage at the output pin (pin 1) of IC1A to be at about 2V less than the supply voltage. 

The Zener diode D1, is used for voltage reference and must be biased correctly threw R1. For correct biasing, the current flow through D1 must exceed a specific value which depends on the supply voltage. That's why, R1's value is related to the supply voltage. In the table below, we present the recommencement value of R1, for 4 possibly supply voltages. R1 must be able to tolerate as much as 1/2W. For all the other resistors in the circuit (except from R9 and R10),  you can use 1/4W type resistors. R9 and R10 resistors should tolerate about 5W. 

Initial adjustments 

If you have an oscilloscope, a signal generator, and a 4 Ohm dummy load, you will be able to adjust the circuit in no time. Just apply an audio tone at the input, and adjust trimmer resistors for achieving maximum dynamic range without clipping (distortion). Before adjusting, it is recommended to set all trimmer resistors in their middle positions. If again, you do not have an oscilloscope and a dummy load, just connect a speaker at the output and then follow the steps below to adjust the amplifier by simply using a multimeter:

Turn trimmers R3 and R11 and their middle positions and measure idle current at the positive supply line (+) using a multimeter (as ampere-meter) in the range of 100mA. Turn on the power supply and then adjust R11 until you get about 30 to 35mA at the multimeter.

Then, measure the voltage in R3's wiper using a voltmeter. Adjust R3, so that the voltage at its wiper be equal to 18V or 33V, for supply voltages of 20 and 35V, respectively (i.e., 2V lower than the supply voltage). After R3 adjustment, check again if the idle supply current remains at 30 to 35mA. If not, adjust R11 again, to restore it.

Performance

The amplifier is designed to yield maximum power at a 4 ohm load. If you choose to use an 8 ohm speaker, the power will be less than the maximum. The output power for different supply voltages and different load resistances, is shown in the table below. Don't forget to use a relatively large heat-sink for the transistors. Additionally, if you wish to build a stereo version of the amplifier, you should build two identical circuits; one for left and another one for the right audio channel.

List of components

R1= see text

R2, R4, R5,R6, R12 =10K

R3, R13=47K trimmer

R7,R14=100K

R8=2K2

R11=270 Ohm trimmer

R9, R10 = 0.1 Ohm, 5W

C1,C5,C6,C7=1uF, polyester

C2,C4,C8=100nF

C3=1000uf/50V

C9=4.7uF/50V

C10=100pF

C11=4700uF/50V

D1=5.1V zener diode, 1W

T1=BDX53

T2=BDX54

 IC1,IC2= NTE778A or NE5532