Rangkaian Audio Power Amplifier Berbasis Op-Amp
The biasing to Q1 & Q3 is provided by R6, R7 & diodes D1 & D2. This arrangement biases the transistors just above cut-off and reduces crossover distortion. R6 must be adjusted to the highest value which eliminates crossover distortion. Ideally D1 & D2 should be mounted so they are in contact with the driver transistors - not the main heatsink.
To adjust R6 without an oscilloscope, start with R6 set to maximum, set the volume control R2 to get the minimum audible output with a suitable input source (such as a CD player which gives 0.65Vrms at line out), and listen for any "crackle" in the sound, especially that which seems to be riding on low frequency sounds. If a "crackle" is heard then reduce the value of R6 in very small steps, until the crackle (crossover distortion) becomes inaudible (it can't be eliminated, I feel).
As the output of opamp A2 is being pulled up by the biasing circuit, capacitor C5 must be connected between the output of A2 and ground to prevent the circuit from breaking into oscillations. The value of C5 is not critical, any value between 22nF to 100nF will do. The opamp A1 is a simple buffer which isolates the input circuit from the power stage. C1 & R1 are used to set the lower 3db frequency to around 15Hz, and to obtain an input impedance of about 100K. The upper 3db frequency is determined by R4 & C7, which in this case is approximately 30KHz. The schematic shows only one channel of the stereo amplifier. The amplifier can be used to drive speakers with impedance ranging from 3 Ohms to 8 Ohms, higher impedance speakers could be used but the power output will be substantially less.
The voltage gain of the amplifier can be increased by increasing R4 or decreasing R3, as long as the output swing is kept less than or equal to 3 Volts below the supply rails. This is due to the fact that the maximum output voltage of an opamp is always 2-3 Volts less than the supply rails, before it clips.