Circuit Connections –
Volume Control for Audio from Smartphone – For controlling the volume input through the smart phone, a variable resistor (Shown as RV1 in the circuit diagram) is connected between the audio jack and the power amplifier. By adjusting this variable resistor, the volume of the audio from the smart phone can be adjusted.
Fig. 6: Circuit Diagram of Electret Microphone Biasing
The MAX4468 IC has the following pin configuration –
The IC is used as inverting amplifier. The inverting amplifier has negative feedback which makes them better than non- inverting amplifiers. The inverting amplifier changes the phase of output (signal amplitude) by 180 degrees with the input (signal amplitude). However, this phase inversion does not affect the audio signal as human ears are response only to the intensity of the sound. The intensity is the energy flowing through an area in a given time, expressed in joule/s/m2. The energy of the wave is proportional to the square of its amplitude. So for a unit area, the intensity is also proportional to the square of the amplitude.
Volume Control for Microphone – For controlling volume input through the microphone, a variable resistor (Shown as RV2 in the circuit diagram) is connected between the preamp output pin and the power amplifier. By adjusting this variable resistor, the volume of the audio from the microphone can be adjusted.
This IC is basically an operational amplifier whose voltage gain can be adjusted by using a proper RC circuit between its gain setting pins. If the gain setting pins are left open, the voltage gain of the amplifier is internally set to 20 (26 dB). Since the pins are left open, the gain of the amplifier will be 20.
The amplifier circuit looks like as shown in the figure below –
The LM386 also decides the output power of the mixer circuit. It will provide a power of approximately 700 mW for a load of 8 E at a supply voltage of 9V.
Fig. 18: Graph of Audio Waveform from Microphone as observed on DSO
The audio waveform from the smart phone is observed to be as follow –
The mixed waveform from the power amplifier is observed to be as follow –
The audio mixer designed in this tutorial is a 2-Channel audio mixer. It has an output power of 1 Watt approximately. The audio mixer is provided volume control at both input and output. This audio mixer circuit can be used in public addressing systems, audio recording and for DJ and musical concerts.