When utilizing a single microphone to record sound from many symphonic instruments being played by various players, the only option to vary the sound balance is to change the position of the musicians relative to the microphone. Before you hit the record button on a direct-to-stereo-master tape, you need to make sure that all the voices and instruments sound good. Here is an audio mixer circuit with eight inputs and controls for bass, treble, volume, and balance. You can use these controls to balance the sounds from each source until you get the mix you want. The audio mixer uses up to eight microphones to pick up sounds from different places. Audio mixer circuit Fig. 1: Block diagram of the audio mixer with bass, treble, volume and balance controls The block diagram of the audio mixing system and the audio power amplifier are shown in Fig. 1. The circuit of the audio mixer and a tone controller are shown in Fig. 2. Circuit of audio mixer with bass, treble, volume and balance control

In this case, the dual operational amplifier IC 747 (IC3) is used to mix several inputs that don't affect each other. The bias network and power source for the two internal

amplifiers are the same. The IC is protected against short circuits and has a wide range of common-mode and differential voltages. In this application, IC 747 is powered by +12V and –12V regulated DC supplies. After each signal's level has been changed, the microphone output signals M1 through M4 are mixed together and sent to the differential input terminals (pins 1 and 2). In the same way, the outputs of microphones M5 through M8 are connected across the differential input terminals (pins 7 and 6) of the second amplifier in op-amp IC 747 after each level adjustment. Logarithmic variable resistors VR1 through VR4 and VR5 through VR8 are used to adjust the level when the outputs of each microphone are fed into the inputs of the two amplifiers inside IC 747. The outputs of the two amplifiers, which come from pins 12 and 10, are combined at the intersection of resistors R9 and R10 before being sent to the next stage (the tone controller) through the 10 F capacitor C12. With the help of potentiometers VR9 and VR10, the overall gain of each amplifier can be changed. The amplified mixed signal from IC 747 is sent to the stereo tone controller IC TDA1524A's input pins 15 and 4. (IC4). The TDA1524A is made to be an active stereo tone and volume control for car radios, TV receivers, and other devices that get their power from the wall. It has controls for the bass and treble, the balance, and the volume with a built-in contour that can be turned off. All of these functions can be controlled by single linear potentiometers or by DC voltages. This IC does a good job of controlling the tone. Even though it might work okay with a 9V DC supply, a 12V supply can improve the bass response. For the IC to last longer and work better, it needs a good heat sink.