Modular Audio Mixer with Multiple Input Channels

Figure 1 

The circuit was designed to create an audio mixer that can be assembled in modules while providing 6 or more input channels.

• Audio Mixer – a device used to adjust, route, change, and combine audio level and dynamics from a range of different sources in order to create a final recorded audio product, such as the soundtrack of a movie
• NE5532 – an internally compensated low noise dual operational amplifier with features such as full power bandwidth up to 140 KHz, input noise voltage of 8 nV, common mode rejection ratio, 9 V/us slew rate, high DC voltage gain, 32 V peak to peak voltage swing, wide supply voltage range from 3 V to 12 V, unity gain bandwidth at 10 MHz, and internal frequency compensation
• TL072 – a low noise JFET input operational amplifier with features such as common-mode input voltage range, high slew rate, operation without latch up, compensated internal frequency, high input impedance at the JFET input stage, low noise, low total harmonic distortion, protected from output short circuit, low input bias and offset currents, wide common-mode and differential voltage ranges, and low power consumption

The mixer circuit consists of 6 channels with monophonic channels from 1-4 while stereo channels for 5-6. The number of input channels can be varied while choosing between mono and stereo channels. The potentiometer RV1-6 regulates the sound level and drives each channel. The balancing between two channels is achieved by RV7-12. The two adders IC1a-b adds up all the signals from the input channels. The two trimmers TR1-2 in every channel provides the adjustments of the gain of each IC, with reference to the level of output signal as desired.

The regulation of the three bands is done in the equalizer stage where the output of the mixer comprises of IC3a-b. It has a gain of one and is responsible for the separation of the previous stages from the unit being driven. The use of headphones is possible due to the presence of a circuit made in the region of IC2a-b that produces its output in JF13 connector. The use of stereo VU meter is to indicate the audio level visually. The JF12b connector handles the supply for various circuits, which consists of mixture of voltages.

The modules for mono input channels are using IC1 or other types of ICs for inverting and regulating the gain of the unit for low to high levels of microphones. The input module for electronic balance mic-line has constant regulation of stage by the 22K ohm potentiometer. The preamplifiers of microphone transistors use Zener diodes to regulate the voltages from the main supply. The stereo input channel module has a gain being increased by the value of resistors connected in parallel with the capacitors. Other circuits attached to the main circuits are the stereo phono preamplifier, 3-band equalizer, power supply, and monitor selector from mixer input to headphone.

Figure 2 

The theory implied in this circuit is widely used by musician in mixing boards as a musical instrument. A mixing console is used in public address systems to set microphones to the correct level for different speakers which can add be added in recorded sounds. The mixing console is used by most bands to combine vocals and musical instruments to the accurate level while minimizing audio feedback. A mixing desk is used by radio broadcasts to select audio from different sources like telephones, pre-recorded advertisements, remote feeds, and CD players.

Source:users.otenet.gr/~athsam/Audio_mixer_6_ch_ENG.htm