Electronic VU Meter

A portable CD player, radio, cassette player, a couple of microphones and PC sound card. I am feeding these into the inputs of an audio mixer the output of which is attached to a pair of active loudspeakers. I want to be able to monitor the signal levels going in and coming out of my audio mixer to make sure that I am not overloading the next stage. This could be a tape recorder if I were to record a CD for example.
Electronic VU Meter - schematic

The audio mixer has four inputs each of which have a volume control. There is also a master volume control on the output amplifier. The reason for using an audio mixer is so that I do not have to swap wires around just to listen to the output from another device. I can have the output of four devices each driving an input of my mixer. I can then adjust the volume of each of the inputs so I can only hear, say, my CD player instead of my radio. Table 1 shows some of the audio devices that I have found in the house. I have listed the name of the device, the maximum output voltage and the impedance it expects to drive. From this I can decide how sensitive hoe sensitive the meter needs to be and what the input impedance needs to be. The input consists of a buffer that may provide any gain or attenuation needed. It also provides the required input impedance for the source. The rectifier will turn the audio input signal into a d. c. voltage to feed into the display function. The meter will be connected in parallel with the load impedance that is being driven by the source. This means that the total impedance that the source will see will be lower than the load impedance of the device that is being driven. If the total impedance is too low then distortion can occur because the source is trying to drive too much current. Even if the impedance of the meter is similar to that of the load then the total impedance will be reduced by...

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