Feedback in public address amplifier is something that should be avoided. The ideal solution is by adjusting the microphone and the speaker position, but it’s not always possible in many situation. Using frequency shifter to change the output frequency 10Hz lower than the input frequency can solve the problem, but it’s too expensive for a low cost public address system. Voice operated switch is one cheap working solution (see the circuit’s schematic diagram).
LM346 is programmable operational-amplifier. According to its data sheet, by pulling the SET pin(s) to V- the op amp(s) shuts down and its output goes to a high impedance state. Using this property, the LM346 can be used as a low speed analog switch for VOX (Voice operated switch) application. The figure below shows the schematic diagram of the VOX circuit.
The first op-amp is the buffer for the microphone with gain=2. The second op-amp and the diode 1N914 is configured as peak follower, follow immediately the signal peak but falls down slowly because the charge of the 10uF capacitor can’t be sinked by op-amp output (blocked by the diode) and must be discharged slowly through 10M resistor. This slow discharging ensure the voice signal is not muted at short silence between words and sentences. The third op-amp is configured as a comparator, which its reference is adjustable via 10k Pot. This potentiometer should be adjusted to set the proper level where the sound activate this VOX circuit. The last op-amp doing the voice muting, when the control voltage output drop to zero because no sufficient sound level detected on the microphone. Connect your microphone to MIC IN, and connect the AUDIO OUT to your microphone input of your amplifier. The CONTROL output of this VOX circuit is provided to control other equipment such as relay or lamps. [circuit schematic diagram source: National Semiconductor’s LM146/LM346 Programmable Quad Operational Amplifiers Application Notes]