Low noise microphone preamplifier
The microphone preamplifier circuit design presented in this schematic use SSM2015 produced by Precision Monolithics Inc. (PMI) which offers a high amplification and low noise ( 1.3nV/√f).
Has been designed to work with differential input signals and can deliver 10 to 2000x amplification and it is term of R4 value. For R5 = R6 the amplification is A = (20000/R4)+3.5
The values from the schematic correspond to 1000x amplification. Noise signal ratio is 95dB.
Distorsions were 0.006% at 1kHz and 0dBV and lower than 0.01% at 10kHz. Bandwidth is 180kHz at 3V and 1kΩ impendance.
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Audio duplex communication link schematic
You need two of this circuits to create a duplex communication.
Duplex circuit adjustment: connect those 2 circuits thru a cable and inject a sinusoidal signal at 1KHz and 5Vef voltage into one circuit, the others circuit input must be shortcircuited. Adjust P1 for a minimum value at K2, then increase signal frequency to 10kHz and adjust C5 for a minimum value on K2. Repeat this operation for the other circuit.
The duplex audio communication schematic:
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Simple VOX
VOX is a voice controlled switch, used often for microphones replacing the classic switching button. The actuating threshold is set thru the volume potentiometer of the amplifier which command the VOX circuit. The VOX switching threshold is 600mV ( on R2 ) and the maximum input voltage must not be higher than 40Vpp.
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