The Big E Stereo Parabolic Microphone Circuit

This is a design circuit for a stereo amplifier for a high sensitivity stereo parabolic microphone. This circuit can be used for listening to distant sounds. Typical parabolic microphones are mono phonic, this unit has a stereo audio path that helps produce more realistic sounding audio. This is the figure of the circuit.

The mini condenser microphone converts sounds into an electrical signal. Resistor R1 provides bias for the condensor microphone's internal amplifier transistor. The 2N3906 PNP transistor acts as a low noise microphone input amplifier. The 10K gain potentiometer is used for adjusting the audio signal level. A stereo 10K audio taper potentiometer can be used for adjusting both channels simultaneously, or individual 10K trimmers can be used for fixed gain applications. The preamp output signal is fed into the 1458 op-amp, which boosts the audio to a level that is sufficient for driving an 8-ohm headphone or a tape recorder input. The 1458 amplifier stage is fixed gain (10X) in the inverting configuration, it drives the headphone speakers. Capacitor C9 provides DC isolation from the 1458 op-amp output, which sits at half of the supply voltage. Resistor R13 provides impedance protection for the op-amp output and reduces audio distortion when driving low impedance headphones. DC bias for the 1458 op-amps is set at half of the supply voltage by the R16/R17 voltage divider. Capacitors C13 and C14 filter the DC power supply for the op-amp stage. The DC is further filtered for the input preamp transistors through resistor R15 and capacitor C11. Diode D1 and resistor R18 protect the circuit from reverse battery polarity.

AM RF Amp for Internal Antenna Circuit

This is a circuit for amplifier antenna. This circuit is work based on transistor. This is the figure of the circuit.


Any NPN transistor can be used. The author used a 2N3904, but a 2N2222A should work just as well. A good, low noise transistor would be even better. Some radios only have three connections to their ferrite bar antenna: the antenna connection to the tuning capacitor, ground (two wires twisted together) and secondary (the smaller number of turns). In this case, treat the twisted wires as ground, and the secondary wire as the base connection to the transistor. The transistor will then receive its base bias from the radio. R1 changes with the supply voltage. If your radio is 9V, use 56K. If it is 6V, use 47k. If it is 3V, use 33k. The goal is to bias the collector of the transistor to one half the supply voltage. The schematic shown is for a negative ground radio. For a positive ground radio, substitute a PNP transistor.