MICROPHONE AMPLIFIER,
LPF AND BALANCED MODULATOR
The microphone amplifier has a gain of more than 20dB. The amplifier is followed
by an 2.7KHz active low pass filter. A dual op-amp is used for both stages. The
choice of op-amp is not critical. I use an LF353 but most dual op-amps should
work in this circuit.
Like all the other circuit boards in this transceiver, the PCB was drawn by hand.
I used double-sided PCB. All ground connections are made to the top PCB foil (Ground
plane). The holes for any component leads that are not grounded, must be reamed
out (countersunk?) with a suitable tool. I used a 3mm HSS drill bit.
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