here.
The AF board contains microphone preamplifier, receiver audio amplifier and analog switch.
The microphone preamplifier is made from the OPA134A hi-performance audio OP amplifier. It provides low distortion and noise. The input is optimized for use with the electretic microphone. The gain is low to prevent ADC overdriving (it is important since the MIC gain control is implemented in the DSP software).
The analog switch (CD4053) is used to select the external source for the ADC. It allows to bypass the MIC preamplifier and feed signal directly to ADC when operating digital modes.
The receiver audio amplifier used TDA2003 with the typical schematics.
The schematics of the AF board is here.
