There are three stages of low-pass filtering and one stage of high-pass filtering. The first
three stages retain the 2.5V “midrail” bias all the way through from the input transformer T1.
The final stage IC9A is biased using the 5V supply (avoiding a few extra components to
create a real 6V mid-rail at half the supply).
The CW filter also provides a measured 18dB of gain.
Sidetone is injected at the input to the CW filter, so as to make is sound nice and clean by
cutting off the squarewave harmonics leaving a clean 700Hz sidetone.
4.8 Audio amplifier
The final stage in the receiver signal path is the audio amplifier, to drive earphones at a
comfortable listening level.
There is a 5K potentiometer on the front panel, used as the gain control. With the wiper fully
clockwise the receiver is at full volume. As the potentiometer is turned anticlockwise it forms
a potential divider which attenuates the audio signal from the CW filter output. This
potentiometer is located on the front panel PCB and is not shown on this diagram section
(below).
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