One disadvantage of the DDF-1 is you have to adjust the receiving radio's speaker volume to a level where the
doppler performs the best. This may be too quiet or too loud for your environment. So I added a small audio
amplifier board with volume control just before the speaker / audio out. So now I can adjust the radio
volume to make the doppler happy, and adjust the volume to the speaker after that, to a comfortable volume.
I made my own LM386 board initially since I had some 386's on-hand. (SOP-8, needed to use an adapter board)
but I found them cheap on Amazon and went with those for the latter DDF-1+'s I rebuilt.
I ended up needing to install a voltage-divider on the input to the board because the knob already had the volume
maxed out on the board when only 1/8 of a turn up. (the divider was a 100k and 1k resistor)
