Like all other modules this amp is disigned with standard components, to be as simple as possible, easy and cheap to construct.
The mic amp has two stages, first the amplifacation, and then the clipper with output power driver
The soft clipper works with two schotkey small signal diodes, they have a forward voltage at about 200mV
Then the +-200mV is amplified by 10 in the output stage,
This output is loaded with 50 Ohm (the BFO mixer in TX mode) no normal op-amps can generate several volts peak-peak into this strong load,
That is why the push-pull stage was added. The 2 diodes in the push-pull npn-pnp transistor stage removes the cross-over distortion without adding quesent current
I have used a 500 Ohm dynamic microphone, I'm sure you could use whatever you have on stock
The Mic amp, notice the PNP transistor that makes the +12V TX from the negative TX signal from the mic plug.
The AF Pre-Amp, has about 30dB Gain
-3db @ 100Hz and -3db @ 3.2Khz
OpAmps that will work only super low noice types
That means: OP27, NE5534, TL072, best first..
Pin configurations OP27 and NE5543: IN- 2, IN+ 3, OUT 6, GND 4, POWER 7
My test was made using TL062 then TL072 and now OP27
The TL062 was WAY to noicy, The TL072 can be used, but the OP27 and NE5534 is BEST.
The first made filter has 50 Ohm driving and 10K loading
Measures: -3 dB @ 7Khz and -10 dB @ 15 Khz
This filter works fine to cut away the 10.7Mhz signal,
But is too wide for the Audio and will allow some noice to pass thru.
Now first resistor is changed to 470 Ohm the mixer loading resistor is now 56 Ohm
The Opamp input resistor is 1K and the feedback 47K parallel with 820 pF
Measures: -3 dB @ 4.6Khz and -57 dB @ 15 Khz
This will reduce the noice alot, and make the signal to noice level better !
The AF Power Amp, has about 90 times gain
-3db @ 150Hz and -3db @ 7Khz
This TDA2003 has about 50mA quicent current, So I will find another one next time.