The audio circuits are quite conventional. The frequency response of
the
system is about 20Hz to about 15kHz which is too wide, in my
set-up
I use a high level of R.F speech compression and audio tailoring. If
you
wanted to restrict the bandwidth try placing 1000pf capacitors across
the
47k resistors on the first two amplifiers, this will roll off the
response
above about 2kHz. The L.F response can be reduced by lowering the value
of
the 2.2uf input capacitor.
For a copy of the schematic, click here.
Oscilloscope pictures
Before even thinking of building this circuit, get yourself an oscilloscope, there is no other way of setting the modulation without one.
The first picture below shows the typical band with no modulation, this takes up two full divisions on the screen, this gives a reference for the following pictures.
This picture below shows the typical pattern with symmetrical modulation, taking up to a total of four divisions, which most AM stations use.
When the schematic at the top of the page is used this is what you can expect from the modulation waveform. In the picture below you can clearly see there is a significant increase in the positive peak, nearly six divisions on the screen, this has the effect of giving much greater 'talk power' when compared against the symmetrical modulation above. You will notice you will be able to modulate greater than the six divisions, this is fine, as using this circuit its very difficult to over-modulate because as the audio is increased the positive peak will keep increasing without the negative getting very much lower. I would not recommend any larger than about 130% upward modulation because any greater than this the receiving end AGC will start pumping like mad, and it won't sound very nice. To give you some idea of the peak powers obtainable, on 80 Metres running 50 Watts carrier power, the peak sideband power is about 300-350 Watts, so make sure the succeeding linear amplifiers are able to take that sort of power.
Well, thats it, if you are going to give this circuit a try, give me a call either on Top Band around 1963kHz or on 80 Metres around 3605-3620kHz.
Good Luck
Paul.