Audio Amplifier Stage Template and Handout: SolderSmoke Direct Conversion Receiver by N7PHR

 

Above is Steve N7PHR's template for the audio amplifier board, the board that gave a lot of people a lot of trouble.  But it SHOULD have caused a lot of trouble!  It is the only gain stage in the receiver -- it provides at least 80 db of gain.  It deliberately uses no feedback -- we did it this way to keep the circuit simple.  So if you let too much of the output find its way to the input, this amplifier will turn into an oscillator.  Then you will have to tame it. And almost all builders did. This was a valuable lesson:  All amplifiers aspire to be oscillators.  They have to be denied the Barkhausen criteria. 

Above we see the placement of Rex's Me pads, and the transformer. (Note that Steve DOES ground one of the output pads.)  I think that Steve's layout was really good -- it sounds like they did not have a lot of the feedback and motorboating problems that were reported by others.  This is a good reminder of the importance of careful layout. 

Here is Steve's handout: 

Steve's messages have caused me to reflect a bit on this project.  I will try to write about this in the days ahead.  

Thanks Steve. And thanks to all the builders.