There have been two audio amplifiers that I have used for this receiver. One using a TDA2002, built like the one in the Beginner and Experimenter's Receiver, that worked best for receiving single sideband. See the following diagram:
For CW, I like the audio amplifier described in "A High-Performance Receiver for 160 Meters", Solid State Design for the Radio Amateur, by Wes Hayward and Doug DeMaw, Page 138. This audio amplifier is rated at 3.5 watts and uses a pair of hefty transistors in a push-pull output. He states on page 138, "There has been no aural evidence of distortion at any signal level while using the circuit of Fig. 40". This includes very weak DX signals for which this audio amplifier really does its best. From my experience, this is the best audio amplifier that I have ever built. I also found it was not picky about the type of output transistors, any PNP-NPN pair in a TO-040 to TO-043 package advertised for audio circuits worked very well. Below is a diagram from the article.
Use landscape setting on your printer when printing this diagram. Right click on the diagram and select View Image, then when it opens in a new browser window, click on Print Settings, select Landscape layout in the appropriate folder, select Fit to Page, and then click on Print.
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