This is the second in the series on the Norcal 40a study and build. The intention is not just to build the radio but also study each block in detail to understand what is going on..

We will be building the receiver first and also be building backwards so that we can "listen" to the result of building each stage as we progress..

Audio amplifier

Norcal 40a uses the popular LM386 IC for audio power amplifier. It is a rather straight forward build and once we build it and apply power and touch the input, I could hear a 50hz hum. Success!

Here are the parts:

  • J4: audio jack
  • U3: LM386-M1. I used a 4x2 IC socket. Since the vias are not present in my build, I had to solder a couple of pins on the top. The 4th pin (ground) is to be soldered in the top side. Pin 3 also needs to be soldered in the top.
  • R8: Circuit says 500 ohms trimmer. I had a 470 bourns one, so I used it.
  • C26: 10uF electrolytic
  • C23: 2.2uF electrolytic
  • C27: 100uF electrolytic
  • R7: 47k
  • R22: 1.8K
  • C55, C22: 10nF (also marked 10000pf on the parts bag)


To test it, I wired up a audio socket that can plug into J4 and then hooked up a JBL speaker into it, powered it up with a 9V battery and moved the R8 to one side (anti-clockwise), not fully, but somewhere 1/4 from the end. After that, touching the R7 or C22 gives me a hum..