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Saturday, December 30, 2017

Building the Ceramic Discrete Direct Conversion Receiver - Part 3 -- The Audio Amplifier

Once you have achieved JOVO and have the oscillator covering the frequency range you desire, it will be time to build the audio amplifier stage.  Now, I'm sure many of you will be tempted to just throw an LM386 or 741 op amp in there.  But don't do that!   You want to build the WHOLE receiver.  Don't let that IC manufacturer rob you of the FULL HB EXPERIENCE.

The DISCRETE amplifier circuit we will used is based on a design that several of us used to replace the LM386 in the BITX20 receiver. 

And here is a tutorial that does a GREAT job of explaining how this circuit works.  I strongly recommend you study the tutorial carefully.  This site will allow you to really understand how your amplifier works.  

It is not that complicated.  But here you have to take care to make sure that your amplifier does not turn into an oscillator.  Keep the outputs away from the inputs.  Keep the leads short.  Some planning is needed here.  Click on the picture below for layout ideas.  The right 1/3 of the board contains the AF amp circuitry. 

After you build the circuit, TEST IT!  If you have an AF signal source and a 'scope, great.  But if not, just hook some earbuds or headphones to the output, connect a 9 V battery and PLACE YOUR FINGER on the input of the first AF amplifier.  If you hear a noticeable increase in hum when your finger makes contact, congratulations, your AF amplifier is amplifying. 

The Complete Schematic

When you get this stage and the oscillator working, you are 3/4 of the way to completion.  Next we will built the mixer, and then the front end filter and amplifier. 


  1. Quick question, do your final transistors get hot? My final transistors are 3A devices and they get too hot to touch. Thanks for your input.

    1. Dean: I don't think mine get hot. Maybe check to see if it is oscillating. Check the bias voltage on the bases. Bill

    2. HI Bill... I built this 2watt amp yesterday and very happy with it. Clean crossover at the output. When I have a problem it's usually a wiring error. Second don't substitute the output transistors. Jeff


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