Make a simple LM386 audio amplifier that you can use with most radio related projects. I use it as a replacement for the high impedance headphones.
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I always had issue finding high impedance headphones for the simple radio receivers I experimented with. Initially I wanted to build an audio amplifier based on transistors only. But a circuit using the LM386 IC is so simple, that I decided to use this one instead. Is great for crystal radios and similar simple schematics. Right now I am also using it together with the (tr)uSDX Transceiver and also the Pititico CW Transceiver.
Off the topic note, sending a big thanks to Anuj from Feedspot for placing DX Explorer in his Best 30 Ham Radio Websites blog article. It means a lot to me as I spend quite a lot of time making the videos and writing the blog articles. It feels that all the work is appreciated.
The schematic of the LM386 audio amplifier is pretty standard. There are just minor changes to make it more useful for the use in radio projects. It has a simple switch that will switch in or out the 10uF capacitor between pins 1 and 8 of the LM386 IC. This way you have the option to have a hi gain or low gain setting. I also placed a very simple audio low pass filter on the input that can be switched in or out depending on your needs. Keep in mind is just a basic low pass filter so it won’t make miracles.
It does make the CW signals sound a little better ( but is not a CW filter ). The low pass filter will basically attenuate any audio signal higher than 720Hz. At least that is what the online filter calculators told me. You can always feel free to play with the value of the R1 resistor and the C2 capacitor to see if you can get it to sound better to your ears. If you use a really tiny speaker, you can also replace the value of C5 to 100uF only.
THE PCB DESIGN
The idea was to make it as small as possible easy to carry it around when doing portable work. I did my best to design the PCB board as tight and compact as possible. Remember though, I am not a professional in PCB design. In case you do like it, on the bottom of the article you can download the folder containing the PDF files both of the schematic and the PCB design. With the toner transfer method it should be really easy to make a few LM386 audio amplifier PCB boards in no time. I made a handful of them to use them in the future.
BUILDING THE AUDIO AMPLIFIER
The best tip I can offer, is to build the LM386 audio amplifier in an aluminum enclosure. You will get rid of many possible interferences. Most of the capacitors are electrolytic capacitors. C6 placed on pin 6 of the LM386 IC can be a simple ceramic capacitor. For C2 and C4 is important to use mylar capacitors. They seem to work so much better than any other capacitors in audio circuits. Even if they have the same value as a simple ceramic capacitor, the way they react in the circuit seems to be a little different. The audio amplifier sounds way better with these. You can even use one for C6 if you like, it would not hurt at all.
Paul ( M0BSW ) just reminded me in the comments on the YouTube video about the LM386-4. This one has a 1W output power at 12v. So if you want a little more juice out of the LM386 audio amplifier, you may want to use that one. Here in Romania I had no luck, as all we seem to have in the stores are the LM386 N or M.
I built mine in a temporary PVC case covered on the inside with aluminum foil. But I will build a proper aluminum enclosure soon. Another idea would be to build the amplifier in the same box with a speaker. This way you will have a tiny powered speaker that you could use in many of your future radio builds. For me is a simple way of simplifying a circuit, not having to build the audio amplifier part over and over in each circuit. Is a nice way to save parts, using the same audio amplifier for all the circuits you build.
73 DE YO6DXE