A Simple Passive CW Audio Filter Experiment

Build a simple passive CW audio filter with only four components. You can always turn it into an active CW filter if used with a simple LM386 audio amplifier.
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This LC passive CW audio filter is by no means perfect or excellent at all. But I found it really easy to build and adapt to any other external audio amplifier, or any external powered speaker. You decide if you like the results and it’s worth your time to build one. It’s really nice to experiment though, and how changing the values of the components is influencing the audio output.


After building the first version of the Sputnik Regenerative Receiver, I decided that it would be nice to build a simple LM386 external amplifier to drive a speaker. The first version of the receiver did not drive a speaker. So I thought about using an LC filter between the audio output of the audio amplifier and the headphones. The intention was to use it as an audio filter for CW, thanks to an article written by KI3U.


I was ok with about the way it works, but I didn’t liked it because I was loosing a lot of the audio output. I tried to find a way to install it between the amplifier and the receiver’s audio output. This way I would have an active CW audio filter. Or well, more like an external LM386 audio amplifier incorporating a passive CW audio filter. Here you have the schematic for both the basic design, as well as the design I ended up using in my latest design ( not presented in the video ).



I had all parts on hands except the inductor, so I made one from scratch. Since I could not measure the inductor’s value, I decided to test the audio filter using all sort of values for the capacitors. I was doing it by trying them in different combinations, while measuring the audio spectrum with an app installed on my phone. I tested them until I found the combination that sounded good to my years.

The coil I made had 550 turns of 0.2mm enameled copper wire. The more turns, the higher the value of the inductor and the lower the center frequency was. That is what I noticed in my experiments. I’m sure there is a way to calculate this. The ferrite core I used had the center core of 18 mm. Recently I measured the inductor, and the value is 160mH.


In the video I explained that I had no way of measuring the performance of the filter. Down bellow you have a simulation I did recently in Elsie, using the components I have now in the final schematic. Since the value of the inductor will change the center frequency, I think I will use this in my advantage and build a coil with multiple taps at different values of the inductor.



Feel free to let me know what you think in the comments down below. Especially if you built it and had a chance to test it. I honestly like it very much, even if it’s far from perfect. I did noticed two things after using it for a longer time after publishing the video on YouTube. First issue was that the filter as it is in the schematic diagram presented in the video was great, but really sharp. That means it was hard to tune the receiver perfectly, since the filter had no adjustment. I could use the potentiometer to adjust the width of the filter, but then it won’t be so effective.

The other thing was that using the values in the schematic presented in the video, made me end up with a center frequency around 460 Hz approximately. That was a little too low. To fix both issues, I changed the values of the capacitors. This way the filter has now a center frequency around 600 Hz, and also is not as sharp, so it’s easier to tune the receiver in order to hear the signals well through the filter. I want to use use a coil with more taps, having different inductance values. With the help of a multi positions switch, will be able to change the center frequency of the filter. I have an 8 positions switch and I think I will also implement this into the final version.


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