A Simple Passive CW Audio Filter Experiment

Play Video about Experimenting With A Simple Passive CW Audio Filter

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, to see how changing the values of different components is influencing the filter 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. Soon I thought about using an LC combination between the audio output of the audio amplifier and the headphones. The intention was to use it as passive audio filter for CW, thanks to an article written by KI3U.


I was ok with how 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.

Schematic - Simple Passive CW Audio Filter


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 the diameter of 20 mm and 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 as seen in the photos I added in the video of 18 mm. A few months later I did got the chance to measure the inductor, and the value was 160mH.


In the YouTube video I explained that I had no way of measuring the performance of the filter. Thanks to the amazing people in the amateur radio community, Marcos Bazzo – PU2MBZ was kind enough to take his time and did some filter simulations for different inductors values. Thank you one more time Marcos. Down bellow you have the screenshots he sent me.

Passive CW Audio Filter - Simulation L 60mH
Passive CW Audio Filter - Simulation L 70mH
Passive CW Audio Filter - Simulation L 80mH
Passive CW Audio Filter - Simulation L 90mH
Passive CW Audio Filter - Simulation L 100mH
Passive CW Audio Filter - Simulation L 130mH


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 was great, but really sharp. That means it’s 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, made me end up with a center frequency around 460 Hz approximately. That was a little too low. To fix both issues was to actually swap the capacitors between them. This way the filter had a center frequency around 700 Hz, and also it was not as sharp, so it was easier to tune the receiver in order to hear the signals well through the filter.