A Homebrew Active CW Filter

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Please note that in the video I referr to QRP Notebook for the filter circuit. This is an error. It is actually another book by Doug called "W1FB;s QRP Notebook" a similar book. The circuit is on page 66. Sorry for my error.


The old workhorse FT747GX radio at my local radio club has no internal CW filter fitted. When i attend the club I like to operate CW, but the wide 2.4 kHz filtering makes it difficult to pick out signals from the noise. So, I decided to build my own active audio filter to rectify the problem. This video describes my project.

There is audio attenuation in the filter and so to compensate for this I added a small audio amplifier using an LM386 chip. The filter unit is based around a single low-noise operational amplifier. I just had a 5534 chip so I used that instead of the TL081 specified in the circuit diagram which I lifted from DougDeMaw,s W1FB's QRP Notebook. Any single operational amplifier chip should also work well.

The unit plugs into the headphone socket of the radio. There is a three=pole rotary switch in my project. This allows the unit to be powered off, amplifier only activated with filter bypassed or both amplifier and filter activated..

The speaker can be muted to allow headphone listening or both can be left on. I added a 10K potentiometer in series with the 10uF capacitor between pins 1 and 8 on the LM386 chip. This allows the gain of the amplifier to be adjusted and set.

I also bypassed each of the external connections to earth to avoid any RF getting into the unit. The project is housed in a diecast metal box, which I drilled to accommodate the controls and connectors. The unit is powered by an external 12V DC supply.

To simulate the lack of filtering on the club radio, I set my Icom IC7300 to the widest SSB setting and deactivated its Noise Reduction feature. The video demonstrates the results.

I thought about using a double-pole filter, but this single op-amp version performs adequately.

I hope you like this video clip. If you have any feedback. comments or questions then you can contact me by email at [email protected]

Best 73 de Tony, EI5EM
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