I have been playing around with some older simple direct conversion transceivers and receivers for CW.
Some examples are the Pixie kits and the Rockmite kits, and some Chinese kits, like the "Frog Sound".
All these have one drawback, the receiver audio is rather wideband and has a good deal of high frequency hiss.
There are a ton of excellent CW filters both active (more complex) and passive. I wanted something really simple, and took a look at this simple CW filter from YO6DXE: https://dxexplorer.com/passive-cw-audio-filter/
Take a look at his article, and his demonstration of the filter on Youtube (linked in the article). With a total of 6 passive components a simple, yet pretty good CW filter can be made. Two different resonalt frequencies, and variable bandwidth.
It is a simple series resonant circuit with two switched capacitors for the resonances, and a potentiometer partly bypassing the off resonance frequencies.
I love the simplicity, and in the video the filter demonstrated a good sounding filter effect. It may take a while, as I am busy with improving my QO-100 setup, but I will get to it.
For the inductor in the filter I have some old multi tap inductors that were used in tone signalling equipment in the 60s, 70s and 80s. sound perfect for my purpose.
Now I need time to build this, and I intend to add a simple (PCB module) audio amplifier, LM386, or the like, so I have speaker sound out of it.
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