This filter for receiving audio in telegraphy was found in a magazine 73 for radio amateurs, in an edition of the 80s. The circuit is tuned to 700 Hz and has a 200 Hz bandpass.
This 1980s circuit uses components of the era that can be replaced by modern equivalents. For the FET we can...
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This is a traditional version of electroscope with FET. We found it at Popular Electronics. The...