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Ed Wetherhold's CW Audio Filter Kit

Ed - W3NQN - designed his famous passive cw audio filter in the 1980's. His articles in QST magazine described how to build this filter using surplus parts. Then he began to supply the hard to find surplus toroidal coils in the filter. After several more articles in QST and other publications, and a write up in the ARRL Handbook, many Amateur Radio Operators wanted one of his filters. The photo at right shows the cover of the Handbook, and Ed's CW filter being tested in the ARRL labs.

He finally put together a complete kit, with all the parts, coils, case, and directions to build the filter. The filter is still available from Ed himself today! You can't buy this filter kit anywhere else. In fact you can't buy the filter. You have to build it.

The filter is a passive device. It uses LC networks (wire coils and capacitors) to filter the audio passband. The passband width of the filter is 250 Hz, which is just the width of a Continuous Wave Morse Code Transmission (CW). The signals and noise above and below this passband are eliminated from the audio. The filter is perfectly quiet because it inserts absolutely no noise in to the audio. The W3NQN filter does not require power of any kind. It simply plugs into the audio out port of any radio receiver.

Read all about the filter here on this web site.
Buy your own kit today and have fun building and using one yourself!
Total kit price is $70 that includes shipping USPS Priority Mail. (USA)
The kit price for orders outside the USA is $90. Includes shipping.

  • No Power Needed
  • Easy to Build
  • All Parts Included - except a phone plug and cord.
  • 1000's in Use
  • Works With All Radios







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