Amateur Radio Tower
Ed Wetherhold's Passive CW Audio Filter

Building W3NQN's Famous CW Filter

The kit is delivered with the following items:

  • One high quality plastic box with one DPDT switch and one audio jack pre-installed.
  • W3NQN Filter Parts
  • One stack of five 85-mH inductors in a cardboard wrap with the inductor leads connected to external solder terminals. The inductor stack is pre-installed in the plastic box and held in place with a plastic mounting clip fastened to the side of the box.
  • Two separate 85-mH inductors that are to be modified by the builder to provide the custom center frequency.
  • Five matched capacitors with values selected for your chosen frequency.
  • Two or three resistors for equalizing the audio level with or without the filter.
  • Two audio 8/200-ohm transformers.
  • Rubber feet for the box.
  • Hook-up wire.
  • Miscellaneous Hardware.
  • Complete written instructions.
  • The 1/4 inch phone plug, as seen in the photo, is not included. The builder is to provide his own properly sized plug and cord.

Tools and Materials Needed

  • A soldering iron and solder is required for basic wire connections.
  • An ohmmeter is recommended to test the correct wiring of the filter.
  • GE Silicone II, Clear Sealant in 2.8 oz. tube, for fastening components within the box.


Building this kit is straight forward and within the capabilities of most people. Care and attention does need to be given to correctly wiring the filter for proper operation. Basic soldering skills are necessary.

85mH Toroidal Inductors The two separate 85-mH inductors (toroidal coils) are modified by removing several turns of windings. The number of turns removed is determined by your chosen center frequency. When you order a filter kit you will choose a center frequency of 546 Hz, 600 Hz, or 700 Hz. (See the Order page.) Ed will send the filter kit to you with specific components that provide this center frequency. The two separate inductors are then mounted on the ends of the inductor stack assembly with silicone sealant, and the wire leads are soldered to the correct terminals.

Matched Capacitors The capacitor leads are connected to the appropriate stack terminals. Because of the small size of the capacitors, their leads provide sufficient support to keep them in place.

The wiring of the transformer leads is completed, and then the transformers are installed in the box.

The switch and audio jack are wired in.

The wiring of the inductor stack is tested with an ohmmeter. The instructions include diagrams and charts that show exactly how it is done.

8 to 200 Ohm Audio Transformers The user should obtain a suitable plug from Radio Shack, or other supplier, and add two wires of a suitable length to reach between the filter box and receiver.

Install the lid and the rubber feet.

It is done and ready to be used!

The detailed instructions included in the kit are in-depth and comprehensive. The instructions take you through the process step by step.

Build it with AK3W wire by wire see photos and download schematics and diagrams.

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