ElectroResales Active CW Filter KIT Filter is centered at 600 Hz | eBay
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N5ESE's Version of the NESCAF (20070117)
The 4SQRP Hi-Per-Mite CW Audio Filter – Amateur Radio Station WB0ISG
A Simple Passive CW Audio Filter Experiment
KH1 Hand-Held, 5-Band Transceiver – Elecraft
How to assemble a narrow CW filter for your Yaesu FT-817/FT-818/FT-857/FT-897 (an affordable YF-122C equivalent) | Q R P e r