CALF the CW Audio Limiter Filter


*** RETIRED ***

Is QRM and QRN getting you down?  Don’t have a cow, get a CALF!

CALF – the Configurable Audio Limiter Filter – Combines an audio limiter circuit along with up to 6 stages of bandpass filtering to aid in SSB/SWL and CW receiving. Filtering bandwidth ranges from 700Hz to less than 100Hz while the limiter circuit can be used to reduce large static crashes or aid in weak signal reception.


  • Limiter helps to reduce crashes from QRN or can aid in weak signal reception
  • 6 stages of Op Amp filtering, 5 switch selectable, offering various bandwidths from 700-100Hz on a 700Hz center frequency
  • Built in 1W Audio amplifier drives headphones or a small speaker.
  • Controls mounted on a front panel PCB for ease of use
  • Experimental options:
    • Stages can be split and re-arranged via jumper blocks – Want to put the limiter in the middle of the filter stages? No problem!
    • Optional IC Sockets are included to change and experiment with different R/C values and filter modes – Make Bandpass, Lowpass or Highpass filter stages!

This kit was designed using through hole components and enlarged PCB solder pads for ease of assembly.

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The story of how CALF was born, links to more pictures, details, audio samples, schematics, assembly manuals, and even drawings for a PCB type enclosure can be found on the CW Reception page.

Read more about CALF in this ARTICLE published in the QRPARCI April 2015 issue of the QRP Quarterly.

New (as of 2018-08-15) CALF-V2.3-Manual

Note: Assembled CALF is made to order.  Please allow 3-4 days for assembly.

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Additional information

Weight N/A
Dimensions N/A

Kit, Assembled


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