An Active Audio CW Filter

The 4 States QRP Group (http://www.4sqrp.com/index.php) put out many very goods kits and one of them is the Hi-Per-Mite active audio CW filter.

filter_02I purchased the kit a few weeks ago after seeing some very impressive videos of it on the net.


The kit is very well produced like all 4 States kits, and this was easy to assemble, even for me. The power requirements for the filter is 5 to 13 volts, I included a small step down adjustable power model to supply the recommended 9 volts. ( See details below)

filter_01The filter is very sharp and excludes all but the CW signal you are tuning to.

Center Frequency: 700 Hz
3 dB Bandwidth: 200 Hz
Signal Gain: 0 dB to 50 dB, user selectable
DC Power: 5 to 13 VDC, <15 mA
Audio Power: 500 mW into 8 ohms, from 9v supply


All in all a great little project, US$26.00 delivered and lots of fun in the process.

Here are the details of the power supply module.



3x LM2596S DC-DC Step down Adjustable Power Supply Module 4-40V to 1.5 V-35V 3A

LM2596S step-down power module, with wide 4-40V input range and the output voltage is adjustable from 1.5V to 35V, max. output current at 3A. Stable and high efficiency DC-DC step-down power module, perfect voltage switching regulator with wide applications.


  • Input Voltage: 4 – 40V (input voltage must be 1.5V higher than the output voltage)
  • Output Voltage: 1.5 – 35V (adjustable)
  • Output Current: Maximum 3A
  • Dimension(L x W x H): Approx. 52 x 20 x 11 mm (With potentiometer)

EBay US$7.57

73,   Garry VK2GAZ / VK2YA

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