FROM: http://web.telia.com/~u85920178/use/filt2.htm

FROM: http://www.appliedphysicstech.com/CWF.html
CWF-001 Active CW Filter

FROM: http://www.morsex.com/misc/filter.htm
Vectronics CW Filter Kit with 1W Audio Amp

FROM: http://www.hanssummers.com/radio/cwfilter/index.htm
http://downloads.hanssummers.com/cwfilter.pdf
Phase-Shifting CW Filter by Radio Society of Great Britain


FROM: http://midnightscience.com/kits.html

HI-PER Audio Filter: 200 Hz wide; centered at 700 Hz Gausian active filter
with minimal group delay...no ringing... I have used this filter for over a decade
and it is very good with qrq cw. It is to be placed just before the final audio amp.
Please see the attached PDF file for the full article about it.

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If you would like to share your favorite cw filter please feel free to upload some images and descriptions by clicking on the 'Reply to This' tab underneath the CW FILTER SHOWCASE ARTICLE. (The green rectangle in the "REPLY" menu bar will add pictures.)
http://newenglandqrp.org/nescaf

see eHAM reviews of this filter HERE

DELUXE COMMUNICATIONS AUDIO BOARD by David Curry wd4pli

This is one of the best external Audio filters with a very clean amp and lots of excellent features:
Q-mulitplier
notch filter
cw regeneration
variable bandpass


see PDF file for full article

Attachments:
PASSIVE CW FILTER KIT from Xtal Set Society, Inc

This filter was easy to build from a kit, and has a very effective bandpass design for cw.
Please click on the attchment WAVE FILE to hear this filter in action.

Attachments:

THE NEW HI-PER-MITE CW BANDPASS FILTER 

 UPGRADED DESIGN   -  SEE WEBSITE FOR MORE INFO  

http://www.4sqrp.com/HiPerMite.php

Here is a spectrum view of the bandpass of this filter for 700 hertz center frequency

Here is the view of the waveform sweep in AUDACITY

Here is what it sounds like sending cw dits from 300 hertz to 1200 hertz

Here is an audio example of how well the HI-PER-MITE filters a harsh square wave sidetone

Here is a very good W3NQN, passive, "GAUSSIAN" designed - cw audio bandpass filter - specifically designed for QRQ CW by W3NQN himself -The Gaussian design is used to preserve the integrity of the shape and sound of the cw notes...so that you don't get ringing, or echoing, or that tunnel sea-shell kind of sound etc...or warping of the shape of the cw edges...

Chebyshev and Butterworth cw audio bandpass passive filters usually do not have the ideal passband phase response for copying QRQ CW....they tend to add "coloration" to the cw notes and can even cause ringing and other audio artifacts....However, this unique W3NQN passive cw audio bandpass filter prototype uses a GAUSSIAN FILTER calculation for its inductors and capacitors...the Gaussian bandpass filter does preserve how the CW is supposed to "sound" - it does not color the "shape" of the cw note....

this video...demos the effectiveness of the filter by:

1. listening to a 40 meter cw qso on a web sdr rig.... the filter is bypassed to show how much qrm/qrn the filter is capable of reducing...

2. another test is a spectrum sweep, using a tone generator, a spectrum analyzer will store the sweep and show how well the filter tunes out the frequencies above and below the center(700 hertz) design bandpass frequency...

3. FLdigi is used at 120 wpm to show how well this W3NQN GAUSSIAN filter preserves the shape and sound of the original FLdigi tone... THE green wave on TOP is the original FLdigi wave, the RED wave on the bottom of the scope is the FLdigi tone AFTER it goes through the W3NQN GAUSSIAN cw audio bandpass filter...

4. then a textfile is sent by FLdigi at 120 wpm so you can hear how well the filter keeps the original sound of the FLdigi CW audio intact at qrq cw speeds...the filter does not add echo, ringing, or allow any altering of the shape of the original FLdigi cw notes...

here are the design blueprints on this project by W3NQN:
https://imgur.com/zauJdeV
https://imgur.com/fxZoPUz

if you are interested in learning more about this filter, please contact W3NQN about this Gaussian Filter prototype

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