[HBR] El-Cheapo (and El-SIMPLO!) CW Filter

[Hugo W. Catta]

EP Swynar wrote:
> Hi All,
>
> Saving --- and READING! --- past issues of old radio magazines can be certainly worth the space and effort...
>
> To-day, in the October 1970 issue of HAM RADIO (pp. 44 - 46), I stumbled upon a short piece by W4NVK entitled, "The Simplest Audio Filter".
>
> And simple it is, indeed...! 
>
> Following the parameters therein for a 380-Hz CW audio frequency, I connected an 88-mH torroid coil in series with a 2-ufd. capacitor in the speaker lead of my Drake R-4 receiver...the results are REALLY quite amazing, and surprisingly effective.
>
> There are parameters given for CW frequencies of 1200-Hz, down to 380-Hz, but my ear prefers the lower tones, so I stuck with those particular values.
>
> If you have a spare 88-mH toroid laying about, along with some extra capacitors, this is a GREAT, easy-to-add-on adjunct that will only enhance the CW performance of your vintage commercial rig, or homebrewed creation...and it's OH SO EASY!
>
> ~73~ Eddy VE3CUI - VE3XZ
> ************************************
> Visit the HBR Receiver Web Site with over 100 pictures of receivers and
> construction notes...... via http://www.qsl.net/k5bcq/
> there is also a mirror (faster response)at http://k5bcq.edebris.com/
>
>
> Retrieve reflector archived data via http://mailman.qth.net/mailman/listinfo/hbr
>
>
>   
Years  ago, before dsp era. I used two small (3 ~ 5 Watts) audio output 
transformers> Each hi-z secondary in parallel with a capacitor. Plug one 
of the primaries to the radio and get the signal from the other xfrmer 
thru a capacitor to a pair of hi-z headphones, and leave the low-z 
winding of the second transformer alone. The coupling between the two 
transformers was inductive. Just an inch or two was enough to get a very 
hi Q, ....adjust to your liking. The result was excellent quality, no 
dsitortion, just some loss of course but easily compensated via the 
volume control.
Experiment with the caps. I'd start with  0.47 mf not ceramic, of course.


73
Hugo - AA1XV