[AMRadio] Question for K4KYV


Donald Chester k4kyv at hotmail.com
Mon Sep 4 00:35:34 EDT 2006


>I am not Don, but since he hasn't answered, I remember he found a filter at
>some surplus place in the Washington, D.C area back several years ago. 
>Seems
>like the cutoff was 3400 Hz on the nose. Not a very expensive part then, 
>but
>IIRC fron his post of this last year, there are unobtainum now.
>Joe W4AAB

That is correct.  I found the filter in 1973.  I have seen ones similar to 
it, but always with a different cutoff frequency.

But I think there are circuits available today for simple solid state active 
filters that would be just as good.

I preceed the filter with a pre-emphasis circuit that begins a presence rise 
at about 800~, and steadily rises up to about 8 dB at 3000~, and then cuts 
off sharply at 3400~.

Two microphones are used, in phase and mixed together.  An electrovoice 
dynamic with crapped out high frequency response is used with a D-104, using 
a homebrew two-channel audio mixer with the pre-emphasis curve on the D-104.

I paid only a few bucks for  that filter in Washington DC.  If I had known 
how good it was, I  would have bought every one they had.  I think they had 
about half a dozen in stock.

I have a second filter with about a 5000~ cutoff, much more gradual, that in 
switchable with the 3400~ one.  I use it when the band is  less crowded and 
the other person can open up the selectivity.

Don k4kyv

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