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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