Re: Looking for a circuit design for bandpass active audio filter

Mike Reed

There was a kit that had the capability of user adjusted band pass . It was switchable. I built one, works well.
I think if you search on eBay for the "CALF filter kit" it might be what you want.
I don't have the stuff right now where I can get to it, possibly a search may find it.
Mike - N7ZEF

On Fri, 25 Dec 2020 20:27:37 -0700, Donald Hellen <> wrote:

Mike . . .

On Fri, 25 Dec 2020 18:53:36 -0700, "Mike Reed" <>

There is one. Try "VEC-830 SSB Filter" I have one, but have not had the
chance to try it.
Mike - N7ZEF

That would not be dual-purpose and wouldn't work for CW.

I'm looking for a bandpass filter that is something like this circuit
or a multi stage one that has a sharper cutoff:

or this one:

or done in 2 stages with a low pass then a high pass or vice-versa
like this:

It would have multiple selections on a switch for R1 and R2 (instead
of a single value for each) to change the cutoff frequencies on both
the high and low end. Maybe 2 or 3 for CW and 2 for SSB or vice-versa.
I don't have much experience with home brewed circuits but I dabbled a
bit when I was a kid. I made a power supply for a cassette tape
recorder from some spare parts, and an electronic mosquito repeller
back around 1970-73. But I have the parts, a great soldering station
(and a hot air rework and soldering station also), and some
breadboards so I could assemble this with the dead bug type of layout
(upside down ICs look like a dead bug sort of).

But, if there was a ready made board that did the same thing at a
decent price, I might spare myself the time and just make
modifications to it, running wires from the two control resistors that
determine the cutoff frequencies to either two potentiometers (for the
high and low end), or two multi-pole rotating switches that would let
me select the pre-set cutoff frequencies.

I have the holes in the speaker enclosure because there are RF gain
and AF gain controls that could feed back to the radio if they were
still connected. Right now, the audio input goes directly to the
speaker. I want to take advantage of the room inside the speaker
enclosure to add the selectable bandpass filter that would work for CW
or SSB. If it was just one or the other, and if I didn't need to be
able to change the bandpass cutoffs on the high and low end even for
those, I would just use a preset filter board.

Donald -- AD8DY
Formerly KJ3I

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