Topics

Looking for a circuit design for bandpass active audio filter


Donald Hellen
 

I think what I called it in the subject line is what I'm looking for.

I want to build an active audio filter that will have the ability to
cut the high end and the low end of the audio from a ham radio
receiver with switchable cutoff points, so that I can tailor the audio
based on band conditions. My speaker enclosure has room inside to add
such a device and two knobs on the front panel (audio volume and RF
gain that can be connected back to a Yaesu radio) that can either be
replaced with pots or switches to change the high-cut and low-cut
parts of the audio signal. I thought I'd use the existing pots or
change them to 3 or 4 position switches to allow switchable (or
tuneable) bandpass cutoff points. Sort of a very simple 1 band
equalizer.

If there's a ready-made kit for something like this I might be
interested, but hand-crafting such a device is OK also.

I would like to keep the cost low (like most of us try to do). I want
to use active components like op-amps along with resistors and
capacitors, and hopefully be able to run this off a wall wart type of
power supply once I either find one that's not noisy for RF or that I
can tailor to make it so. If I can't find that, I can find tap 12V
from my power supply as a power source since it's quiet (that may be
the best approach).

I can add a one or two watt (or more if needed) audio amplifier after
the filter if needed to make the audio level suitable to my ears. I
think the LM386 can make a decent low power amp.

Your ideas?

Donald -- AD8DY
Formerly KJ3I


----------------------------------------------------
Some ham radio groups you may be interested in:
https://groups.io/g/ICOM https://groups.io/g/Ham-Antennas
https://groups.io/g/HamRadioHelp https://groups.io/g/Baofeng
https://groups.io/g/CHIRP https://rf-amplifiers.groups.io/g/main


ka0kaf@bellsouth.net
 

I do not know if the kit from Vectronics would solve what you are looking  but it may be worth a try.

73 



On Thursday, December 24, 2020, 10:07:38 PM EST, Donald Hellen <donhellen@...> wrote:


I think what I called it in the subject line is what I'm looking for.

I want to build an active audio filter that will have the ability to
cut the high end and the low end of the audio from a ham radio
receiver with switchable cutoff points, so that I can tailor the audio
based on band conditions. My speaker enclosure has room inside to add
such a device and two knobs on the front panel (audio volume and RF
gain that can be connected back to a Yaesu radio) that can either be
replaced with pots or switches to change the high-cut and low-cut
parts of the audio signal. I thought I'd use the existing pots or
change them to 3 or 4 position switches to allow switchable (or
tuneable) bandpass cutoff points. Sort of a very simple 1 band
equalizer.

If there's a ready-made kit for something like this I might be
interested, but hand-crafting such a device is OK also.

I would like to keep the cost low (like most of us try to do). I want
to use active components like op-amps along with resistors and
capacitors, and hopefully be able to run this off a wall wart type of
power supply once I either find one that's not noisy for RF or that I
can tailor to make it so. If I can't find that, I can find tap 12V
from my power supply as a power source since it's quiet (that may be
the best approach).

I can add a one or two watt (or more if needed) audio amplifier after
the filter if needed to make the audio level suitable to my ears. I
think the LM386 can make a decent low power amp.

Your ideas?

Donald -- AD8DY
Formerly KJ3I


----------------------------------------------------
Some ham radio groups you may be interested in:






Donald Hellen
 

On Fri, 25 Dec 2020 12:26:41 +0000 (UTC), "ka0kaf@bellsouth.net"
<ka0kaf@bellsouth.net> wrote:

I do not know if the kit from Vectronics would solve what you are looking but it may be worth a try.
Thanks, but if it's the ones on the MFJ web site, it's a CW only
filter. I was hoping to set up something that would cut the SSB
bandwidth as well as CW, possibly one with tuneable starting points
for the slope of the filter.

It also comes with an enclosure and I'd want to build it into the
speaker enclosure. So I'd be paying for an enclosure. I think a simple
active filter for this could be made with some 741 op amps and a few
other components.

My 7300 would do just fine but my older radios would benefit from a
decent audio filter.

I appreciate you mentioning this though.

Donald -- AD8DY
Formerly KJ3I


----------------------------------------------------
Some ham radio groups you may be interested in:
https://groups.io/g/ICOM https://groups.io/g/Ham-Antennas
https://groups.io/g/HamRadioHelp https://groups.io/g/Baofeng
https://groups.io/g/CHIRP https://rf-amplifiers.groups.io/g/main


Mike Reed
 

There is one. Try "VEC-830 SSB Filter" I have one, but have not had the chance to try it.
73
Mike - N7ZEF

On Fri, 25 Dec 2020 07:48:09 -0700, Donald Hellen <donhellen@roadrunner.com> wrote:

On Fri, 25 Dec 2020 12:26:41 +0000 (UTC), "ka0kaf@bellsouth.net"
<ka0kaf@bellsouth.net> wrote:

I do not know if the kit from Vectronics would solve what you are looking but it may be worth a try.
Thanks, but if it's the ones on the MFJ web site, it's a CW only
filter. I was hoping to set up something that would cut the SSB
bandwidth as well as CW, possibly one with tuneable starting points
for the slope of the filter.

It also comes with an enclosure and I'd want to build it into the
speaker enclosure. So I'd be paying for an enclosure. I think a simple
active filter for this could be made with some 741 op amps and a few
other components.

My 7300 would do just fine but my older radios would benefit from a
decent audio filter.

I appreciate you mentioning this though.

Donald -- AD8DY
Formerly KJ3I


----------------------------------------------------
Some ham radio groups you may be interested in:
https://groups.io/g/ICOM https://groups.io/g/Ham-Antennas
https://groups.io/g/HamRadioHelp https://groups.io/g/Baofeng
https://groups.io/g/CHIRP https://rf-amplifiers.groups.io/g/main



--
Using Opera's mail client: http://www.opera.com/mail/


Donald Hellen
 

Mike . . .

On Fri, 25 Dec 2020 18:53:36 -0700, "Mike Reed" <n7zef@tribcsp.com>
wrote:


There is one. Try "VEC-830 SSB Filter" I have one, but have not had the
chance to try it.
73
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:

https://www.electronics-tutorials.ws/filter/filter_7.html

or this one:

https://www.electronicshub.org/active-band-pass-filter/

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

https://www.instructables.com/Active-Band-Pass-Filters/

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



----------------------------------------------------
Some ham radio groups you may be interested in:
https://groups.io/g/ICOM https://groups.io/g/Ham-Antennas
https://groups.io/g/HamRadioHelp https://groups.io/g/Baofeng
https://groups.io/g/CHIRP https://rf-amplifiers.groups.io/g/main


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.
73
Mike - N7ZEF

On Fri, 25 Dec 2020 20:27:37 -0700, Donald Hellen <donhellen@roadrunner.com> wrote:

Mike . . .

On Fri, 25 Dec 2020 18:53:36 -0700, "Mike Reed" <n7zef@tribcsp.com>
wrote:


There is one. Try "VEC-830 SSB Filter" I have one, but have not had the
chance to try it.
73
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:

https://www.electronics-tutorials.ws/filter/filter_7.html

or this one:

https://www.electronicshub.org/active-band-pass-filter/

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

https://www.instructables.com/Active-Band-Pass-Filters/

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



----------------------------------------------------
Some ham radio groups you may be interested in:
https://groups.io/g/ICOM https://groups.io/g/Ham-Antennas
https://groups.io/g/HamRadioHelp https://groups.io/g/Baofeng
https://groups.io/g/CHIRP https://rf-amplifiers.groups.io/g/main



--
Using Opera's mail client: http://www.opera.com/mail/


Mike Reed
 

Did some searching and found the site I mentioned-
https://kc9on.com/product/calf-cw-audio-limiter-filter/

Maybe that may help.
And the Vectronics kit I also was thinking about is the VEC-841k CW/SSB . It is out of production but maybe you can find something there too...

73
Mike - N7ZEF

On Fri, 25 Dec 2020 20:27:37 -0700, Donald Hellen <donhellen@roadrunner.com> wrote:

Mike . . .

On Fri, 25 Dec 2020 18:53:36 -0700, "Mike Reed" <n7zef@tribcsp.com>
wrote:


There is one. Try "VEC-830 SSB Filter" I have one, but have not had the
chance to try it.
73
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:

https://www.electronics-tutorials.ws/filter/filter_7.html

or this one:

https://www.electronicshub.org/active-band-pass-filter/

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

https://www.instructables.com/Active-Band-Pass-Filters/

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



----------------------------------------------------
Some ham radio groups you may be interested in:
https://groups.io/g/ICOM https://groups.io/g/Ham-Antennas
https://groups.io/g/HamRadioHelp https://groups.io/g/Baofeng
https://groups.io/g/CHIRP https://rf-amplifiers.groups.io/g/main



--
Using Opera's mail client: http://www.opera.com/mail/


Richard in NC
 

Have you looked at a Autek QF-1 audio filter? I have one that I have owned for 30+ years. All solid state and work great. There are several of eBay right now for around $40. Time wave DSP-9 is a great audio filter as well and there is one of these on eBay right now as well. Autek filters are variable but the Wavetek I think is better for contesting. I have the QF-1A which added some features. I don't remember exactly what as I did not ever own the QF-1 version. 

Hope this helps 

73 Richard W4MCD 

On December 26, 2020 7:41:11 AM "Mike Reed" <n7zef@...> wrote:

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.
  73
  Mike - N7ZEF

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

Mike . . .

On Fri, 25 Dec 2020 18:53:36 -0700, "Mike Reed" <n7zef@...>
wrote:


There is one. Try "VEC-830 SSB Filter" I have one, but have not had the
chance to try it.
73
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:

https://www.electronics-tutorials.ws/filter/filter_7.html

or this one:

https://www.electronicshub.org/active-band-pass-filter/

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

https://www.instructables.com/Active-Band-Pass-Filters/

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



----------------------------------------------------
Some ham radio groups you may be interested in:
https://groups.io/g/ICOM   https://groups.io/g/Ham-Antennas
https://groups.io/g/HamRadioHelp    https://groups.io/g/Baofeng
https://groups.io/g/CHIRP  https://rf-amplifiers.groups.io/g/main







-- 
Using Opera's mail client: http://www.opera.com/mail/




Donald Hellen
 

Thanks.

That would probably do it but it's not just the board, it's a kit with
the enclosure, and I'd be paying for a nice enclosure that I wouldn't
be able to use.

I'd like to see his circuit though.

If I'm going to use anything ready-made or in a kit, I would want a
board-only device, with no enclosure. I think one of the common ham
radio band pass filter circuit diagrams I found would work, using one
or two 741 IC op amps. I have some quad op amps handy, and probably
some 741C's as well.

Donald -- AD8DY
Formerly KJ3I


On Fri, 25 Dec 2020 22:44:15 -0700, "Mike Reed" <n7zef@tribcsp.com>
wrote:

Did some searching and found the site I mentioned-
https://kc9on.com/product/calf-cw-audio-limiter-filter/

----------------------------------------------------
Some ham radio groups you may be interested in:
https://groups.io/g/ICOM https://groups.io/g/Ham-Antennas
https://groups.io/g/HamRadioHelp https://groups.io/g/Baofeng
https://groups.io/g/CHIRP https://rf-amplifiers.groups.io/g/main


Donald Hellen
 

On Sat, 26 Dec 2020 08:30:55 -0500, "Richard in NC via groups.io"
<w4mcd=yahoo.com@groups.io> wrote:

Have you looked at a Autek QF-1 audio filter?

This is probably close to what might work. Instead of using a high
pass and a low pass filter together it has a variable selectivity pot
to widen or narrow the bandwidth.

I think I can make a better one with steeper slopes but I'll have to
read about this one some. I could mount this inside the speaker
enclosure and move the controls to the front panel, then add holes for
the two toggle switches. It could be made to work.

I'll have to decide if I want to roll my own and have that feel-good
feeling of doing it myself or buying one.

Donald -- AD8DY
Formerly KJ3I



----------------------------------------------------
Some ham radio groups you may be interested in:
https://groups.io/g/ICOM https://groups.io/g/Ham-Antennas
https://groups.io/g/HamRadioHelp https://groups.io/g/Baofeng
https://groups.io/g/CHIRP https://rf-amplifiers.groups.io/g/main