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NESCAF Audio Filter


NESCAF Audio Filter

A couple of months ago, I dug out an old kit from the shelf to give the grandsons something to build while they were spending the day. They soldered most of the components on it, but didn’t have enough time to finish it.

So now, I cleared off the bench once again, and finished the NESCAF Switched Capacitor Audio Filter. This is a primarily a CW audio filter, with some unique characteristics. You can smoothly vary the bandwidth and center frequency for the filter, which makes for very easy operating.

NESCAF Audio Filter

The NESCAF audio filter is kitted by the New England QRP Club. It is intended to be a CW filter with continuously variable bandwidth from about 90 hz to about 1500 hz. The center frequency is adjustable from about 150 hz to about 1000 hz.

The filter uses a Switched Capacitor Audio Filter Chip (MF10, LMF100 or LTC1060 chip). It uses a 555 timer to create a square wave to feed the Switched Capacitor chip at about 100 times the center frequency. The output of the SCAF chip is fed to a typical LM386 audio amplifier chip for final output to a speaker.

After building the circuit board, I prepared the aluminum enclosure by drilling 6 holes for the controls and connectors. Next, I drilled and tapped four holes to accept some 4-40 screws that hold the board in place. I used nylon spaces to keep the board off of the conductive floor of the case.


Next, I used a dremel tool with a cutoff wheel to grind down the ribs inside the case wherever I wanted to mount the connectors.


I then mounted the board inside the case, and connected the wires to the various connectors and controls.


When that was complete, I simply set the gain with R14 and set the center frequency at the detent with R9.


The back of the filter has a 12-volt power connector, and connectors for Audio In and Audio Out.


Here is a video demonstrating the use of the filter…

 Migrated Comments:

7 Responses to “NESCAF Audio Filter”

  1. BOB LECH Says:

    I have the NE-SCAF filter and because of that filter I bagged D that I was not even able to make out.
    The SCAF filter also is a real GEM when I use the FOx-3 transceiver kit.
    The filter is worth every cent
    BOB AF2Q

  2. BOB LECH Says:

    This has to be the best filter I ever used.Things get very tough working CW with the FOX-3 tranceiver but after hooking up the SCAF filter,I was able to narrow down the BW and bag contacts I was not able to hear with my FT 857-D rig.
    Best little kit I ever built.
    BOB AF2Q

  3. BOB LECH Says:

    I keep looking at your kit build.
    I built mine up and ave anoter one redy to go together but fish are biting good 🙂
    I worked FRance with 4 watts about 6 weeks ago with my indoor mag loop made from 10# wire but would not make the QSO without the SCAF filter.
    I just seems to eat up the noise.
    Grear job you did.
    72/73 BOB AF2Q AF2DX pending

  4. wb0smx Says:

    Hi Bob. Yes, it is a great filter. Too bad it’s no longer available. Those looking to duplicate it will have to make it (or something simolar) from scratch, I think…
    Thanks for the cudos… Randy WB0SMX

  5. BOB Says:

    That filter works so well I think the NE club should ask how many hams want another kit build.
    I dug out a lot of DX that I was not able to hear with the 300Hz CW filter on my FT 857

  6. BOB LECH Says:

    Hi Randy,
    I found another kit and just finished that last week.
    Like the first one I put in a tiny 1 watt audio filter for the China rigs.
    That SCAF filter really digs the weak ones out of the mud
    72/73 BOB AF2DX

  7. BOB AF2DX Says:

    Randy WB0SMX
    I did get another kit and on the VK mailing list they had a small group buy
    When it comes to getting really narrow the NE SCAF cant be beat


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