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.

Tapping threaded holes for the board.

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.

Ground down ribs inside case


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

Board wired up

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

Front of NESCAF

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

Rear of NESCAF

Here is a video demonstrating the use of the filter…