Building the W3NQN CW Filter Kit Step by Step

Hi, my name is Matt. I am a ham radio operator with the call sign AK3W. One cold winter night, reading the ARRL Handbook, I saw the article about Ed Wetherhold's Passive CW Audio Filter. It was a construction project, and a kit was available! I enjoy CW Morse code operation but noise and static on the bands was fatiguing for me. When the CW bands are busy, adjacent signals sometimes make it difficult to comprehend the intended signal. So...I ordered a kit.

Follow along as I build this nifty kit. Click on the photos for a larger view. The schematics and diagrams are available in PDF format below.

The first step in the construction is wiring the inductor stack. The stack comes delivered with some short wires already connected to each of the 2 rows of terminals. See photo. Only enough wires are needed to interconnect the stack terminals. I was a bit confused (that isn't hard to do!) when I read the included instructions and they said "remove about half of the wires". After reading ahead and looking at what was needed it became clear. I removed every other wire. Look ahead to see how the terminals need to be connected. They run in one long series circuit.

Orient the stack properly by keeping the end with the vinyl sleeve to the left during the entire building process.

The wires have paper insulation on them. I tried to strip the insulation but it did not cut well, however, I found that the paper insulation would pull completely off the wire, if it were gripped at the end with pliers. I ended up stripping all of the insulation off the terminal wires.

The wires are then connected to adjacent terminals, in order to connect the stack inductors in the proper order. This is important. If these leads are not connected correctly, the unit will not operate. Check the supplied diagram of the stack wiring (I had to do it many times) and connect the terminals and solder them. Double check them.
I have included below links to a diagram and the schematic of the filter wiring, in PDF format.
A PDF Diagram of stack wiring.
A PDF Schematic of filter wiring.
A resistance chart for a filter with a 600Hz center frequency.

Capacitors C1 and C5 are connected to 1 terminal each as shown. Connect capacitor C3 on the stack between C1 and C5. Solder 1 lead each on C1 and C5, and both leads on C3. Note the second leads for C1, C5, are raised for later connection.

Now we deal with the transformers. There is a 200-ohm side, and an 8-ohm side. The audio transformers are marked on the face. 8 ohm for the 8-ohm side, and "P" primary, for the 200-ohm side. 3 leads extend from each side. The 3 leads extending from the P side are the 200-ohm coil leads. The center wire is the center tap. The center tap is not used, and the instructions call for them (both sides) to be folded back. I clipped them off short so that they would not contact any other lead.

The transformer ground leads are now connected. One lead from the Primary (200-ohm) side is connected to one lead from the secondary side (8-ohm). Both leads need to be on the same side (left or right in the photo). It does not matter which side you choose. Solder a 6-inch lead to these leads and you have your ground. Do this for both transformers.

Next, 2-inch leads are connected to the remaining 8-ohm leads (one on each transformer. Long leads (6-inch) are connected to the remaining 200-ohm (high side) leads (one on each transformer). The photo shows the ground leads connected together, along with the 6-inch ground lead connected to them. The 200-ohm leads in the back are not seen in the photo.

The ground leads of the transformers are connected to the phone jack ground lug (this will be the system ground). The remaining 8-0hm leads are soldered to the lower switch contacts (one on each side of the switch). The 200-ohm leads are connected to capacitors C1 and C5, as per instructions.
Attach the transformers inside the box. A large blob of silicone is placed in each corner and the transformers are pressed into the glue. Take care to place the transformers against the end wall as far as possible, to make plenty of room for the stack to fit into the box. I placed my transformer a small distance from the wall (1/4 inch), and later had to move it to fit the stack into the box. The Primary side of the transformers should face out (helps to keep things straight).

The two 85-mH inductor coils included in the filter kit have 2 windings on each core. They are separated by two fiber dividers. The start lead of each winding has a sleeve on it. Do not remove the sleeves. Each winding has a start lead and an end lead. Use an ohmmeter to determine which end lead is connected to which start lead.

The important task to accomplish with these inductors is to remove the correct number of turns of wire from each winding to provide the right center frequency of the filter (the number of turns will be included in instructions). My filter was designed for 700 Hz center freq. I was instructed to remove 77 turns from each winding (two windings per core). I could have just started removing turns and counting. I would have ended up pulling 5 feet of wire through the little center hole many times, then I would have likely lost count!

Ed recommends a nifty method of removing ten turns, snipping off the wire, coiling the removed wire and setting it aside. Count your turns removed by counting your removed coils. See Photo. It works wonderfully! I actually had miscounted my turns removal on one winding, but was able to recoup my mistake by re-counting my removed coils. There can be a lot of wire that needs to be removed (there must be a quarter mile of wire on one of those inductors!) and it does take some time. Get comfortable and take your time. I listened to an 80 meter DX net on the East coast contacting Russians and Europeans at midnight. Note: While removing turns from one side of the core, crimp the end of the wire on the other side in a recognizable way, so that you are only taking turns off one side at a time. Work on one winding at a time. Leave about 6 inches of wire on each lead for later connections. I cut mine too short.

My pile of removed windings. 308 turns.

When the proper number of turns are removed from both inductors, the next step is to connect the start lead of one winding to the finish lead of the other winding. Like so...then solder.

The leads of the inductors all eventually need to be soldered, so the insulation needs to be scraped off the ends. It is best to get it all off as well as possible. Fine sandpaper may work well.
Next measure the resistance as directed in the instructions. The resistances don't have to be exact.

If the inductors check out, (they probably will) it is time to attach them to the ends of the stack with silicone sealant. Let the glue harden.

The leads of the end inductors are connected to the proper components. At this point the filter wiring can be checked by checking the resistance values at various points with an ohmmeter. A diagram in the instructions shows the values that we are looking for. If you have a filter with a 600Hz center freq. the resitance values are a little different. Click here to view table. The actual values measured can be off by as much as 20% or so. A significant difference means a broken wire, or a wiring error. Use a digital ohmmeter if you have one. The resistances are low, i.e. 4.4 ohms. Photo shows my inductor stack fully wired, before placing into the box.

Next wire the switch and phone jack and phone plug. The user should obtain a suitable plug from Radio Shack, or other distributer, and add two wires of a suitable length to reach between the filter box and receiver. The ground lug of the phone jack is the filter system ground. The bottom two terminals of the switch are attached to the transformer ground leads. The center two switch terminals are attached to the phone plug tip lead, and the phone jack tip lug. The top two switch terminals are connected together with a resistor, or a wire lead. Grounds are connected to the ground lug of the phone jack. See the instructions and the diagrams. Tie a knot in the phone plug cord, inside the box, to act as a strain relief on the phone plug.

Phone plug wired and ready to install the stack in the box.

Carefully slip the stack into the clip, watching to protect the wiring from breaks or crimps. There are a couple of connections yet to be made to finish it up. Finally, plug it in and test it. If you have a problem or a question, Ed, W3NQN, is always ready to help. Just send him an email from the Contact Us page of this site.

Here is my modest amateur radio station with the CW filter in place. I also have an active audio filter that I have installed inline with the passive W3NQN filter. I can switch them both in or out, or use each alone. The filters allow me to manipulate the audio for all conditions: noise, static, loud adjacent CW signals, contest situations, or weak signal DX reception. I have worked the world from Tahiti to Antarctica, Greenland to Chile with this rig. I use vertical antennas and 100 watts.

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