A CW Sidetone Bypass for the NESCAF Audio Filter
By: Bertrand Zauhar, VE2ZAZ
Last updated: 13/04/2012

The NESCAF audio filter is a small and low cost circuit that works great. It is a good addition to any amateur radio setup. It is especially useful with radio receivers or transceivers that lack an optional CW filter. I have made a product review of the NESCAF audio filter on my Amateur Radio Blog.

One feature this filter lacks though is a bypass path for the audio when transmitting in CW. This would allow to hear the CW sidetone geneated by the radio through its speaker. So I have come up with a simple solution to this issue.


Referring to the
altered NESCAF audio filter schematic diagram below, all added connections and components are shown in wider/bolder black color. The bypass path is provided by a reed relay. The relay simply bridges the audio input signal to the output audio amplifier. The relay coil is energized via the ground return path provided by the Morse code key. The silicon diode and the zener diode insure that interaction with the radio is kept to a minimum. On the audio path, a 10K resistor is inserted to reduce the gain and match the audio level seen when the filter is inserted. A coupling capacitor is also inserted to block any DC current from flowing. Grounding the "Morse key" terminal will operate the bypass relay.

Click on figure to enlarge it


Nothing is really critical in the implementation, and you may want to experiment with the component values to suit your needs. The following are things you should consider when adding this feature to the filter.
  • Use a reed relay with a 5V coil. Try to select one with the least amount of current drawn from the coil. Remember that the U2, the 9V regulator, is limited to 100mA of current. The relay I use is a DIP package reed relay (Clare PRMA 1A05) I had on hand. You should be able to find an equivalent relay very easily.
  • The 5.1V Zener diode is important, as it insures that the relay does not get much more than 5V on its coil, keeping the coil currrent low. Additionally, it limits the floating voltage at the morse key to less than 5V, which is normally the floating voltage on modern rigs. You may verify this on your radio and select a different Zener voltage diode to suit your radio.
  • The silicon diode blocks any voltage from flowing back into the relay coil, which would have a loading effect on the radio's CW key signal.
  • All discrete components are soldered right onto the relay pins, in other words the relay pins supports all discrete components.
  • I cut off the relay pins; just keeping the wide part of the pin is sufficient to solder onto.
  • The relay is simply glued to the top of the filter chip with flexible goop.
  • All wires are soldered to the PCB on the bottom side. I used 30-gauge wire-wrapping wire to make the connections.
  • I used 1206-sized surface mount resistor and capacitor. The surface mount version is optional but it keeps the overall solution small.


The following picture shows the added circuitry, with the reed relay (in blue) sitting on top of the filter chip. All connections that are part of the add-on circuit are made with the thin red wires.

Click on picture to enlarge it

Operating the add-on bypass circuit is a matter of connecting the "Morse key" terminal of this circuit in parallel with the radio's key input (hot wire and ground) and on the the morse code key. Please observe polarity.