Many thanks to Sam (WN5C) for sharing the following guest post:
A Compact CW Filter and Speaker Build for the TR-35
by Sam Duwe, WN5C
I recently built a Penntek TR-35 and, like seemingly everyone, I love it.
Once the rig passed the smoke test I was having too much fun and wasn’t quite ready to put away the soldering station. I had two non-essential wants for this project: a narrower CW filter for listening comfort, and an external speaker. Here’s a quick description of how I crammed both of those into an Altoids tin. Nothing is new or groundbreaking here, but it has been a fun and useful project for me and hopefully will give some inspiration for others.
The heart of the project is a Hi-Per-Mite 200 Hz CW filter, designed by David Cripe NM0S, and sold as a nice kit for $28 by Four State QRP Group. Hans Summers G0UPL uses the circuit in the QCX so many will be familiar with the filter’s sound. It’s nice and narrow with no ringing, and makes using my base station (a Kenwood TS-520 with the 500 Hz crystal filter) a joy.
To be clear, the existing narrow filter in the TR-35 is great, but I like the option of going narrow(er). It’s a Pixie-level build difficulty so it should come together in an easy couple of hours. I originally built mine in an Altoids tin using inspiration from Phillip Cala-Lazar K9PL’s review and it worked very well. It sips current and is powered by a 9-volt battery. With a DPDT throw switch connected to both the audio path and the power you can easily switch the filter on and off.
A neat aspect of the TR-35 is that there is a lot of audio gain so you can drive a non-amplified speaker. I have a little Bluetooth speaker that does this trick when I want to use CW to annoy people, but I figured if I’m already hauling an Altoids tin to the field maybe I could get it to talk, too. I looked around my junk box and found a broken Baofeng speaker mic and salvaged the speaker. It works really well: a robust but comfortable volume.
I’m sure any little speaker would do the trick… nothing fancy here, it gets hot glued it to the lid of a mint tin after all.
After I built the Hi-Per-Mite here’s what I did: first I ate a tin of Altoids and felt a little sick. Then I drilled some holes. The one on the left is for the audio input, the one on the bottom for the headphones (both of these are 1/8” stereo jacks), and two on the right for two mini DPDT switches. I also drilled holes in the lid for the speaker sound to come through. I gave the tin a good sanding and tried to remove sharp edges, and then sprayed the lot with black primer and spray paint.
I then prepared the two stereo jacks to make them mono by soldering some magnet wire between the tip and ring tabs. I performed the wiring of the jacks and switches out of the tin but you could do it once mounted, it just might be tight. I wired the audio input jack to the IN and GRD pads on the board. I also wired the battery negative wire to ground, too.
I used two DPDT switches. The first (located on the bottom right), described above, is mounted horizontally and switches on (or alternatively bypasses) the filter. The power side of the switch is wired with the positive wire of the battery soldered to the common terminal. When thrown, a wire connects to the 9V pad on the board (the other terminal is unused, so no power). The audio path goes through the other side of the switch. The common is the audio output (which is piped to the other switch to select between the headphones or speakers).
When the filter is off the audio on the board is bypassed and I just routed a wire to the Audio IN pad on the board to gain access to the unfiltered audio. When the filter is on, simply wire the other pole to the audio OUT pad.
The second switch ports the audio to either the positive on the headphone jack or the positive on the speaker. The common of course comes from the common terminal on the audio side of the previous switch. I oriented this switch vertically to distinguish the two in use. Both the phones and speaker negative terminals can be grounded at a convenient location on the board.
Once you’ve tested everything and you’re happy you can finalize the build. I used 3M double-sided foam tape to secure the board and the battery. My headphone jack came pretty close to the board so I applied some clear packing tape strategically to ensure it didn’t ground out. Then I hot glued the speaker into place. I plan on carrying the filter/speaker in a small camera bag, where I can also store the audio patch cable and some headphones, etc.
**One final note: the Hi-Per-Mite is an amplifier, so it will work as a 200 Hz CW filter to drive rigs with lower audio output. This is nice because I can use it with my QCX mini just fine. I built my HPM with the default “0 dB” setting and things were FB, but you could also change some resistor values and make things louder if needed.
Thanks for the great community and 72,