Sam builds a compact external speaker and 200 Hz filter for the Penntek TR-35

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 Hi-Per-Mite

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.

The Speaker

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.

The Build

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,
Sam WN5C

21 thoughts on “Sam builds a compact external speaker and 200 Hz filter for the Penntek TR-35”

  1. Hi Sam, great project. I have a HPM (currently built into a uBitx, but not used since I acquired an IC-705 ?). Working on packaging up a KD1JV Slop Bucket II rig for 20 m, and that could use both 200 Hz filter and ext speaker!
    A couple of suggestions:
    1. Could eliminate your speaker/phones switch by using a switched stereo jack for phones. Wire so that with nothing plugged in, signal is routed to speaker. When phones plugged in, speaker is switched out and signal us routed to phones.
    2. With one switch gone you could mount the other sw entirely *within* the Altoids tin. Rotate sw 90 deg, and elevate 45 deg (hot glue in place inside tin). By elevating the toggle, it is readily accessible but lid still closes. During operation, leave the tin lid open and switch the filter in or out as needed. When done, close lid. This configuration would make the tin more packable and durable.
    I’ll have a go at building one this way and report back to the group.
    72, Scott VO1DR
    PS- Question of the day – which has more uses: Altoids tins or duct tape…? ?

    1. I like the idea of internal switches, or maybe slide switches, to keep the tin small and robust, but I do also like the idea of a manual switch for the speaker. It’s nice to be able to allow someone to hear what’s going on, while still listening on headphones (I have much more trouble copying code on a small speaker, especially if there is room noise or echo). Also, fyi, there are 9v li-poly batteries around now that have more capacity and are about half the weight of a 9v alkaline (besides also being rechargeable). Might be worth while if you are keeping your little qrp kit light.

      It would actually be funny to build this into one of those baofeng speaker mics. It would be funny seeing a speakermic plugged into someone’s cw-only qrp radio. 

      1. Hmm, so a switched stereo jack and the bottom, and a small slide switch for the filter on the side might be the answer here for portable operation. I do want to get some of those rechargeable 9v batteries, they would also work to power my ATU 100 in the field too I think (I saw 4-packs for pretty cheap on Amazon), thanks for the ideas!

      2. I love the idea of building this into a baofeng mic, wonder if it’d fit? You could maybe use the PTT as a straight key, too?

    2. I didn’t think of using a switched stereo plug, that’s a good idea! I do agree that that the mini toggle switched are a bit of a pain to pack and are a weaker point. But, I’ve been using this speaker in the shack, too, where the switches are really nice to access and use. Maybe I should build another, haha.

      1. I never pass up an opportunity to put a toggle switch on something. I’ve been known to remove other switches from appliances and replace them with toggles from my copious hoard. The mini ones aren’t quite as satisfying, but electronics have become uninspiring since we dropped toggle switches. You want that positive “click!” You want the manhood of throwing that switch.

        So good on you, Sam, for figuring out a design that uses not just one, but two. One more reason for me to build it.

    1. Thanks! The paddles are from N0SA and are called the SSP (super small paddles). They are an older version of the SOTA paddles that Thomas uses (similar design but with a cage to better hold the paddles with your other hand). My elmer bought a couple back in the day because he liked them so much and gifted me on (thanks Perry N5PJ!). N0SA sells the SOTA paddles occasionally in small batches and I can’t say enough good things about his work. Also, I’ve recently discovered the joys of 3M dual lock tape, its the best for holding down paddles in the field (I put a piece on my battery and currently use that for a base).

  2. Brilliant! I haven’t built my TR-35 yet (would be interested in your experience with that), but it happens I’m in the research stages of adding a speaker and an audio filter to my old OHR 100, and this design is exactly what I need.

    Two observations:

    Where did you get your old-school 1/8″ chassis-mount stereo jacks? I kid you not, I can’t find them anywhere. The only thing I’ve been able to source is either board-mount or the cheap cylinder style, and even those are ridiculously expensive.

    Also, the best way to deal with that candy they inexplicably ship inside ham project enclosures is to find a child. I prefer Ice Chips for enclosures. The box is the same size and shape as an Altoids tin, the candy comes in a variety of flavours (I’m partial to lemon), and the company doesn’t emboss its name on the lid.

    Thanks again for a timely and useful post!


    1. Thanks! You can add all the gain you would need by changing the resistor values in the HPM (there is a ton of info on the 4 State QRP HPM site), so I agree this would be a welcomed addition to your OHR. I’ve been curious about those rigs since I saw one at a hamfest, very cool,

      Well, I can’t speak on their long-term quality, but here are the jacks I found on Amazon, at least they’re cheap!:

      And thanks for the suggestion on other tins with delicious candy. I found out that my five-year-old doesn’t like Altoids hence why I had to eat them. The Ice Chips might make us all happier.

      1. Thanks, Sam! Unfortunately the Amazon link says “currently unavailable”, which is what I’ve been smacking into everywhere.

        The OHR is a great rig, and the only one I have at the moment. It’s old, of course, both in literal terms and technologically, and it’s a monobander (20m in my case), but it works great. I get about 8 watts on my 14+v bench supply.

        I think Ice Chips are going to solve your issues. With all the flavours they sell, you’re bound to find something your 5-year-old (and maybe even you) can dispose of painlessly.

        1. Thanks for the offer, Sam. In the end, it’s not that dire. (If this is the worst problem I have, my life is good.) I’ll just keep looking for a commercial source… or redesign… or take up needlepoint… or something.

    2. Oh, as for the build: AE5X posted a good discussion of this on his site, but he is a more experienced builder than me. However, it took me, being very careful, two evenings. Each step has its own bag of parts and the documentation is good. I actually like winding toroids (I’m weird) but you don’t even need to do that as they are pre-wound. I think the most nerve-wracking aspect was that this wasn’t a cheap kit as is going to be my only field radio so there was a lot of pressure to not screw things up. I can’t imagine the pressure of building an Elecraft kit! But all said and done, it was very doable and and fun experience.

      1. Thanks for the good word, Sam! That’s exactly my situation as well. The challenge is daunting, which is why I haven’t dived in yet. I’ve got the new optical encoder too, which is touchier than the mechanical one.

        Only one way forward though, eh?

        1. I bought the optical encoder, too. This was the only tricky soldering part as the pads are small and tight, so a thin soldering tip is helpful. I haven’t used the standard encoder, but mine is smooth. Now on to find the perfect bigger tuning knob…

  3. Nice Build !! Its always fun building gear… keep up the good work !!
    ** As for speakers…

    A little hidden secret… go to your local thrift store and pick up used ‘center’ speakers from TV surround sound systems.. they work great on HF rigs. they are designed for voice.. you will be so amazed you will sell off your expensive matching speakers and buy another rig with the funds !! LOL.
    73 MIKE VE3MKX

    1. Excellent. I have found that the Habitat for Humanity Restore has been a treasure trove for cables, etc. but I also see a million surround sound speaker for nearly free. I’ll pick some up!

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.