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Computer (PC) Headset Microphone and Headphone Adapter
Jim  Feldman (W6JMF)

We're going to build a little adapter box that will plug into our HF rig and allow us to use a computer headset (mic and headphones). In fact it's not limited to just HF rigs, but more about that latter.

So as usual I was home one evening and a bit bored. I was flipping through some online ham ads when I saw one for a popular and well known ham headset. I was thinking to myself, "Hey, I have one of those for my computer, why can't I use that?". Well, you can, but...and here's where it gets a little tricky. If you have the type that plug into a USB port, stop reading now or be prepared to buy a set for this use.
The kind we're looking to use are the older "sound card" style that have separate mic and headphone jacks. The other tricky part is that a lot of ham gear expects a dynamic microphone. They actually produce a low voltage. Computer mics are usually electret style and the need a little bit of power. Lucky for us most rigs seem to have a low voltage pin on the mic connector that we can steal the little bit we need. In fact, if your rig doesn't have that, a small battery would probably work since the current is almost nil. We will also take a brief and somewhat unexpected turn into the area of RFI abatement.

Google was my friend here and there were some sites where people had already tried this. http://www.kingsqueak.org/?page_id=174 is a good place to start for reference.

The part count is low, and you may have a few bits of this lying around. All my parts were Radio Shack sourced (they were close), but you may be able to scrounge/recycle bits from other things lying around.

To start with: Parts List

.small project enclosure. 1x2x3 inches roughly

.4.7kohm resistor. Not real critical, we're using it to drop the voltage for the mic. On my Kenwood, I was dropping 8vdc to about 6vdc

.47uF 35V electrolytic cap to block the dc from the rig

.2 of the 3.5 mm stereo panel jacks. No internal switches.

.a panel mount, SPST (two contacts) NO (normally open) momentary contact push button switch for keying the rig.

.short (12-14 inches) of cat5 cable. Use that network cable that you have stashed in the drawer with the broken ends. Turns out that twisted pair is just as effective as shielded cable. You want the stranded vs the solid wire.

.Mic plug appropriate to your rig. Mine was the round 8 pin Kenwood/Icom
( http://www.qsl.net/g4wpw/date.html )
This guy has the BEST web page on mic connections. Do not lose it.

.Headphone plug for your rig. Mine was a 1/4" mono.

.2 small tie wraps

.Assorted tools, small wattage soldering iron, solder, drill and some small bits, hobby knife, band aids and number to the emergency room. Lets just say not all of my projects end well.

.Split core ferrite ring. We'll get to why later.

Literally, the most time consuming part of this project was prepping that stupid little box. Followed up by the fun of soldering the pins on the rig mic connector. Don't let either throw you off. Unless you live next to the parts store, you'll spend more time shopping than assembling. You might want to skip the second cup of coffee before doing the mike connector. They don't call it "jitter juice" for nothing.

The key to using cat5 twisted pair is to use twisted pairs. Use the colored pairs such as green/green stripe for the headphones to get the benefit of the twisted pair common mode current rejection. You'll probably want to write this out to make it easier. Try to keep your wires twisted together as close to the end as possible. I made the convention that the stripped wire as ground or return.

I did the following:

.green/green stripe - headphone

.orange/orange stripe - PTT button

.blue/blue stripe - microphone

.brown (I only needed one) for power for the mic

At the rig end, I took enough of the jacket off that I had a loop for the headphone plug as you can see in the photo below (which means I cut back the other leads much shorter before soldering to the mic jack. Pay close attention when wiring the microphone connector pins so that you know them by pin number and NOT where they look like they should be by the drawing (there lies madness). Solder up your pins on both connectors and maybe use a meter to look for shorts.

Finished Ends

Moving onto the box end. The box will need 4 holes drilled. One for the wire entrance, one for the PTT switch, and two for the panel jacks. I laid mine out so that cat5 came in from one of the ends, the PTT button was on the cover, and the panel jacks were stacked on one of the long sides. Nothing religious here, just remember you'll need to get into the box for assembly so lay things out to make that easier on yourself. Not that I did it that way, of course. I stuck the mic connection at the bottom where it was harder to solder the parts and put the easy headphone one at the top. I could have soldered them out of the box, but it was easier for me to have them mounted.

Circuit Diagram

Reference Pictures below:

Internal component locations


I had to notch the lip of the lid a little to make room for the headphone socket body (if I'd measured better when I drilled the holes...). The wiring isn't pretty, but do try and keep your leads short, and you shouldn't need a perfboard to mount things on. The stereo plug/jacks have three connections, designated tip, ring, and sleeve. I made an assumption that the mic plug used the tip and the sleeve which turned out to be correct. Solder up the blue stripe wire to the sleeve tab. Take the resistor, and trim the leads to maybe a half inch, just enough to put a small loop at one end and put the other through tip tab of the mic jack at the other. Solder the brown (or power supply) wire to the free end of the resistor, but don't solder the other end just yet. The cap is polarized which means you need to look for "+" and "-" signs on the cap body. Take the "+" lead and put that on the same jack tab as the resistor. Now you can solder that. Take the "-" lead and trim it to about a half inch, put a little loop in it and solder the blue wire to that end. The orange pair get soldered to the push button mounted in the lid.

I used a pair of tie wraps to lock the cable in place, but it can still twist. I need to poke a little glue in there to keep that from happening. The box is really light, so you may want to use hook and loop fasteners to stick it down somewhere where you can rest your hand (to push the button). You'll want to mark which jack is for which plug. Lettering, colored tape, a little dab of paint, whatever. The convention is usually red for mic and black or green for headphones.

The way I've wired the headphone gives me a common feed to both sides. I adjust the mic gain at the rig. Test the key button to make sure you light up your transmit indicator and you should be good to go.

Most computer headsets should work with this. You want to look for one with good ambient noise rejection. I bought an "open" back gaming headset from Plantronics . I think the mic tends to pick up a bit more background noise than I'd like.

While the setup works great, it has caused me to rethink the design a little. I'd like to add a trim pot to match the levels if needed. I'd like to put a panel mount rj-45 type socket in too and make up a cable to go to my 2 meter mobile rig. I'd lose access to the buttons on the mic though.


Now for the RFI epilogue. I'd been using this setup on 80 and 40 meters with no problem, even with a 1.2kw PEP linear. So one Saturday, I figured I'd go cruise 20 meter and play (nicely) with the serious contesters and see where my signal was getting to. Major complaints with my audio, and even I could hear it a little in my monitor circuit. What the heck! I'd been using the same setup on the lower bands with no problems. And that was my clue. The rather long cable on my headset was picking up my signal and feeding it back to the rig. For better or worse, when you go high power, you need to be prepared, and I had ordered a bunch of Fair-Rite split ferrite beads from Mouser. I took one of the big ones and wrapped my headset cable around it 4 times (near the plug end) and got it tightly closed. My audio reports went back to 59 and it sounded much cleaner in the monitor circuit. Further research revealed others had problems too and had addressed them with coiling the wire into a air choke or using a ferrite like I did.

73 JIM Feldman W6JMF


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