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Author Topic: Kenwood TS-820 headset/mic  (Read 3116 times)
KI4KMP
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« on: August 06, 2011, 04:35:03 PM »

Anyone have success wiring a standard communication headset with a condenser mic to the 4 pin connector? I have a dynamic mic that works, but I can't seem to get the condenser boom mic to generate very much when soldered to pins 1 & 4. Is there a difference b/w dynamic and condenser? Does a condenser mic need a pre-amp?

Edit: After some research it appears these cheap electret condenser mics normally receive 5v of "phantom power" on the ring of the phono jack. It seems like introducing dc voltage on the Kenwood's pins isn't a good idea.
« Last Edit: August 06, 2011, 04:54:28 PM by KI4KMP » Logged
AC5UP
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« Reply #1 on: August 06, 2011, 06:31:05 PM »

Electret microphones require phantom DC power, typically 8 vdc @ 10 mils. Phantom just means the pre-amp B+ shares the same circuit as the audio 'invisibly'.

In your case you have three considerations: Matching levels, matching impedance and interfacing to the radio. None of these are difficult. Search the web for electret pre-amp circuits and you'll find they range anywhere from one transistor simplicity to an op-amp chipgasm. Keep yours simple as SSB isn't THX surround sound. The clever placement of a DC blocking capacitor allows the phantom thing to work and once you've seen a schematic it will make sense. Matching levels is easy as most pre-amp circuits include a trimpot on the input or output side. Adjust the level so the new mic has the same gain as the stock mic. That way you don't have to reset the radio mic gain whenever you switch them. Matching impedances is also easy since the TS-820 expects a Hi-Z mic and a simple 5k load resistor on the output is adequate. You should have more than enough gain.

Depending on how handy you are an outboard interface box will probably be your best bet. I would NOT modify the radio internally. Headset plugs into the box, pigtails from the box plug into the radio mic connector and headset jack, inside the box is the pre-amp, 9 volt battery and power switch. Add a small LED if you're concerned about wasting battery by leaving it on overnight. Better you should waste battery lighting the LED...

Here's a typical pre-amp circuit for a dynamic element. You may find something much better on the web:

http://www.shure.com/americas/support/user-guides/discontinued-products/microphones/us_pro_590t_ug

And here's an example of how simple it is to do the impedance conversion with a single FET (which may not add any gain):
(Note how the test circuit uses a 20k load resistor. This could be a 20k trimpot with the wiper feeding the mic input on the TS-820)

http://www.shure.com/americas/products/microphones/other/us_pro_wl50_51_mc50_51_en_ug


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KI4KMP
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« Reply #2 on: August 07, 2011, 07:12:40 AM »

Wow thanks for the reply. I think I am just going to buy an old Shure 444. But I still want to build an external inline pre-amp. I was thinking something JFET or MOSFET based like this:

http://www.till.com/articles/GuitarPreamp/

Maybe with a 10k POT on the (input to ground??) to control/match gain. I assume the two electrolytic caps are to block DC and protect the radios input? Seems like a 9v battery would last almost forever in this circuit.

I rebuilt this transceiver and finally got it working. Modifying it internally would be sacrilegious to me. I think Kenwood got it right the first time.

There is a Fry's up the street so parts are readily available.
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N4NYY
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« Reply #3 on: August 07, 2011, 07:24:29 AM »

Quote
Wow thanks for the reply. I think I am just going to buy an old Shure 444. But I still want to build an external inline pre-amp. I was thinking something JFET or MOSFET based like this:

I have one of those. I got it for one reason. Someone had unloaded about 4 1960's tube CBs on me ro repair/sell, and I needed an high impedance mic. I got very good reports on it with no amp. I got both the Black 444 and the preamped 444 for about $40 combined, from a Silent Key sale.

My suggestion is to try the 444 without a preamp. I think you will be surprised. These seamed tailored for PA systems, which made it perfect for boatanchors.
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AC5UP
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« Reply #4 on: August 07, 2011, 08:27:33 AM »

There is a Fry's up the street so parts are readily available.

Never been to a Fry's but I understand it's an experience...

Anyway, the circuit you found is intended for a guitar pickup. Should be no problem but I would add a DC blocking cap to the input, something like a nice 16 volt mylar in the .22 to .47 range. Same deal on the output. C1 is intended to ensure the output is clean audio with no DC component. 4.7 uf is too heavy IMHO and an electrolytic is polarized which is a no-no to an audio purist. A .47 mylar should work mo'better since a clean midrange is all you care about on the radio. C2 is intended to stabilize the battery voltage during peaks, especially when the battery is near flat and could use a little reserve charge from an electrolytic. 100 uf is a good value. Make R4 a 50k trimpot and connect the wiper arm through a .22 uf mylar to the mic input of the radio. Make it somewhat accessible since that's the adjustment for output level.

As for the Shure 444, that is my favorite desk mic and I adapted a Lo-Z version to an Icom IC-756 that came stock with a phantom powered electret hand mic. Used a slightly revised version of the Shure schematic I linked for you. Works FB, no outboard box, built into the base of the mic and is phantom powered by the radio. Almost looks like I planned it that way. If you decide to try that circuit the transistor is a generic NPN like a 2N2222 or 2N3904 and it's very un-fussy.

Finally, 'NYY is correct in that if you can find an original Hi-Z 444 (grey case) all you need do is wire a connector. That's exactly the type of mic the radio was designed for. If you end up with a Lo-Z 444 you will need a pre-amp otherwise the mic will be a real scream to work with..............
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K8AXW
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« Reply #5 on: August 16, 2011, 08:28:14 AM »

KMP: 

Many years ago I bought an inexpensive Telex headset with boom mic at a hamfest for like $5.00.  I could never get the mic to produce enough audio for the TS-830S and so it set in my junk box.  Eventually, I got tired of using my hand mic and looked for a solution to the boom mic problem.  I opened the mic housing and did some measuring and then got the dimensions for the Heil mic elements, HC-4 and HC-5.  I found the Heil elements would just fit inside the housing.

I ordered one, connected it up and put it in the housing and BINGO!, I have a first class boom mic.

As an aside, I later built an "audio enhancer" that was written up in QST.  This circuit split the mono audio from the transceiver into two channels, each with two roofing filters and two separate and adjustable audio amplifiers.  I then split the wiring in my modified boom mic headset into a stereo arrangement and then I was able to hear and understand SSB better.  I'm still using this homebrew setup.
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