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Author Topic: Mating a helmet headset with a Yaesu FTM 350AR  (Read 1918 times)
K1TAZ
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« on: March 10, 2013, 07:41:32 PM »

Good Evening,

I am trying to connect a motorcycle helmet headset to my Yaesu FTM-350AR.  So far the results are less than satisfactory.

I have been testing a headset by talking with others on the repeater.  Everyone says the helmet mic sounds much fainter than the OEM hand mic.

The radio manual says the mic impedance for the OEM mic is 2k ohms.  I have talked with the tech line at Yaesu, all they can say is the radios input impedance is matched.  The tech assumes the input impedance is therefore 2k as well.

The headset mic impedance should be on the order of 200 ohms.  I don't know for sure, but the better helmet mics are dynamic mics in that range.

From what I have read, a low impedance mic should work well with a high impedance input.  It appears I have missed something.

My question is what does it take to be able to use one of the helmet mics with the radio? 

Thanks,
Jeff
K1TAZ
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AC2EU
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« Reply #1 on: March 10, 2013, 08:44:44 PM »

What you are missing is the specs for the helmet headset. what make/model is it?
Yaesu uses dynamic mikes. Most headsets have electret mics which require a minor modification at the jack assembly to work properly (or at all). I use a a standard cheap computer headset with my FT 817 using the additional resistor and capacitor mod with excellent results.
You need to know what type mike that you have. A dynamic type has a coil/magnet , so you would measure continuity across the terminals with an ohmmeter. an electret would be open.
If its just a weak mike, I believe that there are gain adjustments in the user menu.
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AC2EU
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« Reply #2 on: March 10, 2013, 09:03:50 PM »

I just read the spec sheet for the radio. Yaesu has a bluetooth headset designed for that model.
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K4JJL
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« Reply #3 on: March 11, 2013, 05:22:47 AM »

Unfortunately, it won't fit in a helmet.

A few friends have installed the Sena headsets in their Shoei helmets and connected them to the FTM-10 radios.  They seem to be pretty happy with them.  I use an LG Tone headset in my helmet just connected to my phone for listening to tunes.  It works pretty good until I get up to about 70 MPH, then I have a hard time hearing it.
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AC2EU
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« Reply #4 on: March 11, 2013, 06:22:26 AM »

Unfortunately, it won't fit in a helmet.

A few friends have installed the Sena headsets in their Shoei helmets and connected them to the FTM-10 radios.  They seem to be pretty happy with them.  I use an LG Tone headset in my helmet just connected to my phone for listening to tunes.  It works pretty good until I get up to about 70 MPH, then I have a hard time hearing it.

That's easy to fix. Stay at 65 MPH!  Grin
It's like an audible speed governer!
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AA4PB
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« Reply #5 on: March 11, 2013, 06:54:51 AM »

"From what I have read, a low impedance mic should work well with a high impedance input"

That's not necessarily true. A low impedance mike will have less voltage output than a high impedance mike. Whether the low impedance mike works with a high impedance input depends on how much gain the preamp in the radio has. If the radio has a mike gain adjustment you may be able to turn that up to make it work.

An electret mike has an internal preamp thus it requires a DC voltage on the mike wire in order to power the preamp (unless the mike contains a battery). If the radio doesn't supply the DC voltage on the mike wire then you need to use a dynamic mike.

You can use a small audio transformer to match impedance (and thus voltage levels) between the mike and the radio input. You need to use one designed for microphone usage in order to get the required magnetic and electrical shielding or you may end up with hum and/or noise being injected by the transformer.

Another approach is to build your own external mike preamp to boost the output of the mike to match the radio's requirements. If you use solid state devices then you won't have the magnetic coupling issues, but it will require power of some sort. The power needs to be very clean, which can be an issue if you take it from the available 12VDC ignition system. A small internal battery is easier, but then you have to be sure to keep good batteries in the preamp.

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K1TAZ
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« Reply #6 on: March 11, 2013, 07:39:41 AM »

All, thanks for the responses!

AC2EU  Thanks for the idea, but as K4JJL pointed out, the Yaesu headset is just not suitable for a motorcycle helmet.

I have looked at the SENA unit, and a friend even uses one.  I won't go into details, but it's just not suitable for my needs. 

AA4PB 
I have tried adjusting the mic gain, but it does not seem to make any difference, at least according the ham on the other end of my test conversation. 

I keep thinking there has to be something I can put between the mic and the radio, but I just do not know enough about electronics to feel comfortable experimenting with a new radio. 

I have read a bit about impedance matching transformers, even found a few on Mouser.com.  I am not sure if they are suitable, though.  They offer a 1:1 and a 1:4 ratio.

I would be open to an amplifier, i just don't know how to build one, yet!  I can solder and follow a diagram, but the design of such beasties is beyond me at the moment.

I am not totally sure of the radio's requirements.  The manual says the OEM hand mic is 2k ohms impedance, but they say nothing about the impedance on the radio side.  All the tech support line can say is they have matched the impedance.

Thanks again for the responses and your help. 
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AA4PB
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« Reply #7 on: March 11, 2013, 08:04:00 AM »

If the mike impedance is 200 ohms and the radio impedance is 2000 ohms then you need a transformer that is designed to match something close to those impedances. A 1:1 transformer wouldn't change the impedance at all. A 4:1 would change your 200 Ohm mike to 800 Ohms (which is closer). A 10:1 would change your 200 Ohm mike to 2000 Ohms BUT you also have to make sure that the transformer is intended to be used with something near those impedances and that it is designed for microphones and audio use.

Jensen is a top line mfg of microphone transformers. Take a look at: http://jensen-transformers.com/mic_in.html
One of the 150:15K transformers might be suitable.
« Last Edit: March 11, 2013, 08:20:35 AM by AA4PB » Logged
WA3SKN
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« Reply #8 on: March 11, 2013, 08:41:32 AM »

When switching microphones, you need to match both impedance and levels.  Since you are working with a tranceiver, you also need to plan supervision... or switching between transmit and receive.  AND since you are working with a motorcycle, noise pickup/sensitivity is also an issue.  I would assume that since you are working with a helmet, size is also an issue.
It can be made to work.  Fitting it in the required space may be the biggest problem!
Good luck with the project.
73s.

-Mike.
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K1TAZ
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« Reply #9 on: March 11, 2013, 09:36:35 AM »

AA4PB,
It's not a 10:1 but the specs on this seem to match.  Is this the kind of thing I am looking for?  http://www.hammondmfg.com/pdf/140PEX.pdf.
I don't mind ordering one and giving a try if it won't fry something on the radio!

Addendum:  OK, I just noticed on mouser that this little jewel is $67!  Still something I will happily buy if it works.  I guess for that kind of money I would like a little more confidence this is what is needed!

WA3SKN
Could you elaborate some more on what you mean my 'supervisoin'?

There are a number of excellent purpose-made motorcycle headsets out there.  J&M and Edsets are two that come to mind.  They both use low impedance (150-200 ohm) dynamic microphones.  My problem is getting one to connect to my radio.  Both J&M and Edsets assume you will be connecting your headset to an onboard intercom system.  These intercom system (standard on Goldwings and some of the higher end H-D's) will interconnect radios, GPS units, phones, and your passenger.

Since my bike is about half the size of a 'wing, I don't have an intercom system.  There are companies that offer aftermarket intercom systems (Kennedy products at cellset.com), but there has got be a way to hook a microphone to a radio without putting $300 worth or electronics between them!

Thanks again for all the responses! I do appreciate it.
« Last Edit: March 11, 2013, 09:49:16 AM by K1TAZ » Logged
WA3SKN
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« Reply #10 on: March 13, 2013, 07:09:44 AM »

Supervision is control.
Connecting to a transceiver, you will want to switch between transmit and receive mode.  with a two point intercom, this is not a problem,  The mic is connected to the other station headphone... and there is full duplex communications, no need for switching.  Some intercoms will have a signalling device (I doubt that a motorcycle one will, though!).
So headsets for an intercom may not have a switch available, while headsets for transceivers would... or at least a "VOX" option.
Matching impedances mean getting the 200 ohm mic up to the 2000 ohm radio impedance.
Then there are the levels.  As I recall, Yaesu "electret" type mics nominally put out about 5 millivolts peak (or was it RMS?) into the 2000 ohms of the radio input.  Get the headset levels up to that level and you should be good-to-go, audio wise!
73s.

-Mike.
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K1TAZ
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« Reply #11 on: March 14, 2013, 08:25:07 AM »



Well, I ordered a couple of Audio transformers from Mouser, thankfully I found a more economical alternative to the $70 Hammond unit.  Page 2855 of the current online catalog has a whole bunch of audio transformers for about $2.  I got a 600CT x 4kCT transformer.  Using the center tap on the 600 side matched the impedance on the microphone, and the center tap on the other side matched the 2k impedance for the radio.  Results were better than without the transformer, but not as good as the OEM hand mic.  Switching the radio side to 4k helped more, but it was still needing more.  Thanks to N5FSJ for talking with me.

I am going to try a transformer with a higher value on the radio side, as well as this:
http://www.frys.com/product/6232950?site=sr:SEARCH:MAIN_RSLT_PG

it's a small microphone preamp available at Frys.
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AA4PB
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« Reply #12 on: March 14, 2013, 08:43:51 AM »

Microphones have different output levels in addition to different impedances so it is quite possible that even when the impedances are properly matched with a transformer that the output level of the mike is too low. Sometimes there is a mike gain control inside the radio that can be turned up to provide more amplification for the microphone signal - but then the original mike will overdrive it.

The little preamp you are looking at should solve the problem and you probably won't need to use a transformer with it. As long as the preamp input impedance is equal to or higher than the microphone impedance AND the preamp output impedance is equal to or lower than the radio input impedance you should be okay. In audio work it is common practice to have the load impedance higher than the source impedance. In audio work its all about proper voltage levels rather than matching impedances for maximum power transfer as is common with RF work.
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K1TAZ
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« Reply #13 on: March 15, 2013, 06:36:35 AM »

I tested the amplifier last night, and it works very well!  Thank you everyone for your input!
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