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G8MNY  > TECH     14.06.19 10:30l 128 Lines 5623 Bytes #6 (0) @ WW
BID : 8261_GB7CIP
Subj: Hands Free Mobile Mic
Sent: 190614/0820Z @:GB7CIP.#32.GBR.EURO #:8261 [Caterham Surrey GBR] $:8261_GB

By G8MNY                                 (Updated Sep 17)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

This homebrew design has the advantage that it has clear audio, as the treble
has been lifted. Most /M mics have a flat response & when placed near the mouth
for background noise reduction (as needed for /M work), the LF bass is far too
loud & VERY WOOLY audio is the result. Indeed with some preamps the LF engine &
rumble noise from the car can cause problems if no LF filtering is used!

         T A I L O R E D   R E S P O N S E
+10dB´                              _..-----
     ³                 Woolly   _.-~
  0dB´LOUD CAR NOISE   Voice_.-~  Clear
     ³                  _.-~     Treble
     30     100     300      1k      3k     10k     Hz

Here a 3 terminal electret mic is used (not common), it is mounted (soldered)
on to a thin wire (flower arranging type) stalk inside a thin AF lead for about
20cm. The end of the wire is placed in plastic insulation tube & shaped into a
loop & tied off. I wear the loop around my ear. As the mic is mounted right in
front of my mouth, there is next to no background noise.
      __                   ||     ||
     / \\                 //     //
 Mic \_//                ||     // Ear
Insert \\________________//   ,// loop

              Drain                                 .-Ä-.
 +3-5V ÄÄÄÄÄÄÄÄÄÄÄ´       ³|                       /D   S\
     Internal FET ÃÄÄÄÄÄÄÄ´|Electret              ³       ³
Output ÄÄÄÄÄÄÄÄÂÄ<´ Gate  ³|Pressure               \=gnd=/
              1k Source    |  Mic                   'Ä--' 

In my circuit the treble has been lifted by the 0.2uF tantalum cap on the
electret mics' internal FET transistor, source to ground.

       screened           ³   ³     screened          +³D
        lead              ³  ===     lead            _3term
                        ÷3k3  ³10n                + /S mic
                          ³   ³                0.2u=== ³case
                          ³   ³                     ³  ³

This simple treble lift is only possible with a 3 terminal mic capsule. The
normal +ve rail drain input now becomes the output as in a 2 terminal mic type.
A shunt R of about 3.3k (selected on test) across the mic, sets the AF output
level, & a 10nF across the mic in the gear lever switch to roll off the AF lift
above 3kHz.

The result is a clear punchy mic that has very little background noise. A small
sponge covers the face of the mic to reduce popping/blow etc.

It is comfortable to wear, & I have to make sure I remove it before getting out
of the vehicle. The only problem I have found is while eating Sandwiches!!

If your rig does not supply power for a 2 terminal type mic & the PTT operation
you will need a C & R as well.

smooth DC                  R
           - C +           ³

RIG PTT<ÄÄÄÄÄÄ¿                   after PTT
              / PTT SWITCH        as above

The C needs to be about 1uF (10-100nF if not treble lifting), & the R between
1k-33k select for the gain you need.

Cable ties                  /      \ 
        \ÚÄ¿                \      /
soldered ³Ü³PTT Slide        \    /          This gives a tactile & easily
on nut-[³³²³switch soldered   ³  ³_ Ü_       operated PTT switch at one's
     ___³³_³to strip.         ²²²²ßßßßÝ      fingertip, but not too easy
    /    ³\screw under switch ²²²²(/)³       to mis-operate.
   /gear ³                     ÝÞ ~~//~~
  ³ stick³tin/copper/          ÝÞ  //        Keep signal earths isolated from
   \    / brass strip          ÝÞ // AF      gear leaver as that will have
    \__/ shaped to fit.        ÝÞ ||cables   the noisy engine earth on it.


Here is another approach that works with 2 terminal electret mics if you have
enough rig mic gain...
   .-~~-.                             ÚÄÄÄ´ÃÄ¿
  /      \ Head band or      Mic ===(Ä´      ÃÄ)======= Rig MIC
 |        | neck loop.              ³ ÀÄÄ22kÄÙ ³       
          | Commercial              ³          ³   / 
    @._  /. Mic system              ÀÄÄÄÄÄÄÄÄÄÄÙ  o oÄÄ Rig PTT
       ~~  '.                                     ÀÄÄÄÄ
             :      It assumes that the rig powers the mic & that with the
                    close in mic, there is enough level to add a series 22k.
                    Then the treble can be boosted with the 15nF.


Doepie, ZS1HK @ ZS0HER says these electret mics can turned into noise
cancelling mics by drilling 3-4, 0.8mm holes through the insulation between the
terminals of your mic. This will influence the gain but makes it a high quality
noise cancelling mic. This will take away all back ground noise.

I suggest you only try this if you have a spare insert!

Why Don't U send an Interesting Bul?

73 De John, G8MNY @ GB7CIP

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