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G8MNY > TECH 14.07.03 19:53l 76 Lines 3248 Bytes #999 (0) @ WW
BID : 5637_GB7CIP
Read: DB0FHN GUEST OE7FMI
Subj: Hands Free Mobile Mic
Path: DB0FHN<DB0RGB<OK0PPL<RZ6HXA<HA3PG<JE7YGF<7M3TJZ<ON0AR<VK3TE<GB7YFS<
GB7CIP
Sent: 030714/1728Z @:GB7CIP.#32.GBR.EU #:5637 [Caterham] $:5637_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY
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 filtering is used.
Here a 3 terminal electret mic is used, 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 and
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
\________________/ / loop
---/
/
/
cable
Electrically the treble has been lifted by a 0.2uF tantalum cap on the electret
mic's internal FET transistor, source to ground. This simple treble lift is
only possible with a 3 terminal mic capsule. The normal +ve rail drain input
now becomes O/P as in a 2 terminal mic type. A shunt R of about 3.3K (select on
test) across the mic sets the AF O/P level, & a 10nF across the mic in the gear
lever switch to roll off the AF lift above 3kHz.
RIG MIC<------ PTT Switch --------------------------
screened lead ! ! screened lead !
! !10n _ 3 term
~3K3 === + / mic
! ! 0.2u=== !
! ! ! !
RIG GND<--------------------------------------------
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 ware, and 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.
+5-12V >-----------------------!
smooth DC R
- C + !
RIG MIC<--------!!--------------------------
RIG PTT<---------! after PTT
/ PTT SWITCH as above
!
RIG GND<------------------------------------
The C needs to be about 1uF (10n-100n if not treble lifting), & the R between
1K-33K select for the gain you need.
------------------------------------------------------------------------------
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!
/QSL
73 De John, G8MNY @ GB7CIP
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