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G8MNY > TECH 01.09.06 07:37l 78 Lines 3346 Bytes #999 (0) @ WW
BID : 44462_GB7CIP
Read: DL1LCA DL1RX GUEST OE7FMI
Subj: Hands Free Mobile Mic
Path: DB0FHN<DB0MRW<DB0PV<DB0FSG<DB0RGB<OK0PPL<OK0NAG<LZ4NY<DB0RES<ON0BEL<
GB7CIP
Sent: 060901/0023Z @:GB7CIP.#32.GBR.EU #:44462 [Caterham] $:44462_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (new Graphics Sep 03)
(8 Bit ASCII Graphics use code page 437 or 850)
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<ÄÄÄÄÄ PTT Switch ÄÂÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
screened lead ³ ³ screened lead ³+
³ ³10n _ 3term
÷3k3 === + / mic
³ ³ 0.2u=== ³case
³ ³ ³ ³
RIG GND<ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÙ
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 >ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
smooth DC R
- C + ³
RIG MIC<ÄÄÄÄÄ´ÃÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄ
RIG PTT<ÄÄÄÄÄÄ¿ after PTT
/ PTT SWITCH as above
³
RIG GND<ÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ
The C needs to be about 1uF (10-100nF 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!
Why Don't U send an Interesting Bul?
73 De John, G8MNY @ GB7CIP
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