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G8MNY > TECH 16.07.10 08:10l 113 Lines 4740 Bytes #999 (0) @ WW
BID : 58172_GB7CIP
Read: GUEST
Subj: Hands Free Mobile Mic
Path: IZ3LSV<IK2XDE<DB0RES<DB0EEO<DB0GOS<ON0AR<UA6ADV<GB7CIP
Sent: 100715/2206Z @:GB7CIP.#32.GBR.EU #:58172 [Caterham] $:58172_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (Updated Jan 09)
(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
Level
+10dB´ _..-----
³ Woolly _.-~
0dB´LOUD CAR NOISE Voice_.-~ Clear
³ _.-~ Treble
-10dB´_____________..--~
ÅÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÂÄÄ>Frequency
30 100 300 1k 3k 10k Hz
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 &
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
~~~~~Stalk~~~~~~~(':::'-'
|
\
cable
STANDARD 3 TERM MIC
Drain
+3-5V ÄÄÄÄÄÄÄÄÄÄÄ´ ³|
Internal FET ³>ÄÄÄÄÄÄ´|Electret
Output ÄÄÄÄÄÄÄÄÂÄÄ´ Gate ³|Pressure
1k Source | Mic
Ground ÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÙ
Case
Electrically the treble has been lifted by a 0.2uF tantalum cap on the electret
mics' 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 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 that 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 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.
+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.
-----------------------------------------------------------------------------
Here is another approach that works with 2 terminal electret mics if you have
enough rig mic gain...
15nF
.-~~-. 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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