Message 26095 in TECH.WW
From: G8MNY
Date: Sun, 19 Jul 26 08:07:00 Z
Newsgroups: TECH.WW
Subject: Phantom Bal Mic Preamp+Gun Mic
Message-ID: <65272_GB7CIP>
Path: N2NOV|N2MH|N2MH4|GB7BED|EI2GYB|GB7CIP
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From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Jul 18)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
Any 8 dil dual opamp e.g. CA3240, LM358N, MC1458C, NE5532N, NJM4558,
OPA2604AP, TL072, TL082, etc,
Gain of each phase = R7orR8/10K
Typical values R7=R8= 100K, gives 10x gain/phase, x20 overall gain (26dB).
The polarity of the C2 in series with the electret mic depends on the DC bias
level needed for the perticular mic capsule. So test the relative DC levels
before inserting capacitor.
The value of R9 & R10 can be changed to better suit the phatom powering voltage
e.g. 12V use 4K7, 48V use 22K.
LF tayloring can be incorperated if C2 in series with the mic can be reduced to
about 0.1uF. Also the 2 O/P Cs (C5,C6) are reduced, values depend on external
load.
WHAT DOES WHAT.
R9&R10 with C4 decouple the DC phantom IC supply from out going line AF.
R2 with C1 decouple the DC supply for the mic.
R1 supplies DC current to the mic.
C2 passes mic AF into R5 load, & hence determins LF responce.
R5, R8 & R6, R7 provide the 2 opamp voltage Negative Feed Back, setting gain.
C5 & C6 pass O/P AF into the line.
- ve PHANTOM POWER
If you phanton supply is -ve voltage, just re-connect R9 & R10 from C4 +ve to
the bottom rail & rewire the earth to the +ve of C4. Reverse the polarity of C5
& C6.
SIMPLIER PHANTOM POWERED CIRCUIT!
With this simple circuit DC is only taken from the live phantom. The capules is
insulated from its hum avoiding earthed surround shield. The 47K live to ground
is a balancing R to ensure equal +/- signals.
MAKING A GUN MIC
If the Mic capsule is inserted tightly (sealed) into a long 30mm dia tube that
has holes or slots made down it's length, a very directional gun mic is made,
(over the frequency range where the sound waves are columnmated in the tube).
: : : : : : : : : : : : : : : : : : : sponge windshield
: : -- --- --- --- --- --- --- --- ----
: : [] mic capsule
: : -- --- --- --- --- --- --- --- ---- slotted pipe
: : : : : : : : : : : : : : : : : : :
See also "Phantom Balanced Mic Preamp" & "DC sources for coax/AF" bul.
Why don't U send an interesting Bul?
73 de John G8MNY @ GB7CIP
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