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G8MNY > TECHNI 30.12.18 10:20l 78 Lines 3054 Bytes #999 (0) @ WW
BID : 51703_GB7CIP
Read: GUEST
Subj: Phantom Balanced Mic Preamp
Path: IW8PGT<IZ3LSV<IK6ZDE<VE2PKT<OK2PEN<GB7CIP
Sent: 181230/0915Z @:GB7CIP.#32.GBR.EURO #:51703 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (New 31/12/18)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
Here is my design for a low noise mic amp. It makes unbalanced mics that are
unsuitable for long leads, as good as any balanced mic.
SPECIFICATION
Freq Response 20Hz-100kHz @-3dB
Reasonably RF immune!
Balanced Floating output 0dBm Max!
Output Z 600ê
Input Z 15K
Gain 10dB (3x)
ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ - 3
³ typically @ +28V +48V Phantom powered
ÃÄÄÄÄ680ÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄ + 2 (e.g. +48V from 2x 10K)
68 68
ÚÄ2M2ÄÄ´ ÃÄ2M2Ä¿
Low Z ³ ÃÄÄ¿ ÚÄÄ´ ³
MIC u1 ³ ³/ ³1n 1n³ \³ ³
INÄÄÄÄ())Ä´ÃÄÁÂÄÄÄ´ === === ÃÄÄÄ´ XLR
Jack RFC ³BC ³\e ³ ³ e/³BC ³
³ ³108 ÀÄÄ)Ä150Ä)ÄÄ´ 108³
³ ³ ³ ³ ³+6V ³+
³ === ³ ³ 1K === 1u
³ 1n³ ³ ³ _³_ ³ 25V
³ ³ ³ ³ \_/LED ³
ÀÂÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÁÄÄÁÄÄÄÄÄÁÄÄÄÄÄ gnd 1
_³_
///
HOW IT WORKS
Based on a simple long tail pair amp, but with the balanced output heavily
shunted. The gain is determined by the series emitter 150R, compared to the
shunt 680R, & it is enough to mask cable noise, but not overload mixer mic
inputs.
The shunting brings down the high output Z to about 600ê to drive screened
balanced cable well, & also gives it effectively a floating output, almost like
a transformer, but without the quality issues. (e.g. AC grounding either output
phase, does not alter the balanced output difference level!)
The amp input is RF filtered by the RFC (>6 turns of fine enamelled wire on a
tiny ferrite bead) & a 1nF cap. The amp output leads are also RF decoupled by
2 series 68R & 2 1nF caps.
The LF response is tailored by the input u1 cap, the 1uF cap goes to 1/10 the
frequency, keeping the LF output phases @ 180ø. The 1uF also ensures there is
very little signal lost (unbalancing) across the current sharing 1K.
For a stereo amp the LED can be shared with a 2nd channel amp too, without any
problems.
Construction of the stereo prototype was in a standard old tobacco tin.
_________________________________
/ ~~~~³ XLR ³~~~~ ~~~~³ XLR ³~~~~ \
³ ³ ³ ³ ³ ³
³ ÀÂÄÂÄÂÙ ÀÂÄÂÄÂÙ ³ Once the holes are made for the
³ ³ XLR sockets, & Jacks, the connections,
³ Left Right ³ 4 caps & the LED soldered to the
³ Amp LED Amp ³ metal tin can ground, give support
³ ³ points for simple ugly construction.
³ =|~~~| =|~~~| ³
³ _| | _| | ³
\______|___|___________|___|______/
1/4" Jack 1/4" Jack
Also see tech buls "Phantom Bal Mic Preamp+Gun Mic" & "DC sources for coax/AF".
Why Don't U send an interesting Bul?
73 De John, G8MNY @ GB7CIP
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