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G8MNY > TECH 18.08.20 09:51l 84 Lines 3795 Bytes #999 (0) @ WW
BID : 1779_GB7CIP
Read: GUEST
Subj: Phantom Balanced Mic Preamp
Path: IZ3LSV<IR1UAW<F1OYP<ON0AR<OZ5BBS<CX2SA<N9PMO<JE7YGF<GB7CIP
Sent: 200818/0843Z @:GB7CIP.#32.GBR.EURO #:1779 [Caterham Surrey GBR] $:1779_GB
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Jan 19)
(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 phantom powered 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 +48V Phantom powered
³ typically @ +28V (e.g. +48V from 2x 10K)
ÃÄÄÄÄ680ÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄ + 2
68 68 Kit list for 2 preamps
ÚÄ2M2ÄÄ´ ÃÄ2M2Ä¿ RFC 4x tiny rings & wire
Low Z ³ ÃÄÄ¿ ÚÄÄ´ ³ 2M2 4x
MIC u1 ³ T1³/ ³1n 1n³ \³T2 ³ BC108 4x
INÄÄÄ´ÃÄÄ())ÄÁÂÄÄÄ´ === === ÃÄÄÄ´ XLR 1uF 2x
Jack RFC ³BC ³\e ³ ³ e/³BC ³ 1n 6x
³ ³108 ÀÄÄ)Ä150Ä)ÄÄ´ 108³ 150R 2x
³ ³ ³ ³ ³+6V ³+ 680R 2x
³ === ³ ³ 1K === 1u 68R 4x
³ 1n³ ³ ³ _³_ ³ 25V 1K 2x
³ ³ ³ ³ \_/LED ³ Jack x4
ÀÂÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÁÄÄÁÄÄÄÄÄÁÄÄÄÄÄ gnd 1 XLRs x4
_³_ Jack x4
/// LED x1
Tin x1
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.
LAYOUT
Construction of the stereo prototype was in a standard old tobacco tin, which
gives a easy to solder to sheilded box & room for XLRs fitted internally.
_________________________________
/ ~~~~³ XLR ³~~~~ ~~~~³ XLR ³~~~~ \
³ ³A amp³ ³B amp³ ³
³ ÀÂÄÂÄÂÙ ÀÂÄÂÄÂÙ ³
³ 68 680 68 68 680 68 ³ Once the holes are made for the
³ 1n 1n 1n 1n ³ sockets, clearance holes, screws,
³ T1a T2a 2M2 T1b T2b 2M2 ³ Jack sockets & LED, Then connections,
³ 150 1u LED 150 1u ³ 4 caps & the LED, soldered to the
³ 1n 1k 1n 1k ³ metal tin can ground, all give support
³ RFC RFC ³ points for simple ugly construction.
³ u1=|~~~| u1=|~~~| ³
³ _| A | _| B | ³
\______|___|___________|___|______/
[___] [___]
1/4" Jack 1/4" Jack
Also see tech buls "Ultra Low Noise Mic Pre-amp", "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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