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G8MNY > TECH 07.10.25 15:51l 86 Lines 3920 Bytes #4 (0) @ WW
BID : 42288_GB7CIP
Subj: Phantom Balanced Mic Preamp
Path: JH4XSY<N3HYM<K5DAT<GB7BED<GB7JED<GB7CIP
Sent: 251007/0645Z @:GB7CIP.#32.GBR.EURO #:42288 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Nov 20)
(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 600 Ohm
Input Z 15K
Gain 10dB (3x)
ﾚﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄ - 3 +48V Phantom powered
ｳ typically @ +28V (e.g. +48V from 2x 10K)
ﾃﾄﾄﾄ680Rﾄﾄﾄﾄﾂﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄ + 2
68R 68R Kit list for 2 preamps
ｳ ｳ u1 2x
ﾚﾄ2M2ﾄﾄｴ ﾃﾄ2M2ﾄｿ RFC 2x tiny rings & wire
Low Z ｳ ﾃﾄﾄｿ ﾚﾄﾄｴ ｳ 2M2 4x (low noise?)
MIC u1 ｳ T1ｳ/ ｳ1n 1nｳ \ｳT2 ｳ BC108 4x
INﾄﾄﾄｴﾃﾄﾄ())ﾄﾁﾂﾄﾄﾄｴ === === ﾃﾄﾄﾄｴ XLR 1uF 2x
Jack RFC ｳBC ｳ\e ｳ ｳ e/ｳBC ｳ 1n 6x
ｳ ｳ108 ﾀﾄﾄ)ﾄ150ﾄ)ﾄﾄｴ 108ｳ 150R 2x
ｳ ｳ ｳ ｳ ｳ+6V ｳ+ 680R 2x
ｳ === ｳ ｳ 1K === 1u 68R 4x
ｳ 1nｳ ｳ ｳ _ｳ_ ｳ 25V 1K 2x
ｳ ｳ ｳ ｳ \_/LED ｳ Jack x4
ﾀﾂﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾁﾄﾄﾄﾄﾄﾄﾄﾄﾁﾄﾄﾄﾄﾄﾁﾄﾄﾁﾄﾄﾄﾄﾄﾁﾄﾄﾄﾄﾄ gnd 1 XLRs x4
_ｳ_ 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
collector 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 600 Ohm to drive screened
balanced cable well, it also gives it effectively a good floating output,
almost like a transformer, but without the quality issues. (e.g. AC grounding
either output, 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) & the 1nF cap. The amp output leads are also RF decoupled by
2 series 68R & 2 1nF caps on the collectors.
The LF response is tailored by the input u1 cap, the 1uF cap goes down to 1/10
of the frequency, keeping the LF output phases difference @ 180�. The 1uF also
ensures there is very little signal lost (unbalancing) across the current
sharing 1K.
For a stereo preamp 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.
_________________________________
/ ~~~~ｳ XLR ｳ~~~~ ~~~~ｳ XLR ｳ~~~~ \
ｳ ｳA ampｳ ｳB ampｳ ｳ
ｳ ﾀﾂﾄﾂﾄﾂﾙ ﾀﾂﾄﾂﾄﾂﾙ ｳ
ｳ 68 680 68 68 680 68 ｳ Once the holes are made for the
ｳ 1n 1n 1n 1n ｳ sockets, clearance holes, screws,
ｳ T1a T2a 2M2 T1b T2b 2M2 ｳ Jack sockets & LED, Then connections,
ｳ 150 1u LED 150 1u ｳ 4 caps & the LED, soldered to the
ｳ 1n 1k 1n 1k ｳ metal tin can ground, all give support
ｳ RFC RFC ｳ points for simple ugly construction.
ｳ u1=|~~~| u1=|~~~| ｳ
ｳ _| A | _| B | ｳ
\______|___|___________|___|______/
[___] [___]
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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