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G8MNY > TECH 10.03.24 08:36l 86 Lines 3920 Bytes #999 (0) @ WW
BID : 2820_GB7CIP
Read: GUEST
Subj: Phantom Balanced Mic Preamp
Path: ON0AR<OZ5BBS<CX2SA<EA2RCF<I0OJJ<GB7CIP
Sent: 240307/1011Z @:GB7CIP.#32.GBR.EURO #:2820 [Caterham Surrey GBR] $:2820_GB
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)
旼컴컴컴컴컴컴컴컴컴컴컴 - 3 +48V Phantom powered
� typically @ +28V (e.g. +48V from 2x 10K)
쳐컴680R컴컴쩡컴컴컴컴컴 + 2
68R 68R Kit list for 2 preamps
� � u1 2x
旼2M2컴� 쳐2M2커 RFC 2x tiny rings & wire
Low Z � 쳐커 旼캑 � 2M2 4x (low noise?)
MIC u1 � T1�/ �1n 1n� \쿟2 � BC108 4x
IN컴캑쳐�())컨쩡컴� === === 쳐컴� XLR 1uF 2x
Jack RFC 쿍C �\e � � e/쿍C � 1n 6x
� �108 읕�)�150�)컴� 108� 150R 2x
� � � � �+6V �+ 680R 2x
� === � � 1K === 1u 68R 4x
� 1n� � � _�_ � 25V 1K 2x
� � � � \_/LED � Jack x4
읓컴컴컴컴컴컨컴컴컴컴좔컴컴좔컨컴컴컨컴컴� gnd 1 XLRs x4
_�_ 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.
_________________________________
/ ~~~~� XLR �~~~~ ~~~~� XLR �~~~~ \
� 쿌 amp� 쿍 amp� �
� 읓컫컫� 읓컫컫� �
� 68 680 68 68 680 68 � Once the holes are made for the
� 1n 1n 1n 1n � sockets, clearance holes, screws,
� T1a T2a 2M2 T1b T2b 2M2 � Jack sockets & LED, Then connections,
� 150 1u LED 150 1u � 4 caps & the LED, soldered to the
� 1n 1k 1n 1k � metal tin can ground, all give support
� RFC RFC � points for simple ugly construction.
� u1=|~~~| u1=|~~~| �
� _| A | _| B | �
\______|___|___________|___|______/
[___] [___]
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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