Message 26109 in TECH.WW
From: G8MNY
Date: Mon, 20 Jul 26 06:40:00 Z
Newsgroups: TECH.WW
Subject: Phantom Balanced Mic Preamp
Message-ID: <65340_GB7CIP>
Path: N2NOV|N2MH|N2MH4|GB7BED|GB7COW|GB7CIP
R:260720/0653z @:N2NOV.#RICH.NY.USA.NOAM $:65340_GB7CIP
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R:260720/0652Z 34002@N2MH4.#SENC.NC.USA.NOAM LinBPQ6.0.24
R:260720/0651Z 14566@GB7BED.#21.GBR.EU LinBPQ6.0.25
R:260720/0648Z @:GB7COW.#44.GBR.EU #:44931 [Hartland] $:65340_GB7CIP
R:260720/0640Z @:GB7CIP.#32.GBR.EURO #:65340 [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)
68R 68R Kit list for 2 preamps
/// 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, and 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) and 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
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 and room for XLRs fitted internally.
_________________________________
\______|___|___________|___|______/
[___] [___]
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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