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)
ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ - 3 +48V Phantom powered
³ typically @ +28V (e.g. +48V from 2x 10K)
ÃÄÄÄ680RÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄ + 2
68R 68R Kit list for 2 preamps
³ ³ u1 2x
ÚÄ2M2ÄÄ´ ÃÄ2M2Ä¿ RFC 2x tiny rings & wire Low Z ³ ÃÄÄ¿ ÚÄÄ´ ³ 2M2 4x (low noise?)
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 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 @ 180ø. 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.
_________________________________
/ ~~~~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


-----------------------------------------
ARGEN-X BBS - IPV6 TELNET LU9DCE.DYNU.COM -----------------------------------------


--- Mystic BBS v1.12 A48 (Linux/64)
* Origin: ARGEN-X BBS (4:902/6)