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G8MNY  > TECH     14.06.19 10:15l 138 Lines 5597 Bytes #6 (0) @ WW
BID : 8255_GB7CIP
Read: GUEST
Subj: Mic DJ Preamp
Path: ED1ZAC<ED1ZAC<IZ3LSV<F1OYP<ON0AR<GB7CIP
Sent: 190614/0810Z @:GB7CIP.#32.GBR.EURO #:8255 [Caterham Surrey GBR] $:8255_GB
From: G8MNY@GB7CIP.#32.GBR.EURO
To  : TECH@WW

By G8MNY                                    (Updated Dec 12)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

Here is a preamp from Mic to high line level with gain control, designed for an
unbalanced close in Mic, with top lift & bass cut controls. As it is for close
in Mic so very low noise it not needed, but a good bass roll off is. The bass
cut reduced the woolly sound of close in Mics & the extra treble boost
available also improves clarity if needed.
  __
 / o\CLOSE
³   =~()MIC
 \ ÚÙ
 ³  \                ()DISTANT MIC
 ³   ³bass
 ³   ³out of
 ³   ³phase
               OTHER
             BACKGROUND
               NOISES

A close in Mic right in front of the mouth hears the loudest voice from the
mouth & nose, hence other background noises (including PA feedback) are greatly
reduced by the simple square law. E.g. voice is 6dB quieter for each doubling
in distance.

The distant Mic hears both the noise & voice, but also the large amount of out
of phase "bass" off of the speaker chest, which is how you generally hear a
voice. So a close in Mic sounds very unnaturally bassy, even "woolly".

Cutting the bass, is the method usually used to gain all the advantages of the
close in Mic without it sounding "woolly". But to make it sound really natural
adjustable bass cut & treble lift (& sometimes middle) are needed depending on
the voice characteristics, Mic proximity, & even ones body shape.

Lip Mics as used by broadcast commentators in noisy locations, is an extreme
example of these principles in use, where the mic is designed to be at a fixed
location to the face by a lip bar, & the nose shield helps reduce nose bass.
The moving coil insert also has built in bass cut using a stiff diaphram.

PREAMP SPECIFICATION
   GAIN.....    5 to 50 times. 14 to 34dB
   TREBLE LIFT  @ 15kHz  0dB to +15dB
   BASS CUT     @ 100Hz -3dB to -15dB
   INPUT Z      5kê suitable for any low Z mic.
   NOISE...     Very low for 1/2 of a simple low noise op amp.
   POWERING     +8V to +36V DC powering @ 3 - 5mA (C6 voltage!)
   RFI          OK for reasonable levels, e.g. Cellphones/VHF/HF radio.

CIRCUIT
           ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄ470Ä<+5to30V
     C1    R1              L1 FB          ³              C6³+    R6
    u47   10k               - -        3³\³8  half        ===100u
  ÚÄÄ´ÃÄÄÄÂÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ(((()ÄÄÂÄÄÄÄÄ´+`\ LF353   +10u  ³
  ³ Polly ³    C2 10u  R4         ³     ³IC1`>ÄÄÄÄÄÂÄÄÄ´ÃÄÄ)Ä68ÄÂÄÄÄ>
  ³       ³     Ú´ÃÄÄÄÄ1kÄÄÄÂÄÄÄÄÄ)ÄÄÂÄÄ´- /' 1    ³   C7  ³ R7 ³   Output
  ³      10k    ³   +    u1===    ³  ³ 2³/³4       ³       ³    ³
50ê-1k    R2    ÃÄÄÄ¿    C4 ³     ³  ³    ³       GAIN     ³    ³
 MIC      ³     ³ BASS     68     ³  ÀÄÄÄÄ)Ä4k7ÄÂÄ>50k     ³    ³
  ³       ³     ³  CUT<Ä¿  R3    ===n1    ³  R5 ³  ³       ³    ³
  ³       ³     ³   5k  ³   ³     ³ C5    ³     ÀÄÄÙ       ³   R8
  ³       ³ u47 ³       ³  TREBLE ³       ³                ³  100k
  ³       ³ C3 ===      ÃÄ>LIFT   ³       ³                ³    ³
  ³       ³     ³       ³   5k    ³       ³                ³    ³
  ÀÄÄÄÄÄÄÄÁÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÁÄÄÄ>0V

WHAT DOES WHAT
R6 & C6 decouples the supply, & makes sure it is quiet to feed into R1.
C1 is a low noise input capacitor, just big enough to let though 100Hz @ -3dB.
   for flatter low bass use C1 = 5uF.
R1 + R2 Bias the amp to 1/2 power rail & determine the input Z.
L1 near IC is 10 turns on a ferrite bead, & with C5 forms a RF low pass filter.
IC1 is low noise, only 1/2 used here, making pins 3 & 4 ideal for C5 mounting.
R5 + 50k Gain with R4, set the gain 5 to 50. (50 is max if U want treble lift)
C2 & R4 give minimum bass roll off. (as did C1). For flatter bass use C2 = 100u
C3 & Bass Cut control give up to -6dB/O additional bass cut.
C4 & R3 + Treble Lift Control, give up to +6dB/O treble boost.
C7 DC decouples output.
R7 sets the output Z to 68ê, & gives some static & RF protection to the IC.
R8 discharges C7 if needed.

TONE CONTROL GRAPHS (measured)
 dBs
+18¿                                    ,ú'  MAX
+15´                         Slope    ,:.
+12´                        +6dB/O ,ú'   :  TREBLE
 +9´                            ,ú'  Gain :  LIFT
 +6´                         ,ú'     @ Max :
 +3´                      ,ú'
  0´               ___...------------------- MIN
 -3´     MIN_.--'"~ ,ú'
 -6´     .-"     ,ú'
 -9´   /"BASS ,ú' Slope
-12´  |  CUTú'   -6dB/O
-15´ |   ú'
-18´|  :'MAX
   ÀÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄ>Freq
    20  50  100 200 500 1k  2k  5k  10k 20k Hz


BALANCED PHANTOM POWERED
By using the 2nd opamp in the IC a balanced output (not floating) can be made
to be power from +48V phantom mixer feed.

ÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÂÄÄÂÄÄÄ¿
      ³        _³_    C6³+ ³   ³
      ³        /_\'15V === R6  ³
   3³\³8  half  ÃÄÄÄÄÄÄÄÙ 10k  ³
ÄÄÄÄ´+`\ LF353 ÄÁÄ        0.5W ³
    ³IC1`>ÄÄÄÄÂÄÄÄÂÄ´ÃÄÄÄÄÄÁÄÄÄ)ÄÄ68ÄÄÄ>
ÄÂÄÄ´- /' 1   ³   ³ C7+        ³  R7   Output+
 ³ 2³/³4      ³   ³ 10u        ³
 ³    ³      GAIN ³            ³
 ÀÄÄÄÄ)Ä4k7Ä>50k  ³            ³
     _³_ R5       ³            ³
                  ³            ³
 Ú100kÂÄÄÄÄÄÄÄÄÄÄÄÙ           10k
 ³   10k                      0.5W
 ³    ³                        ³
 ³    ³5³\   half              ³
 ÃÄ1kÄ)Ä´+`\ LF353  10u        ³
 ³    ³ ³IC1`>ÄÄÄÄÂÄ´ÃÄÄÄÄÄÄÄÄÄÁÄÄ68ÄÄÄ>Output-
 ³+   ÃÄ´- /' 7   ³   +
===   ³6³/'       ³
 ³10u ÀÄÄÄÄÄ10kÄÄÄÙ
 ³
ÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ>0V


See my bul on "Hands Free Mobile Mic".


Why Don't U send an interesting Bul?

73 De John, G8MNY @ GB7CIP


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