Message 26068 in TECH.WW
From: G8MNY
Date: Sat, 18 Jul 26 08:39:00 Z
Newsgroups: TECH.WW
Subject: Mic DJ Preamp
Message-ID: <65217_GB7CIP>
Path: N2MH4|DK0WUE|PI8ZTM|CX2SA|ED1ZAC|GB7CIP
R:260722/0241Z 33862@N2MH4.#SENC.NC.USA.NOAM LinBPQ6.0.24
R:260718/0843Z 32418@DK0WUE.#BAY.DEU.EU LinBPQ6.0.25
R:260718/0842Z @:PI8ZTM.#ZH1.NLD.EURO #:56888 [Den Haag] $:65217_GB7CIP
R:260718/0842Z @:CX2SA.SAL.URY.SOAM #:1976 [Salto] FBB7.00e $:65217_GB7CIP
R:260718/0840Z @:ED1ZAC.EAC.ESP.EU #:20531 [EANET1G.06] $:65217_GB7CIP
R:260718/0839Z @:GB7CIP.#32.GBR.EURO #:65217 [Caterham Surrey GBR]
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
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 the speaker's 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, clothing & 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 an upper lip bar, and 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
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
WHAT DOES WHAT
R6 & C6 decouples the supply, and makes sure it is quiet to feed into R1.
C1 is a low noise input capacitor, just big enough to let through 100Hz @ -3dB.
for flatter low bass use C1 = 5uF.
R1 + R2 Bias the amp to 1/2 power rail and 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 68R, and gives some static and RF protection to the IC.
R8 discharges C7 if needed.
TONE CONTROL GRAPHS (measured)
dBs
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.
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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