Explanation Of Each Operating Principles - Akai GX-747 dbx Service Manual

4. EXPLANATION OF EACH OPERATING PRINCIPLES
Amplifier Block Diagrams
MIC IN
ov-{>-!•---"-"------'
TAPE MONITOR
SWITCH SOURCE
f
LINE
OUT
LINE O U T ( Q t - - - - - - - - - . -
HEAD PHO!£ AMP
METER AMP
VU METER
TAPE
.
L __
4-1 dbx NOISE REDUCTION SYSTEM CIRCUIT
=v
ERASE HEAD
~~
REC HEAD
dBx NR SWITCH
RB AMP
PB HEAD
Fig. 6-8 Amplifier Block Diagram
+B (+I l,6V)
RI Cl
The main circuits composing this system provide the
pre-emphasis, de-emphasis, VCA (Voltage Controlled
Attenuator or Amplifier), RMS (Root Mean Square)
action, level sensor, etc. (Refer to Fig. 6-8). Each
circuit
will
be described below:
N
~
TRI
4-1-1 INPUT FILTER (HPF)
This filter, which is a high-pass filter (HPF) of about
20 Hz cutoff (fcut) frequency and -12 dB/oct slope,
is formed by a positive feedback active filter. Here,
the filter is made into an active filter using a
transistor to improve the shoulder characteristic of
the filter and to act as a buffer to eliminate the
effect of following circuits due to their impedance
characteristics. (Refer to Figs. 6-9 and 6-10)
..,.
er
.--->---o
OUT
-B
H
l,6Vl
Fig. 6-9 INPUT Filter Circuit
[ 12d8/oct
g
Yf
~
I
I
fcut::; 20Hz
Fig. 6-10 Input Filter Charactelis tic
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