Akai GX-747 dbx Service Manual page 14

4-1-2 PRE-EMPHASIS CIRCUITS
(Refer to Figs. 6-11 and 6-12)
r---------
-7
I
RI I I0K C6 I00P
I
I
Rl2 33K
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, - - - - - - , I ______ __. I
I
C3 R5
I I
C7 I00P
I
I
0,01 IOK
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I - - - - - - - - I
I I
IC I }JPCl252H2
I I
FROM
R6 V
CA
H.P.F
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C4 I 33K
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1------~+--➔ TO
REC AMP
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L
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'---+------,►
TO 8, P.F
- - - - LPre Emphosis Circuit _ _ _
_.,,,..I
I
(dB)
10
5
0
., -5
>
~
-10
I I
L _ _ _ _ _ _ _ _ _ J
Fig. 6-11 Pre-emphasis Circuit
I
Pre
-emphasis
\
De-emphasis
-15 ' - - - - - - - - - - ~ - - ~
10 100
IK
I0K I00K (Hz)
Freq
Fig. 6-12 Pre-emphasis and De-emphasis
Characteristics
The primary role of this circuit is to play the same
role as the equalizer used with the record or other
recording media by pre-emphasis of the high frequency
band to reduce high band noise, and to reduce the
breathing phenomenon (Note 1 ).
During playback, the high frequency band is de-
emphasized to return the signal to the original state.
In Fig. 6-11, the time constant of the Low band is
determined in the first stage, and the time constant of
the high band and the emphasis value is determined
in the last stage. C3 and
C4
provide
AC
coupling
to prevent the DC offset of the previous stage from
being modulated by the change in the
VCA
gain.
The first stage also serves to convert the input voltage
to a current source, while the last stage converts the
current to a voltage output. C7 is an oscillation
preventing capacitor. (Refer to Fig. 6-11)
Note I. It is generally well-known that noise (hiss noise)
added by the tape recorder or tape deck has a
high content of high frequency band com-
ponents.
In the dbx system, compander gain changes
according to the level of the intermediate and
low frequency band components which · are the
main components in the input, but hiss noise
also varies according to the gain change. As a
result, hiss noise changes as the level of the
main signal component changes, and "bre8thing"
noise is heard. This is called the breathing
phenomenon.
Of course intermediate and low frequency band
noise changes similarly, but since in term .ediate/
low band signal components are stron. g, this
noise is masked by these strong signals, a.11d it is
not sensed by the ears. Hiss noise consisting of
high frequency band components is not oiasked
by the signal components and is therefore easily
detected by ear.
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