Akai GX-747 dbx Service Manual

4-1-3 DE-EMPHASIS CIRCUIT (Figs. 6-12 and 6-13)
In contrast to the pre-emphasis circuit which pre-
emphasizes the high frequency band during recording,
this circuit de-emphasizes the high frequency band
during playback to restore the
signal to its original
4-14 VCA (Voltage Controlled Amplifier (or At-
tenuator)) CIRCUIT (Refer to Figs. 6-14,6-15,6-16)
.--------- + B ( +11,6 VJ
'---+-------+--- -B (-11,SV)
From R,M.S Sencor
Fig. 6-14 Encoder
GND I set
Vee
GAIN
CELL
IDLING
CURRENT SUPPLY
GC I
I out
GC 3
Fig. 6-16 IC µPC1252H2 Internal Block Diagram
ICl and IC4 (µPC1252H2) are dbx VCA circuit ICs.
Pin (D is a virtual ground, and accepts only an input
signal current. Pin @ is a high impedance current
output.
Fig. 6-13 De-emphasis Circuit
state. The circuit configuration is the same as the
pre-emphasis circuit, but
has some differences in
component values.
INPUT
.--------,;. +9 (+11,SV)
1-----<;; OUTPUT
L......---1-------B (-11.SV)
From R.M.S Sencor
Fig. 6-1 S Decoder
Pin ~ is connected to negative power supply through
the current setting internal bias circuit. The set
current is nominally 2 mA. The range is co:rnpressed
in the encode mode when the RMS sens~r cutput is
positive. In the decode mode, since the RN.IS sensor
output
is fed to Pin a) with Pin @ beini GND, the
range is expanded when the RMS sensor output is
positive, and it is compressed when the MS sensor
output
is negative, which is just the opposi 1e action
to the encode mode.
The circuit consisting of R9(R37),
Rl0[R.38), and
VRl (VR3) can make symmetrical adjusiroent pos-
sible and minimize DC offset voltage -..Nhich is
multiplied by the circuit gain, and controlth.e second
order harmonic distortion. When a fast gain 4;:hange is
commanded, DC offset appears at the Oiltj)Ut. R8
(R36) are provided to reduce this. In tie encoder
(Fig. 6-14), it acts as a logarithmic amplifier, while
it acts
as an anti-logarithmic amplifier in ie decoder
(Fig. 6-15). Fig. 6-16
shows IC µPC125lff2 block
diagram.
---------------- SERVICE MANUAL GX-747dbx ----------------
14

Akai GX-747 dbx Specifications

General

Track System	4-track, 2-channel stereo
Reel Size	up to 10.5 inch reel
Type	Reel to Reel
Heads	2 x erase
Motor	1 x capstan, 2 x reel
Tape Speed	9.5 cm/s, 19 cm/s
Wow and Flutter	0.03% (19 cm/s)
Signal to Noise Ratio	65dB
Input	1 x line, 1 x microphone
Output	1 x line
Dimensions	440 x 480 x 260 mm