Akai GXC-750

1.
LINE IN
OPERATING PRINCPALS OF A.D.R.
(AUTOMATIC DISTORTION
* REDUCTION) SYSTEM
(Refer to Figs. 51 and 52)
1) Refer to Fig. 52
The combined impedance of the B.P.F (Band
Pass Filter) comprised of Diodes D3 through D8
and L2 and C24 suitable for TR8 feedback circuit
is represented by Zo. Zo is comprised of B.P.F.
impedance Z, which is changed by frequency, and
Diode impedance Z, which is changed by the
size of the signal level.
2) Refer to Fig. 53
B.P.F. Z,; impedance resonates in the vicinity of
peaking frequency fo of the recording equalizer
characteristics, and at resonance time, Z, im-
pedance becomes minimum.
3) Refer to Fig. 52
The frequency band is wide at the line input and
when a uniform level signal is introduced, it is
amplified at Amp. | and added to the Dolby Amp.
input. This dolbyized signal then passes TR8 and
is supplied to the recording head. At this time,
when comparing the high range frequency with
the low range frequency of the signal current
flowing to the recording head, the high range
current volume is greater.
4) Refer to Fig. 54
Therefore, in making a high level recording, the
recording equalization will relatively increase and
in the vicinity of fo frequency, the recording
equalizer will reach it’s peak and exceed the
saturation level of the magnetic tape.
However, at this time, TR8 output passes the
A.D.R. element which is comprised of D3 through
D8, and the fo vicinity frequency only is allowed
to pass the B.P.F. and is supplied to point (A)
in Fig. 52.
5) Because the signal to TR8 base and the feedback
signal are reverse phased in relation to each
other, TR8 gain is attenuated and the recording
head electric current is controlled.
Fig. 55 O.L.S. Block Diagram
6) Because the B.P.F. works in relation to fo vicinity,
the current supplied to the recording head is
controlled and at recording time the equalization
level declines. For this reason, the output is
below the saturation level and an undistorted
signal is recorded on the magnetic tape.
2.0.L.8. (OVER LEVEL SUPPRESSOR)
CIRCUIT OPERATION
(Refer to Fig. 55)
1) When a signal is introduced of which the input
level is over ‘0’? VU and the O.L.S. Switch is at
OFF position, the saturation level of the tape is
exceeded and distortion is recorded.
2) Therefore, it is necessary to lower the signal level
to “O” VU in order to record at an undistorted
level and this is accomplished with the O.LS.
circuit. When the O.L.S. Switch is set to ON
position, at “0” VU signal time, current does not
flow to Diode D1! connected to the output side of
transistor TR6. However, when the signal is over
“0” VU current flows to D1 and condenser C22
is charged. Because C22 becomes charged, D.C.
bias is supplied to transistor TR7.
3) When D.C. bias is supplied to TR7, this causes
emitter current to flow and this current is supplied
to the base of TR5 as D.C. bias.
4) At this point, the resistance value between TR5
emitter and collector is changed by the D.C. bias
to TR5. Therefore TR5 is employed as a variable
resistor.
5) Thus, when an input signal of over “0” VU is
introduced, TR5 variable resistor automatically
changes the signal depending upon signal level,
and by means of TR3 input signal control, the
signal level is lowered to “O” VU and recorded
on the tape.
32