Akai GX-747 dbx Service Manual page 17

4-1-7 RMS (ROOT MEAN SQUARE) LEVEL SENSOR
(Figs. 6-21 and 6-22)
R22 22M
,-....,1--<1,__ __
+BC+ 11.6V l
....._ _ _ _ _ _ _ _ _ _ _ _ _ _ _ -BC-ll.6Vl
Fig. 6-21 RMS Level Sensor Circuit
Vee
SQUARELAW
AMPLIFIER
) - - - - - 1 6 ) - - - - - - - - - '
Vee
Is CAP.
Fig. 6-22 IC µPC1253H2 Block Diagram
The RMS level sensor circuit detects the effective
value (RMS) of input signal current, and converts it
to a DC voltage proportional to the logarithm of the
detected level.
IC2 and IC5 (µPC1253H2) are for the dbx RMS level
sensor circuit.
Fig. 6-21 shows the RMS level sensor circuit. For
pre-setting the control signal (OV at zero point level)
for the input level, zero point level control is made
possible by varying the operating current of IC by
means of YRS. (Zero point level=410 mV=-5.5 dB
ref lmW 600 ohms (0.774 volts))
The attack time is 15 ms when there is a IO dB level
change (increase), 5 ms when there is a 20 dB change,
and 3 ms when there is a 30 dB change. The decay
rate (Note 2) is 125 dB/sec as standard. Time
constant is determined by the current (11) flowing
through R26, and by Cl 7.
In the GX-747 dbx, since IT=7.5
µA,
in the equation
for obtaining the decay rate of 125 dB/sec, C=IOµF
from IT=0.75 C.
Cl8 is a by-pass capacitor, and the charging iurrent of
Cl7 flows through Cl8. Cl6 is a capacitorprovided
to obtain stability and low distortion chanc1:eristics
in the high frequency band. R25 serves as ale'-'el shift
for bringing the control value to the center. VR2 is
a volume control for symmetry adjustment.
R22 prevents any ''thump" sound when tle~e is no
signal.
Note 2. Slope of the output voltage change a.Jter an
abrupt large drop of the input signal
lere
1.
Note 3. GND
is
used for common, othenise mis-
operation may result due to the larl,'! current
flowing during the recovery time.
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