PBL 388 13
IL
+ Line
+
20
Speech IC
C +
19
18
2V DC-
supply
1
audio input
23
16
V
c
from Rx
channel
T
X
17
22
2,6V Level shift
-
+
+
-
IE
2
IC
IB
21
V
s
Z ≈ 0
PBL 388 13
R
VRE
VR
RE
IR
- Line
VR
x
R
RE
V
RE = VR;
= I
;
I
I L= I R
x R;
B + I C
VR
x
= I R
x
x
C < 50 IE = 50
= 50
IR
R E
= 0 then IC = 0
VS
Figure 23. Loudspeaker amplifier current supply system.
a). To control the loudspeaker volume
with a DC- voltage from a potentiometer.
Someoptionalfeaturesusing
the dc. set volume control on
the loudspeaker amplifierof
PBL 388 13.
To volume
control
b). To control the loudspeaker volume
with a digital signal ( for ex. 8 - levels ).
19
9
PBL
Resistor that is added
and which determines
the dynamics of the AGC
The DC set volume control has an
wholly internal function to lower the gain at
low supply voltages. This is to avoid that
the power stage dies and causes breaks in
the output signal at long line lengths ie. low
currents in combination with high input
signals. This DC controlled volume is
externally accessible in the PBL 388 13
and can thus be utilized in several ways.
388 13
c). An AGC can be combined with the
volume control by connecting a resistor
from the DC - control pin 19 to the output of
the receiver detector at pin 9. Care has to
betakennottodisturbthespeechswitching
balance. If the resistor is made too low
ohmic the same value has to be applied on
the transmitter detector output at pin 6 as
well as that the capacitors at the detector
outputs have to be made bigger.
c).
To volum
control
19
11
+
pin 4
+
PBL
388 13
10µF
Resistor that sets the
"softclipping" level
+
This resistor sets
the max. attenuation
PBL
d). A ”softclipping” with a fixed level
can be combined with the volume control.
A draw back with the fixed level is that
whensettingit intoinhibit clippingdistortion
at a long line ie. low level, the level will not
increase with short lines even if the supply
voltage would allow it. In the other case
when setting the level for a short line some
amplitude clipping on long line can be
expected.
388 13
19
d).
This resistor sets
the min. attenuation
position on the pot.
Sets the steepness
of the "softclipping"
a).
18
20
PBL
388 13
+
Resistor that sets the
"softclipping" level
PBL
388 13
Weighted
resistors
17
19
To volume
control
e). A ”softclipping” that is controlled by
the ”real” output level that means that the
"softclipping" will follow the line current
changes and will at all times give the opti-
mum distortion limiting performance.
19
Three bit
digital
signal
+
10µF
e).
b).
Figure 25. DC - volume control options.
Figure 24. DC - volume control options.
11
ERICSSON [ ERICSSON ]
