PBL 388 14
Loudspeaker amplifier
used only
with inductive load
0.015 µ
LL 20
The loudspeaker amplifier drives
directlya25-50Ωimpedanceloudspeaker.
The single ended loudspeaker amplifier
hasaninternalgainregulationthatprevents
distortion in case of insufficient supply
voltage. The loudspeaker volume control
can be solved in two different ways. One is
to use a conventional potentiometer that
willactasanacvoltagedivideratthepower
amplifier input pin 23. The second is to
control the gain of the power amplifier by
dc. at pin 21. See fig.16. The controlling
element can be a potentiometer or a digital
control from a µ-processor. See figure 17.
LSP 19
50Ω
Load
V
IVOL
out
Input
PBL 388 14
VOL 21
23 LSP
in
F
µ
16 V
100
V
in
1 µF
+
+
I
+L
+
-
V
A
22
18
V
A
GND
17
GND
A
Figure 15. External power supply options.
+ supply
+ supply
+
+
+
Someoptionalfeaturesusing
the dc. set volume control on
the loudspeaker amplifierof
PBL 388 14. ( Fig. 17. )
0V
50 Ω LOUD
SPEAKER
50 Ω LOUD
SPEAKER
0V
0V
19
22
19
22
21
21
23
23
17
0V
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 low supply currents in
combination with high input signals. This
DC controlled volume is externally
17
12
PBL 388 14
PBL 388 14
12
F6
F6
AC-control
DC-control
0V
accessible in the PBL 388 14 and can thus Figure 16. Loudspeaker volume control. options.
be utilized in several ways.
To volume
control
a). To control the loudspeaker volume
with a DC- voltage from a potentiometer.
supply voltage and will at all times give the
optimum distortion limiting performance.
21
10
PBL
388 14
Resistor that is added
and which determines
the dynamics of the AGC
b). Tocontroltheloudspeakervolume
with a digital signal ( for ex. 8 - levels ).
c). An AGC can be combined with the
volume control by connecting a resistor
from the DC - control pin 21 to the output of
the receiver detector at pin 10. 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
21
+
pin 4
+
+
This resistor sets
the max. attenuation
PBL
388 14
10µF
Resistor that sets the
"softclipping" level
PBL
388 14
21
12
This resistor sets
the min. attenuation
position on the pot.
d).
a).
d). A ”softclipping” with a fixed level
can be combined with the volume control.
Adrawbackwiththefixedlevelisthatwhen
setting it in to inhibit clipping distortion at
lowsupplyvoltage,thelevelwillnot increase
even if the supply voltage would allow it.
Sets the steepness
of the "softclipping"
19
22
+
PBL
388 14
PBL
388 14
Weighted
resistors
Resistor that sets the
"softclipping" level
18
21
21
Three bit
digital
signal
To volume
control
e). A ”softclipping” that is controlled by
the ”real” output level that means that the
"softclipping" will follow the changes in the
+
10µF
b).
e).
Figure 17. DC - volume control options.
10
ERICSSON [ ERICSSON ]
