Specifications

CIRCUIT IDEAS
F
ig. 1 shows a muting circuit, which
makes use of IC LB1403. Sig-
nal from any pre-
amplifier, such as HA1032,
LA3161, or LA3160, is con-
nected to the base of am-
plifier transistor T1. Vari-
able resistor VR1 is used
to control the gain of in-
put signal.
Comparator 2 output
at pin 2 of LB1403 is used
for generation of muting
signal at the emitter (point
A) of transistor T2, which
can be directly connected
to muting pin 4 of ampli-
fier employing IC LA4440.
As long as the audio input
to the circuit of Fig. 1 is
below a certain level (say,
150 mV peak to peak), the output at point
A will be high (the value measured at
EFY Lab was around 4.5V). Once the in-
put crosses this threshold level, the out-
put will be around 0V. Capacitor C4 de-
termines the ‘on’/‘off’ muting delay.
Higher the value of this capacitor,
greater will be the muting delay period.
Slight circuit modification will be
needed if this circuit is used with STK
series amplifiers, such as STK 4141, 4142,
4152, and 4191, because they need nega-
tive polarity voltage for muting. The addi-
tional circuit to be connected at point A in
that case is shown in Fig. 2.
and then press switch S1. As a result,
buzzer PZ2 sounds. Simultaneously, the
side tone is heard in the speaker of hand-
set of phone 1. The person at phone 2
could then lift the handset and start con-
versation. Similar procedure is to be fol-
lowed for initiation of the conversation
from phone 2 using switch S2. In this
mode of operation, a 3-pole, 2-way slide-
switch S3 is to be used as shown in the
figure.
In the changeover mode of operation,
switch S3 is used to changeover the tele-
phone line for use by telephone 2. The
switch is normally in the intercom mode
and telephone 3 is connected to the ex-
change line. Before changing over the ex-
AUTOMATIC MUTING CIRCUIT
FOR AUDIO SYSTEMS
S.C. DWIVEDI
SUNISH P.
S.C. DWIVEDI
2-LINE INTERCOM-CUM-TELEPHONE
LINE CHANGEOVER CIRCUIT
J. SRINIVASAN
T
he circuit presented here can be
used for connecting two telephones
in parallel and also as a 2-line in-
tercom.
Usually a single telephone is con-
nected to a telephone line. If another tele-
phone is required at some distance, a par-
allel line is taken for connecting the other
telephone. In this simple parallel line op-
eration, the main problem is loss of pri-
vacy besides interference from the other
phone. This problem is obviated in the
circuit presented here.
Under normal condition, two tele-
phones (telephone 1 and 2) can be used
as intercom while telephone 3 is connected
to the lines from exchange. In changeover
mode, exchange line is disconnected from
telephone 3 and gets connected to tele-
phone 2.
For operation in intercom mode, one
has to just lift the handset of phone 1
132