12.23.3
Circuit Description
The circuit comprises eight transistors connected as four similar but not
identical long tail pairs. Taking the simplest of these, VT3 and VT4 are provided
with a long tail R7 connected to the +20V line. The base of VT3 is connected,
through R6, to a +9V rail, obtained from R22, R23 and R24 connected across the
+20V supply. The base of VT4 is held at about +7V by R9 and R10, also across the
+20V supply. The collector of VT4 is connected to the 0V line and the collector of
VT3 is connected through R8 to a –3V line obtained from the –20V supply by R25
and R26. Under these conditions VT4 is turned on and VT3 is turned off so that
terminal 8, which is also connected to the collector of VT3, is held at about –3V.
When the talkback button is depressed, the base of VT3 is connected to a
+6V line, also obtained from the chain R22, R23, R24. This turns VT4 off and VT3
on, thereby passing about 4.5mA to the two Amplifiers J in the Control Room
Monitor Cassette, thereby turning; on VT4 in each of these amplifiers, and thereby
quieting the two monitor loudspeakers. C2 serves to slow the operation and thereby
suppress switch clicks.
The circuitry of VT5 and VT6 is identical with that of VT3 and VT4. This pair
is however operated by the closure of the Artist Managers microphone button,
provided that his selector switch is set to Talkback. This circuit quiets the monitor
loudspeakers by turning on VT5 in each Amplifier J.
VT1 and VT2 are required to quiet the monitor loudspeakers either if the
multi-track announce button is operated, or if any individual track announce button
is operated. These cannot, however, be connected directly in parallel, since only
the multi-track announce button must operate the RL5 in each of the Main Channel
Cassettes. D3 is therefore included, to prevent the operation of RL5 by the
individual announce buttons. Since a silicon diode has a contact potential of about
0.5V, D4 is included so that the potential of the base of VT1 is the same, whichever
button is pressed. Since this potential is therefore 0.5V more positive than the
potential applied to VT3 and VT5 for switching, the potential of the base of VT2 is
made 0.5V more positive than the bases of VT4 and VT6 by making R3 24k
Ω
as
compared with 27k
Ω
for R9 and R14.
It will be seen that RL1 and RL5 can also be simultaneously operated by
setting the oscillator key to position 1. Another leaf of this key (not shown) applies
the oscillator to the track announce bus line. RL5 switches this to the tape
machines for setting up purposes and RL1 quiets the monitor loudspeakers.
VT7 and VT8 provide the control currents required for the change-over
switches in the Amplifiers N in the studio loudspeaker circuits. In this case the
current from both collectors is used so they are both fed in the same way. Since
this relay is to be operated, either by the talkback button or the Artist Manger’s
microphone button, hold-off diodes are again required, these being D1 and D2.
Also R19 has the same value as R3. In this case a somewhat greater output
current is required so the tail resistor R18 is reduced to 2.7k
Ω
.
