In series with the operating push buttons is a section of the talkback key. In
the position marked 3 on the circuit diagram (key with lever down) the connection
from the buttons to RL4 is broken, so that talkback is routed, by a second section of
the key, to the alternative talkback system, without any change in the studio
loudspeaker system. In the mid position of the key, RL4 is operated by the push
buttons and the second section of the key applies the talkback signal to the A inputs
of the Amplifiers N, which are now routed to the studio loudspeakers thus giving
talkback on the studio loudspeakers. In the remaining position (marked 1) of the
key RL4 is still operated by the push buttons, but the second section of the key
leaves the A input of the Amplifiers N without a signal, so the studio loudspeakers
are muted. The second section of the key connects the talkback signal to the
alternative talkback channel, thus resulting in alternative talkback with studio
loudspeakers muted (marked ALT.(M) on the panel).
12.24 Solo Relay
12.24.1
Purpose
This is used in conjunction with the solo buttons to mute the right hand
monitor loudspeaker and to feed to the left hand monitor loudspeaker the signal
from only the channel of the pressed solo button.
12.24.2
Circuit
The circuit diagram is shown on drawing No. BE229.
12.23.3
Circuit Description
VT1 and VT2 are a long tail pair with the base of VT2 held at about +0.8V
from the potential divider R1, R2, R3 connected across the +20V supply. R17
connects the base of VT1 to the common emitter point, and therefore VT2 is turned
on and VT1 is held off. The current in VT2, determined by the base potential and
R6, is about 3.5mA. The base of VT1, and the junction of R2 and R3 which is at a
potential of about 6V are connected to the control contacts of all the solo buttons.
When any one of these is pressed, these two points are connected together thereby
reducing the voltage of VT1 base to about 6V. This turns VT1 on and VT2 off. The
collector of VT1, which was previously held slightly negative by R4, R5 connected
across the -20V supply now passes its 3.5mA through R7, R13, R8, R14, which
serve to equalise the sharing to the bases of VT3, VT4, VT5 and VT6, thereby
turning on these four transistors.
Terminals A and B, left and A and B, right are connected to corresponding
terminals in the Amplifiers P in the left and right control room monitor loudspeaker
circuits. This puts these terminals in series with the normal signal paths to the
loudspeakers. Thus when VT3, VT4, VT5 and VT6 are in the “off” condition, normal
signal transmission takes place through the series resistors, which total about 20k
Ω
,
thereby causing the virtual earth amplifier VT4, VT5 of Amplifier P to have unity
gain.
When the four transistors are in the “on” conditions, each channel has a two
stage attenuator each stage of which introduces at least 40dB of attenuation. The
normal signals in both channels are therefore effectively switched off. The solo
