particular case it is scaled from –40dB in the penultimate anticlockwise position to
0dB in the fully clockwise position.
3.8.2 Echo Selector
The output of the level control is fed to an Amplifier B and thence to the echo
selector whereby it may be connected to any of four echo bus lines. There is also
an “off” position in which the amplifier output is connected to the 0V line through a
3.3k
Ω
resistor for the reason described in 3.5.8. The Amplifier B shares a board
with the Amplifier G in the cue circuit.
3.8.3 Echo Bus Lines
Echo bus lines Nos. 1 and 2 connect to echo line output circuits in the left
hand Group Cassette and lines Nos. 3 and 4 connect to similar circuits in the right
hand Group Cassette. (See 2.4.9) If there is only one Group Cassette then only
Nos. 1 and 2 echo send circuits are available.
3.9
Power Supply
A cassette power unit, the circuit of which is described in 12.21 is mounted
on the underside of the cassette. It receives its input at 50V with an earthed centre
tap, 50 or 60Hz on a 6-pin plug. This is of the same type as the 7-pin plug used
elsewhere but pin 2 is removed and socket 2 is plugged so that they are not
interchangeable. Soldered connections are made between the +20, 0 and –20V
outputs of the power unit and the corresponding rails of the cassette. The +20, 0
and –20V outputs also appear on pins 3, 4 and 5 respectively of the input plug for
test purposes.
The maximum rating of the power supply unit is 75mA at +20V and 120mA at –20V.
The microphone cassette actually consumes 58mA at +20V and 82mA at –20V.
4.
GROUP CASSETTE
4.1
Contents
Each Group Cassette contains two identical and independent group channels
and two identical and independent Echo Line Output circuits.
4.2
Group Channel
4.2.1 Input Gain Control
The input to each group channel is a group bus line and therefore requires to
feed into an Amplifier C the circuit of which is described in 12.5. At this point, it is
desirable to have some degree of gain control. Firstly, the level at the input will
increase with increasing number of input channels. Doubling the number of
channels will result in a 3dB increase if the signals are not related or a 6dB increase
of the signals are identical. Secondly, if the signal has been limited in the
