4.2.6 Presence Controls
These are identical with those used in the Microphone Cassette and are
similarly positioned.
4.2.7 Band Pass Filter
This is similar to that in the Microphone Cassette but two of the components
in the part of the circuit which is used for trimming the high frequency response of
the cassette have slightly different values.
4.2.8 Output
The Band Pass Filter feeds an Amplifier B the output of which is connected to
the output selector whereby it may be routed to any of eight main channel input bus
lines. An “off” position is also provided. The Amplifier B is one of two on a B + B
board, the other one being unused. In the event of a fault the unused one could be
brought into use by suitable wire links.
4.3
Echo Output Circuit
4.3.1 Main Level Control
Each echo bus line feeds an Amplifier C2 (fixed gain) the output of which is
connected to a general-purpose attenuator. This is the main control of the level
sent to the particular echo device (chamber, plate, etc.) and it is calibrated from 0 to
–40dB and “off”.
4.3.2 Output Amplifier
This is an Amplifier R and is described in 12.17. Together with the output
transformer, it provides 10dB of voltage gain to give an output of 0dBV into open
circuit. The output impedance is approximately 200
Ω
and it is intended to work into
an impedance of 2k
Ω
or greater. Under these conditions it will handle a level of
+20dBV without clipping. The two echo outputs from a cassette are available on a
5-pin Tuchel socket.
4.4
Metering
The V.U. meter switch in the corresponding Main Cassette when turned to
“Check Group” connects the relevant V.U. Meter Amplifiers to the sliders of the
check switch in the Group Cassette. This is a 3-position switch of which the centre
is an off position in which terminating resistors to the 0V line are provided. The
other positions allow the V.U. Meters to measure the levels either at the outputs of
the main channel Amplifiers C, this being the nearest point to the cassette input at
which a measurement can be made, or at the echo outputs. Since the V.U. Meter
Amplifiers require an input level of –10dB and the output transformers have a step-
up ratio of 1:1.77 (5dB) the metering point is taken from a 5dB resistive
potentiometer across the Amplifier R output. Thus ‘0’ on the V.U. meter
corresponds to an open circuit output voltage of 0.447V.
