Microphone Cassette and if the fader is set to 0, the output of the Microphone
Cassette will be a standard level signal as measured by the limiter meter which is a
peak reading meter. Depending on the signal waveform this may be substantially
less than a standard level signal as measured on a V.U. meter where it will be fed to
the tape machine. In the first case, it is desirable to have less gain available and in
the second case more gain is required. The standard Amplifier C2 has a feedback
resistor of such value that an Amplifier B plus an Amplifier C2 has unity overall gain.
In this case an Amplifier C1 is employed. This has a feedback resistor of greater
value than the C2 and this, when shunted by the input gain control provides a range
of
±
10dB in 2dB steps.
4.2.2 Compressor/Limiter
This is identical in every respect with that in the Microphone Cassette (see
section 3.5.2). The controls and meters are identical and are similarly positioned.
The a.c. coupling, consisting of 6.8
µ
F and 33k
Ω
between the C1 Amplifier output
and the Out-Compress-Limit key, would at first sight appear to be superfluous since
the Amplifier C1, itself has an output coupling capacitor. The Compressor/Limiter
introduces a small amount of bass loss when feeding into the fader. This bass loss
is counteracted by trimming the band pass filter. The a.c. coupling introduces a
bass loss equal to that of the compressor limiter and maintains a similar frequency
characteristic whether or not the Compressor/Limiter is in circuit.
4.2.3 Inject
This is identical with the facility provided in the microphone channels (see
3.5.3).
4.2.4 Fader
The fader used is electrically identical with that in the microphone channels.
In this case, the left-hand fader is provided with an optional mechanical ganging
device. If the two faders are set to the same reading and the projection on the right
hand side of the left-hand control knob is pressed home, the two faders can be
operated as a ganged pair. The two controls can easily be separated by lifting the
ganging member.
4.2.5 Poling
As some channels may be routed directly from Microphone to Main
Cassettes and other channels may be routed through a Group Cassette, it is
essential that a Group Cassette should not introduce a reversal of polarity. As the
circuit of this cassette is one side earthy throughout, each channel must contain an
even number of active elements, which produce a reversal of polarity. The Amplifier
C, Amplifier B and Band Pass Filter all reverse so it is necessary to add a fourth.
Therefore, instead of using an Amplifier E to counteract the 5dB loss of the fader at
its normal operating point as in a microphone channel, an Amplifier K, which gives a
phase reversal, is used for this purpose. The circuit of the Amplifier K is described
in 12.12.
