12.16.4
Mechanical
The unit is assembled on a plug-in board of the same size as the Q Amplifier
Power Unit and is housed in the same frame.
12.17 Output Amplifiers Type R
12.17.1
Purpose
This is a line output amplifier which receives signal at a level of –10dB and, in
conjunction with an output transformer type B35A it delivers a signal at zero level
into a load of 2k
Ω
or greater. It is used for main channel outputs to tape machines,
for echo outputs, for control room monitor and studio playback loudspeaker outputs
and for the auxiliary talkback and intercom outputs.
12.17.2
Circuit
The circuit diagram is shown on Drawing No. AE219.
12.17.3
Circuit Description
As in the Amplifier E, the gain is obtained by two d.c. coupled transistors, the
feedback also being d.c. coupled. The base of VT1 is held at about –11.6V by R2
and R3. R1, R2 and R3 in parallel present an input impedance of 50k
Ω
. C2, R5
serve to prevent r.f. oscillation. The output capacitor must have a value sufficiently
large to avoid tuning with the inductance of the transformer which it feeds and
hence has to be of the electrolytic type. On test the value of AOT 1 is selected to
give a voltage gain of 5dB. The output transformer with which it is used has a step
up ratio of 1:1.78 thereby yielding the desired overall gain of 10dB.
The Amplifier U which is used to drive a V.U. Meter requires and input at a
level of –10dBV. This can be obtained from the –5dBV output of Amplifier R by a
resistive potentiometer R9, R8 , R10 which introduces a loss of 5dB.
The amplifier board is tested without a transformer using a resistive load of
680
Ω
into which it will deliver at least 2.7V r.m.s.
12.17.4
Output
The output is unbalanced, is in phase with the input and its impedance is low.
At the secondary of the transformer the output impedance is about 200
Ω
this being
mostly the resistance of the transformer.
12.18 Correlator
12.18.1
Purpose
The correlator receives signals from a pair of channels and indicates on an
associated meter whether these signals are in phase or out of phase with one
another. No indication is given if the signals in the two channels differ in level by
more than about 20dB.
The correlator unit also includes the circuitry for driving two low level meters
which are calibrated over the range –40VU to –60VU. These levels are with
