that under all signal conditions the emitter follower VT27 operates correctly with an
adequate current margin.
The negative stabilisers are similar but require npn instead of pnp transistors.
12.19 Amplifier Type U
12.19.1
Purpose
This receives signal at a level of –10dBV and provides an output to drive a
standard VU meter to a scale reading of 0 VU. It also has provision for increasing
the gain by 10dB or 20dB by means of the boost buttons.
12.19.2
Circuit
The circuit diagram is shown on Drawing No. AE222.
12.19.3
Circuit Description
VT1 and VT2 constitute a d.c. coupled pair as used in Amplifier E. In the
present case a high input impedance is required so that junction of resistors R6 and
R7 is boot-strapped to the emitter of VT1 by C. The effective impedance from the
emitter of VT1 to the 0 V line is therefore R6, R7 and R10 in parallel (= 2.65k
Ω
).
The voltage gain which is determined by this impedance and R9 is therefore about
10.5dB resulting in a signal level of about +0.5dBV at the base of VT3.
A VU meter has an impedance of 3.9k
Ω
and is intended to be fed from a
600
Ω
line terminated in 600
Ω
, that is from 300
Ω
, through a 3.6k
Ω
resistor. In order
to economise in power, in the present case the identical operating conditions for the
meter are obtained by coupling it through C3 to the junction of R12 and R13. These
in parallel provide the required source impedance of 3.6k
Ω
+ 300
Ω
and signal
current is fed to the junction from the collector of VT3. The magnitude of the current
supplied is determined by the total impedance in the emitter circuit of VT3. This
consists of the parallel combination of R15 and R14 and AOT1 in series. AOT1 is
selected, on test of the board, to give a voltage gain of 13.1dB into a resistive load
of 3.9k
Ω
. AOT2 is then selected so that, when terminal “20” is connected to 0 V the
gain is 33.1dB. AOT3 is then selected so that when terminal “10” is connected to 0
V the gain is 23.1dB.
12.20 Oscillator
12.20.1
Purpose
This enables the performance of the Mixer itself to be checked by direct
injection into the input circuits of the Microphone Cassettes. It can also be switched
through the track announce circuit to all tracks for setting up tape machines. It is
also available for testing other external equipment.
12.20.2
Circuit Diagram
The circuit diagram is shown in Drawing No. BE224.
