12.10.3
Circuit Description
VT1 and VT2 are d.c. coupled as in Amplifier E. The base of VT1 is held at
about –12.3 V by R1 and R2. The d.c. conditions of VT2 are then determined by R4
and R5 whilst the a.c. gain is determined by R4 and R5 shunted by R6 and AOT1 in
series. The output capacitor C5 feeds output A through a 5.1k
Ω
resistor R15.
Since the collector of VT2 is a low impedance point, the output impedance is
approximately 5.1k
Ω
. The output, when used, is always loaded by 5.1k
Ω
thus
introducing a 6dB voltage loss. The voltage gain of the transistor is therefore 16dB
to give the required overall 10dB gain.
VT3 and VT4 constitute a unity gain phase reversing amplifier. It is of the
virtual earth type and has unity gain because the forward and feedback resistors R7
and R9 are equal in value. C4, R10 and C3, R11 serve to prevent r.f. oscillation.
This amplifier is d.c. coupled except in that the output (output B) is provided with a
coupling capacitor C6 and a build-out resistor R16 as in the case of output A.
12.10.4
Output
Each output will deliver at least 1.4V r.m.s. into a load of 5.1k
Ω
.
12.11 Amplifier Type J
12.11.1
Purpose
This provides 10dB of gain to counter half the 20dB loss introduced by the
control room loudspeaker level control at its normal setting and also provides three
independent quieting circuits.
12.11.2
Circuit
The circuit diagram is shown on Drawing No. AE212.
12.11.3
Circuit Description
The amplifier transistors VT1 and VT2 use the same circuit as the Amplifier E
except that the gain adjusting resistor is in series with R5 in this case and in shunt in
the Amplifier E. The input base is held at about –14.4V by R2 and R3 and the value
of R1 has been chosen to obtain an input impedance of 50k
Ω
. The output of the
amplifier stages is taken through coupling capacitor C2 and fed through build-out
resistors R8 and R15 to OUT ‘A’ and OUT ‘B’ respectively. OUT ‘A’ is the main
signal output and R8 increases the impedance so that about 20dB of quieting can
be realised by the shunt circuits provided. The switching is affected by transistors
VT3, VT4 and kVT5 which are turned off or on by control signals from the Active
Relay board (see 12.23) applied to their bases through resistors R10, R11 and R12.
In the “off” condition the transistors have high impedances between their emitters
and 0 V so OUT ‘A’ is unloaded. Where VT3 is switched on its impedance from
emitter to 0 V becomes low, so R9 is connected as a shunt on the output thereby
introducing a loss of about 22dB, this having been found by experience to be a
suitable amount of quieting during track announcements. In the case of operator
and artist manager talkback the amount of quieting is controlled by 1k
Ω
preset
