potentiometers with screwdriver adjustment. Each of these is a 2-gang unit to
control the two loudspeaker channels. The maximum quieting is determined by R13
and R14 and is about 20dB. The minimum quieting is about 8dB.
OUT ‘B’ is connected to the correlator input selector switch.
12.11.4
Output
The output is intended to work into an impedance of 50k
Ω
and it will deliver at
least 1.1V r.m.s.
12.12 Amplifier Type K
12.12.1
Purpose
Amplifier K is used following the fader in Group and Main Cassettes where a
phase reversal is required in order to maintain correct poling through these
cassettes. As in the case of the Amplifier E the gain of the Amplifier K is 5dB.
12.12.2
Circuit
The circuit diagram is shown on Drawing No. AE213.
12.12.3
Circuit Description
VT1 is connected as an emitter follower. Its base is held at about +6.3V by
R2 and R3. R1, R2 and R3 in parallel give the required input impedance of 50k
Ω
.
VT2 and VT3 are connected as a fully d.c. coupled virtual earth amplifier very
similar to that of Amplifier H. For initial test of this amplifier a short circuit is
provided in place of R4 (AOT 1). The value for this resistor is chosen on overall test
of the cassette in order to adjust the overall gain of the channel to the nominal
value.
12.12.4
Output
The output is d.c. coupled and has a potential of about +3V. It must therefore
be followed by a coupling capacitor. It will deliver at least 1.8V r.m.s. into a load of
3k
Ω
.
12.13 Amplifier Type M
12.13.1
Purpose
This amplifier is used in the Studio Playback Cassette. Its input is switched
between track announce signal and the output of the oscillator level control. It has
three outputs. One is at a level of –35dBV and is fed to one stud of each of the
Microphone Cassette input switches. Another is at a level of 10dBV and this is
connected to the V.U. Meter switches in the Main Cassette as a check of the meters
and their amplifiers and also to the track announce bus line whereby either track
announce signal or oscillator signal can be fed to the tape machine. The third
output is at 0dBV and appears on a Tuchel socket for feeding to external
