equipment. The associated switching is such that the track announce signal does
not get fed to the Microphone Cassettes.
12.13.2
Circuit
The circuit diagram is shown on Drawing No. AE215.
12.13.3
Circuit Description
VT1 and VT2 are d.c. coupled as a unity gain amplifier. The input base is
held at about –3.5V by R2 and R3. An input coupling capacitor C1 is provided and
R1, R2 and R3 in parallel present an input impedance of 50k
Ω
, the value required
for the attenuator from which it is fed to give its indicated loss.
The output from this amplifier pair is coupled by C3 to the oscillator output
socket, a 5-pin Tuchel with parallel connections to both channels. Since the
oscillator output (see 12.20) is at a level of +10dBV and since the zero of the scale
of the oscillator level control is at the stud at which 10 dB of loss is introduced, this
output is at 0dBV.
For the –10dBV output a second d.c. coupled unity gain amplifier VT3, VT4 is
provided. The potential of the base of VT3 is established mainly by R8 and R9.
The 10 dB loss is determined by R6 + R7 working into R8 and R9 in parallel, the
precise loss being adjusted on test by selecting a value for R7. Since the potential
drop across R6 and R7 is small, the d.c. conditions of VT3 and VT4 are almost
unchanged by the selection of R7.
The –35dBV output is obtained from the –10dBV output by a potential divider
consisting of R13, R12 and R14. Its impedance is therefore about 68
Ω
.
12.13.4
Output
The output connected to the oscillator socket will deliver at least 2.2V r.m.s
with a load of 2k
Ω
from a low source impedance.
The V.U. and track Announce output will deliver at least 0.7V r.m.s. into a
load of 2.4k
Ω
.
12.14 Amplifier Type N
12.14.1
Purpose
This is used in the Main Cassette for providing the multi-track announce
facility and in the Studio Playback Cassette for providing talkback on the studio
loudspeaker.
12.14.2
Circuit
The circuit diagram is shown on Drawing No. BE216.
12.14.3
Circuit Description
The normal input is coupled to the base of VT1 which is held at about –10.8V
by R2 and R3. Since R10 feeds into a virtual earth amplifier, the collector load is R4
in parallel with R10. Since this is equal to the emitter load R5, VT1 has unity gain,
