ratio normally determined sensibly by the input amplifier alone and which is
therefore as high; as can be obtained by any technique.
12.
CIRCUIT DESCRIPTION. INDIVIDUAL BOARDS
12.1 Presence
12.1.1 Component Parts
There are four sections, the complete circuit being shown in Drawing No.
BE.200.
The active components together with the reactive components for
determining the two highest presence frequencies are assembled on Board B51/3.
(This board with some components different is also used on AE.202).
The reactive components for the remaining frequencies are assembled on
Board B78/2.
The frequency selection is made by switch S1, kHz Presence.
The amount of presence is determined by switch assembly S2, dB Presence,
the circuit of which is shown on Drawing No. B30.
12.1.2 Active Circuitry
The active circuitry comprises two d.c. coupled transistors acting as a virtual
earth amplifier. VT1 is a high gain stage and VT2 is an emitter follower providing a
low impedance output R7, C4 in the forward path and R6, C5 in the return path
serve to prevent r.f. oscillation.
With S2 in its central position the feedback path consists of R5 (20k
Ω
) in
parallel with the total resistance between terminals 1 and 2 on S2, which is also
20k
Ω
. The forward path consists of R1 (20k
Ω
) in parallel with the total resistance
between 4 and 5 of S2 (again 20k
Ω
).
The forward and feedback paths to the base of VT1 are therefore equal
resulting in unity voltage gain.
12.1.3 Reactive Components
The 10kHz series tuned circuit consists of the inductance of the section of L1
between terminals 2 and 3 and capacitor C3.
For the next lower frequency, 6.5kHz, the total inductance of L1 is employed
and the capacitor is C2.
For 4.2kHz the inductance is the sum of that of L1 and that of L3 between its
end 1 and tap 4 and capacitance is provided by C16 and C17 in parallel.
For the remaining frequencies inductance is added successively by two more
sections of L3 and three sections of L2. In all cases except 1.2kHz two capacitors
in parallel are required to obtain the required value of capacitance.
The tolerance on the larger capacitors is
±
5% and the inductance tolerance is
±
1% so that there is a tolerance of
±
3% on the frequency at which the presence
peaks.
