12.7.2 Types
There are four types designated F1 to F4. These involve small differences in
value of two of the components controlling the high frequency cut-off characteristic
to enable the overall response of the cassettes in which they are used to be
trimmed. Type F1 is used in Microphone Cassettes. Type F2 in Group Cassettes,
Type F3 in Main Cassettes and Type F4 in the remaining cassettes.
12.7.3 Circuit
The circuit diagram is shown on Drawing No. AE208.
12.7.4 Circuit Description
The circuit comprises two capacitor-resistor time constants giving attenuation
below about 25Hz and two capacitor-resistor time constants giving attenuation
above about 25kHz. Two d.c. coupled unity gain pairs of transistors serve to isolate
each time constant from the other of similar frequency and overall negative
feedback is applied.
The d.c. conditions for the circuit are established by R6 and R7 which hold
the base of VT3 at about –4.65V. This determines the potential at the output and
hence, through the d.c. feedback path, the potential of the base of VT1, the input
being a.c. coupled.
The low frequency time constants are given by C1 + C2 with R1 + R2
(assuming the source impedance to be low) and C4 with R6 and R7 in parallel. The
high frequency time constants are given by C3 with R3 and C5 with R8 and R9 in
parallel.
The filter is tested as a board with a value of 360
Ω
for R1 and R8 omitted.
When overall tests are performed on the cassette, values for R1 and R8 are
selected to give the best low and high frequency response curves respectively.
12.7.5 Output
The output is at approximately –5V and therefore an external coupling
capacitor is required unless the subsequent unit has an input capacitor.
The output impedance is low. The output will deliver at least 3V r.m.s. into a
load of 3k
Ω
. The output is in phase with the input.
12.8 Amplifier Type G
12.8.1 Purpose
The amplifier G is a unity gain device which is used where it is required to
connect a resistive control and the loading which this would impose cannot be
tolerated.
12.8.2 Circuit
The circuit diagram is shown on Drawing No. AE209.
