For the purpose of temperature compensation, the base of VT23 is held by
the emitter of an emitter follower VT20, the base of which is held at about –15.5V by
R47 and R48.
VT25 has its base held at –6.7V by R57 and R58 and, for normal signal
levels, is cut off. In the overload region it becomes conducting, thereby connecting
R60, in shunt with R54 and R55, as emitter load of VT24. The relation between
control current and control voltage thus changes to about 1.2mA per volt. This
rapid rise of control current with rise of control voltage minimises distortion in the
8dB overload region. When switched to act as a compressor, the function in the
overload region is that of a limiter.
The hold control is a 10k
Ω
linear potentiometer, mounted with its control
coaxial with the recovery control. It is fed, through R56, from the point which is
decoupled from the +20V supply by R49, C11 and which feeds the collector of VT19
and the emitter of VT22. This results in the upper end of the hold control being at
about +10V. The potential from the slider is connected to the junction of D11 and
R50. Thus the hold control sets a potential, below which the effective control
voltage cannot fall. Thus the hold control determines the increase of gain with
falling signal level and therefore controls the amount of compression or limiting.
The earthy lead from the recovery switch is taken to the slider of the hold
control so that the discharge of C12 is exponential to the potential on the slider.
Terminal K is connected to a ganging switch. When this is switched to the
“gang” position this terminal is connected to the corresponding terminal of the limiter
in the other channel of the cassette. In this condition both units are controlled by
whichever control voltage is higher. In this state, only the left hand hold control is
operative, and the recovery controls should be set to the same setting.
The hold control should never be set so as to allow the meter to enter the
lower red band or distortion may result.
Drawing AE378 gives some notes on the use of this limiter.
12.23 Active Relays
12.23.1
Purpose
This contains transistor circuits which, when operated by mechanical
switches, provide the control signals necessary for remote control of audio signals.
12.23.2
Circuit
The circuit diagram is shown on Drawing No. CE227.
In order to facilitate an understanding of the complete system this drawing also
shows the connections to other components. The components on the actual printed
board are those within the large dashed enclosure. The location of the remaining
component is given in the circuit description below.
