The collectors of VT5 and VT6 are connected to the bases of another long
trail pair VT7 and VT8, the long tail being the collector impedance of VT9. This
stage provides a gain of about 7dB for the push-pull signal and rejects the push-
push signal.
A d.c. coupled unity gain pair VT10, VT11 is connected to the collector of
VT8, and provides a low impedance output through d.c. blocking capacitor C8.
For the control chain, another push-pull amplifying stage VT12, VT13 is direct
coupled to VT7, VT8, and provides about 9dB of gain. The collectors of this stage
are a.c. coupled to the bases of a further push-pull pair VT15, VT16. The earthy
ends of the base resistors of VT15 and VT16, are connected to a low impedance
point, held at about –13.3V by an emitter follower VT14, which provides temperature
compensation for VT15 and VT16. These two transistors are connected as a long
tail push-pull pair, and the output is taken from the collectors through diodes D6 and
D7 to the base of an emitter follower VT17. The combination of VT15, VT16, D6,
D7 and VT17 provides a back-off system which requires the signal level at the
bases of VT15, VT16 and hence the output signal level to exceed a predetermined
value before any signal appears at the emitter of VT17. Less back-off is required for
compression than for limiting. To effect this, the compress-limit switch short circuits
a part of the tail resistance (R41, AOT5, AOT6) thereby increasing the current to
VT15 and VT16 and so reducing the collector potential.
VT18 and VT19 are connected as a d.c. coupled virtual earth amplifier. In
the “compress” condition the gain is quite low being about 3dB (the ratio of R46 to
R44). In the “limit” condition R45 with D8, D9 and D10 in series are shunted across
R44, thereby increasing the gain to about 20dB. The purpose of the diodes is to
round off the sharp knee which would otherwise be present in the transfer
characteristic.
The emitter of VT19 is connected through D11 and R50 to C12 so that, when
the emitter swings positively, C12 charges. The charging time constant is about 8
ms, this having been found to be the optimum value, C12 discharges through a
recovery control B247A, mounted on the top panel which provides recovery time
constants of approximately 0.1, 0.25, 0.5, 1, 2 and 5 seconds.
The control voltage across C12 is fed to a d.c. coupled unity gain pair of
transistors VT21, VT22. The current in this stage is determined by VT23 and is
about 1mA. Thus the standing potential of the collector of VT23 is about –14V and
its excursions are identical with the control voltage at C12. VT24 converts these
voltage excursions into current variations, the effective emitter load being R54 and
R55 in parallel. This equals 4.7k
Ω
and therefore a control current of 0.21mA per
control volt is generated. The control current is fed to VT5 and VT6 and thence to
the gain controlling zener diodes D1, D2, D3 and D4. It also passes through a
meter Type A49 which is calibrated in decibels of voltage gain. The range is 0 to 20
with 0 near the top and 20 near the bottom. The scale above 0 is coloured red
since this is an overload region. This red range covers 3dB of an 8dB overload
margin before peak clipping occurs. In the overload margin the total harmonic
distortion at a frequency of 1kHz is of the order of 0.5% but is normally less than
0.2% in the normal working range of levels. Left and Right hand meters are
provided for the two channels in a cassette.
