to realize the required 50k
Ω
input impedance. The first amplifier is therefore
preceded by an emitter follower VT1. R1 and R2 provide the required input
impedance and hold the base at about +6.7V. R3 serves to reduce the risk of r.f.
oscillation. AOT1 is used for setting the overall gain of the channel in the cassette
and is initially a short circuit on the board. The gain of the amplifier VT2, VT3 is
approximately the ratio of R8 + AOT1 to R3 or about 16dB. The output of this
amplifier is coupled through C5 to terminal A and, through a 5.1k
Ω
build-out resistor
R21, to OUT 2 (Anti-phase).
In the normal condition the Solo Relay provides a resistance of 20k
Ω
between terminals A and B. Terminal B is coupled by C8 to the input of the second
amplifier VT4, VT5, the feedback resistor of which, R15, also has a value of 20k
Ω
.
This therefore has unity gain and the output is coupled through C9 and build-out
R20 to OUT 1 (In-phase).
12.28.4
Output
Each output will deliver at least 1.1V r.m.s. into a 5.1k
Ω
load.
12.29 Microphone Power Unit
12.29.1
Purpose
This unit provides a 50V d.c. supply, with low ripple, for phantom powering of
microphone head amplifiers.
12.29.2
Circuit
The circuit diagram is shown in Drawing No. BE238.
12.29.3
Circuit Description
A transformer B189A as used in the Cassette Power Unit is fed from the 50 V
a.c. supply and the whole of the secondary winding is used to feed a bridge rectifier
circuit D1, D2, D3 and D4. The output is fed through a current limiting resistor R1 to
reservoir capacitors C1 and C2 in parallel. The positive line passes through R4 and
a series control transistor VT2 to the output. Two 16V zener diodes d
Ω
and D7,
carrying the emitter current of VT3 and about 1.8mA supplied through R6, hold the
emitter of VT3 at about +32V. The base of VT3 is connected to a potential divider
consisting of R7, AOT and R8 connected across the output. The collector of VT3
has a load resistor R5 and it is connected to the base of VT2, thus forming a high
gain feedback circuit which stabilises the output voltage. The AOT resistor is
chosen to give the required 50V output. C5 provides an a.c. by-pass across the
upper part of the potential divider, and therefore increases the loop gain at a.c. and
so reduces the output ripple.
D5 is a 7.5V zener diode holding the emitter of VT1 7.5V below the rectified
supply. The base of VT1 is connected through a current limiting resistor R3 to the
output line. Thus if the rectified voltage rises in excess of about 8V above the
output, VT1 is turned on and takes current which reduces the rise of voltage. It
therefore acts as a shunt stabiliser.
