12.3 Amplifier Type A
12.3.1 Types
There are three types of Amplifier A. Type A1 had a voltage gain of 10dB
and is used in the re-record circuits of the Track Monitor Cassette.
Types A3 and A4 have voltage gains of 20 and 40dB respectively and they
are used in the microphone circuits of the Studio Playback Cassette.
12.3.2 Circuit
The circuit of the three types is shown on Drawing No. AE.203
12.3.3 Circuit Description
The circuit comprises three d.c. coupled transistors. VT1 and VT2 provide
voltage gain and VT3 is connected as an emitter follower output stage. Overall
feedback is provided by connecting the emitter of VT1 to a tapping across the
output. The voltage gain is determined mainly by the ratio of:
(R5 + (R6 // AOT1) to R6 // AOT1).
The three values of gain are obtained by using suitable values of R6. In the
case of the Amplifier A1 which is used in the re-record signal path, the voltage gain
in set on test to 10
±
0.03dB by choice of a suitable value for A.O.T.1. The value
15k
Ω
given in the table is a typical one. Amplifiers A3 and A4 are only used in the
operator’s and Artist Manager’s microphone circuit where a precise value of gain is
unnecessary so no gain adjustment is made on test. C2 in the forward path and C5
in the feedback path serve to prevent r.f. oscillation. The decoupling capacitor C3 is
reduced in value in the Amplifier A4 to increase the negative feedback at low audio
frequencies to give a bass cut.
The Amplifier A1 feeds a mic. Input bus line which is connected to a resistive
attenuator in each microphone channel. To maintain good bass response the
output capacitor C6 is a 25
µ
F electrolytic. In the other cases a 1
µ
F is adequate.
The direct output A is not used.
12.3.4 Output
The output impedance is low. The output will deliver at least 1.4V r.m.s. into
3k
Ω
. The output is in phase with the input.
12.4 Amplifier Type B + B
12.4.1 Component Parts
This contains two amplifiers which are completely independent apart from the
supply lines. The inputs are “a” and “b” and the outputs “A” and “B” respectively.
12.4.2 Circuit
The Circuit diagram is shown in Drawing No. AE.204.
