Amplifier Section; Electronic Switching; Transistor Switches - Teac C-1 Service Manual

AMPLIFIER SECTION
1. INTRODUCTION
The C-1 is a three head tape deck and therefore has fully
separate record and playback amplifiers. They have been
designed for maximun dynamic range so as to be able to
take advantage of the wide dynamic range made possible by
the use of the add-on dbx unit.
Optional BIAS/EQ cards enable the user to attain optimum
bias, EQ and sensitivity depending on the type of cassette
tape he wishes to use.
The many other features of the C-1 include the use of
Bicon cored heads for enhanced low frequency response,
peak level meters and ganged record gain controls for added
user convenience.
2. ELECTRONIC SWITCHING
To minimize stray signal wires (which contribute to cross-
talk, RFI etc) extensive use is made of electronic switch-
ing. FET switches are used for switching audio signals and
bipolar transistors are used in the equalizer, level adjusting
and muting circuits.
2-1 FET SWITCHES
2SK68A-M N-channel junction type FETs are used for
their excellent noise characteristics. Figure 1 shows the
basic circuit configuration of the FET switches together
with their switching characteristics, R3 sets the voltage
present at the source of the FET to OV. When SI is open-
15V (considerabley more than the pinch-off voltage which
is about —0.5V) is present on the gate and the FET is cut
off. In this condition the drain-source AC impedance is
virtually infinite and thus the electronic switch is OFF.
When SI is closed, Di is reversed biased by the positive
voltage present at the point A and no voltage appears at
the gate of the FET (R1 is sometimes omitted), VGs falls
at OV and the drain-source impedance drops practically to
zero. D1 acts as an isolating diode.
FET
AUDIO
(DRAIN) — (SOURCE)
— AUDIO
SIGNAL —> SIGNAL
INPUT
OUTPUT
R3
Sl
+B—07O
=
~ Ves
(A) FET SWITCH BASIC CIRCUIT
2-2 TRANSISTOR SWITCHES
NPN type transistors are used to control various AC signal
networks. Figure 2 shows both grounded collector and
grounded emitter configurations. When the transistors are
on, the collector-emitter impedance is similar. However,
the DC present at point A is about 0.4 mV in the case of
grounded collector circuits and 4mV in the case of ground-
ed emitter circuits. When the transistors tum off, the DC
voltage drops to zero. Consequentely, grounded collector
circuits are about 20 dB quieter than grounded emitter
Circuits as far as switch-off click is concerned. A negative
voltage applied to the base of the transistors holds them
off completely. In the case of Q108 and Q119, trasistors
with large emitter-base breakdown voltages (VEBO) and
reversed biased bases are required owing to the large voltage
swings of the signals present.
AC SIGNAL LINE AC SIGNAL LINE
CONTROL
SIGNAL
CONTROL
SIGNAL
T-1313
(A) GROUNDED COLLECTOR (B) GROUNDED EMITTER
Fig. 2 Transistor Switches
FET DRAIN-SOURCE
' lo
SWITCH
RESIS TANCE
oe Sao
ON
50
loss
) &
VP 0
r
oe
eee
(B) FET SWITCH CHARACTERISTIC CURVE
T- 1312
Fig. 1 FET Switch
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