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Teac C-1 User Manual

Teac C-1
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24
AMPLIFIER
SECTION
4,
PLAYBACK
AMPLIFIER
P.C.B.
4-1
PLAYBACK
EQUALIZER
AMPLIFIER
The
playback
equalizer
amplifier
comprises
a
differential
input
current
mirror
circuit
(Q101
to
Q103)
and
a
pure
complementary
push-pull
amplifier
(Q105
to
Q107).
Playback
frequency
compensation
characteristics
are
deter-
mined
by
the
negative
feedback
circuit
(R111
to
R113,
C107)
of
the
amplifier.
The
time
constant
T
with
the
EQ
switch
set
to
the
NORMAL
position
(Q104
off)
is
obtained
by
the
following
equation.
T
=
C107
x
(R111
+
R112
+
R113)
=
120
seconds
When
the
EQ
switch
is
in
the
FeCr
or
CrO2
position
(Q104
on)
the
time
constant
T
is
as
follows.
T
=C107
x
(R112
+
R113)
=
70
seconds
The
differential
input
current
mirror
circuit
is
briefly
explained
below.
The
circuit
comprising
Q3,
Q4
and
R1,
R2
connected
to
the
load
of
the
transistor
Q2
as
shown
in
Fig.
4(A)
acts
as
a
current
mirror
circuit.
As
Q4
is
connected
with
the
diode
with
shortcircuited
collector
and
base,
it
can
be
replaced
by
the
diode
D1
as
shown
in
Fig.
4(B).
Fig.
4
Current
Mirror
Circuit
The
current
mirror
circuit
is
established
when
the
base-
emitter
characteristics
of
Q3
are
the
same
as
the
diode
characteristics
and
R1
equals
R2.
This
circuit
is
charac-
terized
thus;
when
the
collector
current
Ic2
is
applied
to
Q2,
the
collector
current
Ic3,
which
is
the
same
value
as
Ic2,
flows
in
Q3.
Ic3
reflects
any
change
of
Ic2just
like
a
mirror.
This
is
the
reason
why
this
circuit
is
called
a
current
mirror
circuit.
Current
mirror
circuits
are
usually
used
in
combination
with
differential
amplifiers.
(Fig.
5)
When
no
signal
is
received,
the
collector
current
Ipc2
and
Ioc1
flow
in
Q2
and
Ql.
Thanks
to
the
current
mirror
feature,
Ipc2
is
made
equal
toIpc1.
If
an
opposite
phase
signal
(shaded
part
in
Fig.
5)
is
applied
to
bases
of
Q2
and
QI,
the
collector
current
IDc2
-
di2
flows
in
Q2
(di2
indi-
cates
the
change
in
current
caused
by
the
input
signal)
and
T-1316
Fig.
5
Differential
Input
Current
Mirror
Circuit
the
same
current
IDC2
-
di2
flows
also
in
Q3
by
the
current
mirror
feature.
The
current
IDc1+dil
flows
in
the
collec-
tor
of
Q1.
IfhFe
of
Q1
equals
that
of
Q2,
dil
equals
di2.
In
an
actual
circuit,
the
input
signal
is
applied
to
the
base
of
Ql.
The
base
of
Q2
is
connected
with
the
NF
circuit
of
the
amplifier
in
the
subsequente
stage.
The
current
IL
flowing
in
the
load
RL
is
the
difference
between
collector
currents
of
Q1
and
Q3,
and
is
expresed
as
follows.
IL
=
(Ip1
+
dil)
—
(Ioc2
—
di2)
=
(Ipc1
—Ipc2)
+
(dil
+
di2)
Where,
IDc1
=
Ipc2
and
dil=di2.
Thus,
IL
is
obtained
by
the
following
equation.
Iv
=
2dil
=
2di2
That
is,
a
current
which
is
two
times
larger
than
the
change
in
Ic1
or
Ic2
flows
in
the
load.
This
means
that
the
gain
is
increased
by
6
dB
by
the
current
mirror
circuit,
compared
with
that
obtained
by
a
differential
amplifier
only.
The
load
is
push-pull
driven
by
Q1
and
Q3,
and
distortion
is
minimized.
IDC2
is
well
balanced
with
Ipc
1
in
this
circuit.
These
features
make
this
circuit
relatively
unaffected
by
noise
and
temperature
rise
in
the
power
supply.
In
an
actual
circuit,
the
signal
from
the
playback
head
is
applied
to
the
base
of
the
transistor
Q101
which
corre-
sponds
to
Q
in
Fig.
5.
The
base
potential
of
Q101
is
as
low
as
about
0.11V,
which
is
low
enough
to
protect
the
play-
back
head
from
DC
leakage.
The
latter
stage
of
the
playback
head
amplifier
is
composed
of
a
pure
complementary
push-pull
circuit,
and
is
designed
with
particular
care
taken
to
suppress
distortion
and
provide
wide
dynamic
range.

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Teac C-1 Specifications

General IconGeneral
BrandTeac
ModelC-1
CategoryCassette Player
LanguageEnglish

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