Playback Amplifier P.c.b; Playback Equalizer Amplifier - Teac C-1 Service Manual

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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.