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CIRCUIT
DESCRIPTION
Design
Philosophy on
430A
and
410A
When
using a
high
gain
wide band
open
loop
operational
amplifiers,
it is
possible to design a
passive
network
providing
a
negative feedback
to control
the
amplifier gain
and
frequency
response.
In
fact
it
makes
easy the
reproduction
and
repititivity
of the
wanted
results.
We
can define
the
gain
of such an
amplifier
as
follows.
Zm
o
'VW-
Vln
O
r
OP
AMP
3
PHONO
IN
R
CH
C4
[PHONO AMPL
tFIER
USING OP AMP]
1)
Vo
=
-Vin-
2)
Rin =
Zf
Zin
(Inverting
Amp.)
OOUT
3)
Vo
= Vin
•
1
+
Zin c
—
R
in
^
Zf
Zl
Zino
•
Gopenloop
G
closedloop
(Non
Inverting)
Where
Ri n
is
the input load
resistor.
Z
m
is
the
open
loop
input
impedance
multiplied
by open
loop
gain divided
by
closed
loop
gain.
Example:
If
open
loop
gain
is
10,000
(i.e)
80dB) and
the
closed
loop
gain
is
100
(i.e
40dB).
For
an
amplifier
having Zin
=
10K
ohm,
the
equivalent
Zin
c
=
Rin
//
1000K
ohm.
Phono
Equalizer:
The phono non
inverting amplifier
equalizer consists
of an
operational amplifier
with
a
feedback
network
to
fullfil
the
RIAA
equalizing requirements.
Zf
=
R8
//
X
C
6
+
RlO
» *c8
Zl
=Ri2
+
Xci2
1
1
where
X
c
=
2TTf-C
6.28
f-c
f
= frequency
c
=
capacitance
in
Farads
R
=
Resistance
in
Ohms
The
above
network
provides
a gain
of
60
at
1kHz
and
a gain
variation
as
function
to
the value of Zf
and Z\
at
any
given
frequency.
For example
at
100Hz
the gain
is
265
or
+12.9dB
referred
to
1kHz
gain, at
10kHz
gain
is
about 13
or -l3.7dB
referred to
1kHz
gain.
Premain
Amplifier
The
premain
amplifier
is
an
operational amplifier
built
by
discrete
components.
It
is
directly
coupled
to the
load
(i.e
speakers).
The
high
gain
open
loop
is
provided
by
the dual
differential
amplifiers
and
the
boostrap
capacitor.
The
complementary
output
drivers/buffers
provide
a
symmetrical
output
drive.
The
amplifier
is
controlled
by
multiple
feedback networks:
DC
feedback (R93 +
R95) on
one channel
(R94
+
R96)
on
the other
are directly
coupled
between output and
negative
input.
The
effect
of
the
DC
feedback
is
for
longterm
stability
and
unity
DC
gain.
The
effect
of
this
feedback
is
minor
at
frequencies over
5Hz.
The
multipole
AC
feedback
network
(with the bass
and
treble
potentiometers
incorporated) provides
a
constant gain with
no
effective
gain variations of
tone potentiometers.
At 100Hz
the bass
potentiometer
allows gain control of
±10dB.
At
10,000Hz
the
treble
potentiometer
allows gain control of
±!OdB.
The
amplifier has a current
limit
network
that
limits
excessive current loading.
A
fuse
is
provided
to
prevent
damage
to
speakers
if
the amplifier
fails.
Tone
Control
The
tone
control
is
a negative
feedback type
whicli
uses the
power
amplifier stage
as
the
active
element.
That
is,
the
gain of the
power
amplifier stage
is
controlled
by
the tone
controls
circuitry.
At
1kHz,
the position
of
the
tone
controls has
little
effect
on
the
gain, as
C5
3
impedance
is
high,
removing
VR2
from
the
circuit,
and CI,
C3
impedance
is
low,
effectively
short
circuiting
VR1.
Bass Control:
As
the
frequency
decreases
below
1
kHz,
the
impedance
of
CI and C3
increases
proportionately.
Thus
at
very
low
frequencies,
the gain
is
mainly determined
by
the
position of the bass control
VR1.
Rotating
VR1
toward
R3
will
boost
the
low
frequencies,
while turning
it
towards
C4
will
cut the
bass.
Treble Control:
At
high frequencies,
as at
1kHz,
\/Rl
is
effectively
short
circuited.
At
these frequencies,
\
owever
C53 and C55
impdeance
decreases, so
that
VR2
Is
comes
the
the
main
control of the amplifier
gain.
Rotating
VR2
towards
C53
cuts
the
treble
response.
5
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