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4
ll
em
Q1y. DescriPlion
I I S1ereophonic Preamplifier Equalizer
Asse
mbl
y
2 2
Knob
3 I Dial Plate
4 I Spacer
s 2 Shafi E
,([
ension 1/1x
l/1
x 1¾
6 2
Bu
shing
7 I Container for
hardware:
2 A scre
w,
llat head #8-32 x I
1/4
long
2 B setscrew, Allen #8-32 x
:~
fo
l
ong
2 C
nut,
hex,
1/s-32
2 D lockwasher, %
I E screw, llnt head, ;/tI0-32 x 11/.t long
2 F screw, hex head, #8-32 x
1/s
long
Circuits
The
BA-36A Preamplifier Equa
li
zer consists
of
rwo
four stage feedback amplifiers,
PW
l
and
PW
2,
and
a self-contained power supply,
PW
3, which are
mounted inside the chassis,
and
output
transformers,
Tl
and
T2
, power transformer,
T3
,
and
two switches,
Sl
and S2, which are mounted
at
the ends
of
th
e chassis.
Either
MONO
or
STEREO mode
of
operation
is
selected
by
sw
it
ch
SI
which is connected to the in
put
terminals of the amplifiers,
PWl
and
PW
2. For
monophonic mode
of
operation the
input
s of
PW
I
and
PW
2 are paralleled;
for
stereophonic operation,
each channel is fed
to
its respective amplifier.
Tbe
amplifiers
PWI
and
PW
2 arc identical a
nd
the fo
ll
ow-
ing description applies to either.
Resistor R
1,
together
with
the
input
impedance
of
Q
l,
provides correct pickup termination.
Tran
sistor
+20
QI
, a common emitter amplifier, is
RC
coupled
to
Q
2.
The
input
impedance of
Q2
is low so
that
most
of
the
signal current from the collector
of
Q l flows into
the
base of Q2.
The
signal
at
th
e co
lle
ct
or
of
Q2 is
coupled
to
the emitter
of
Ql through the feedback
networks C5, C7,
and
Rl
2
or
CG
and
R13.
The
first
network provides
NAB/R
IAA
response; the second,
provides flat response.
The
1000 cps gain is the same
for
either network. Either may be selected by a
strapping change
on
the amplifier board.
The
collector
of
Q2 is direct-coupled through re-
sistor
Rl4
to
the base
of
Q3,
an
emitter follower
stage. Resistor R14 provides a convenient method
for
varying the high frequen
cy
r
es
po
ns
e in conjunction
with capacitors
CL
a
nd
C2.
The
impedance of this
response compensation network
is
high compared
to
the
output
impedance of Q2 so chat the same degrees
of rolloff arc obtained wi
th
either the
NA
B/RIA
A
or
fl
at
response netw
or
k
s.
Bleeder resistors are
pr
o-
vi
ded
on S2
to
suppress sw
it
ch clicks.
The
GA
IN control
and
o
utput
stage Q4 arc iso-
lat
ed
fr
om the equalizing and compens
at
i
on
stages
Q1 and Q2,
by
Q3 which
is
an emitter follower.
The
gain control
RI6
pr
ovides a range
of
gain ad-
justment to obtain the required
output
le
vel with
pick
up
s of widely varying o
utput
levels. Sufficient
range
of
adjustment is
pro
vi
ded
for
nearly all pickups
presently ava
il
able.
The
dynamic range
of
stages Q1
to Q3
is
such chat
di
st
ort
ion does n
ot
increase
at
minimum gain setting
and
the
noise level
at
max-
imum serting is still below the noise generated by
the pickup when r
ep
roducing a record.
The
gain
control is isolated by two capacitors,
CB
and ClO to
I I I I
111
1 I I I
--
t--,..
r---
+1
0
ID
0
z
~
0
z
0
0.
(/)
w
a:
-10
-
20
20
'r-. EQUALIZER
SW
ITCH
S2
"'
i-,..
AT
NORM
POSITION
.............
1\14s
~
~
4s,._
~4Pp
r---!,ltl
c
FLAT
STRAPPING ~
1---...__
---........
i'--......
..........
100 1000
FREQUENCY
IN
CYCLES PER
SECOND
Figure
2-F
re
qu
ency
Res
pon
se
r--......
r-,...,,
~I'
~
~
10000
20000
8
026268
t
