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TABLE 3
Transformer
voltage step-up ratios
Terminals
OUTPUT
Voltage
Impedance
Output impe-
used
as
IMPEDANCE
step-up
step-up
dance
if source
inlet
switch position
ratio
ratio
is
GOO
n
BAL
BAL
GOO
Q
1:1
1:1
GOO
Q
BAL
BAL
150
Q
1:2
1:4
2.4
kQ
BAL
BAL
75
Q
1:2.8
1:8
4.8
kQ
One BAL and
BAL
600
Q
1:2
1:4
2.4
kQ
CENTRE TAP
One BAL and
BAL
150
Q
1:4
1:16
9.6
kQ
CENTRE TAP
One BAL and
BAL
75
Q
1:5.7
1:32
19.
2
kQ
CENTRE TAP
USING THE
OUTPUT
MATCHING
TRANSFORMER TO OBTAIN A VOLTAGE
STEP-UP
54.
The transformer
in
the
attenuator
can
be employed as a
voltage
step-up device at
audio
frequencies
by
applying
a
signal
to the BAL terminals normally
connected
to a
load,
and by
taking output
from
the
front
or rear
INPUT
socket normally
used
as an
inlet
.
One BALter-
minal, and the CENTRE
TAP
terminal
can
also
be used
for
applying an input.
The nominal
voltage
ratios obtainable are
the
square roots of
the
impedance
ratios (Table
3)
.
55.
In most cases, it is necessary to set
all
the
attenu~tion
dials to
0
in
order that the
attenua-
tion circuits should not
load
the output obtained
in this
manner, but the attenuation controls
can
be turned to other positions to reduce and
adjust the
output
if required.
56.
The
input
voltmeter of the attenuator can
be
used
to
monitor
the output, but its shunt impe-
dance
(approximately
30
kQ)
is
significant
in comparison with some of
the
va lues
of output impe-
dance
which are obtained by this inverted use of the transformer, and consequently
it
may
lower
the
voltage a
nd
effective
output
impedance obtained.
To disconnect
the
voltmeter circuit, turn
the
METER
RANGE
switch to OFF.
57.
The
transformer
is
not
particularly designed
to
operate in this
fashion,
and in doing
so
it
has a relatively poor frequency response, which
is
worst at the highest ratios.
However,
the
voltmeter, used
to monitor the output, facilitates
making adjustments
to compensate for varia-
tion
in the output.
The transformer also causes distortion to
become serious
at
low
freque
n-
cies
(below
400
Hz)
when high step-up ratios are
used.
Chap. 3
Page
1-!
Nov.
79
t
1
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