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--
The
rear- panel
NORM/TEST
switch ( essentially
an
artifact
of
the
8100A)
has
been reconfigured to bypass Accessory Port # 1 if desired, maintaining continuity
of
the
signal
path
if
the
Orban
8182A/SG
is
not
connected
to
the
Port.
--
A matrix
has been added to the Output Amplifier Card (# 7) to supply optional
L+R
and L- R sends
to
the stereo generator. ( If
the
Orban 8182A/SG
Stereo
Generator is in use, L+R/L-R sends are mandatory.)
The text below describes the 8182A:
1.a) Input Amplifier: ( on Cards # 3 and #4)
The
audio
is
applied
to
an
RFI
suppression
network
and
pad,
the
latter
strappable for 0 or 20dB attenuation. The RFI-suppressed audio is then applied to
a low- noise
true
instrumentation
amplifier,
whose "+"
and "-"
inputs
are
symmetrical
and high
impedance.
The gain
of
this
amplifier
is
adjustable
from
0.88 to
approximately
10.5 ( a 21.5dB
range).
If
this
range
does
not
yield
the
desired amount of gain reduction, the input pad should be restrapped.
In
order
to
avoid
distortion
due
to
imperfections
in
the
large- value
coupling
capacitors
that
would
be
necessary,
the
input
is
DC- coupled.
Therefore,
only
small
amounts of differential DC should be applied to the input. Ordinarily, the
input is fed by the output of a transformer or capacitively- coupled amplifier, and
no
difficulty
should
arise.
Slight
amounts
of
DC
offset
are
eliminated
in
the
30Hz highpass filter following the input amplifier.
1.b) 30Hz Highpass Filter: ( on Cards # 3 and # 4)
The
output
of
the
input
buffer
is applied to a third- order Chebychev
highpass
filter
with
30Hz
cutoff frequency ( 0.5dB down) and 0.5dB ripple.
This
filter
is
not
conveniently bypassable. It was felt that the advantages of this filter ( i.e.,
elimination
of
modulation- wasting subsonic energy
from
stage rumble and other
sources, elimination of subsonic energy's introducing distortion by modulating the
compressor
control
voltages,
and
prevention
of
destabilization
and/or
distortion
introduction in exciters' AFC's) merited the filter's inclusion as a standard part
of the system.
The cutoff
frequency
of
the filter
is sufficiently low
that
the only commonly-
found
musical
instrument
producing
lower
fundamental
frequencies
is
the
pipe
organ.
Most
records
cut
off
at
30Hz,
and
no
rock-and-roll
instruments
have
fundamentals below 40Hz.
The
ringing introduced by the filter is insignificant. The ear
is very insensitive
to
ringing
in this frequency range. Further, the
ringing
is
comparable
to that
introduced
by
a well- designed
vented
box
loudspeaker
with
30Hz
cutoff.
If
there seems to be an on- air problem with bass response, please don't blame
this filter! First investigate such problems as obviously measurable bass rolloff in
the chain up to OPTIMOD-TV, excess numbers of transformers in the audio chain,
non-linear group delay in phone lines, and rising bass harmonic and IM distortion
at program levels ( which are, in general, at least 14 dB higher than tone level at
"0"
VU).
The
30Hz
highpass
filter
does
not
cause
significant
loss
of
bass
"tightness", and certainly does not introduce " thinness".
A-2
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