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Kurzweil K2000 - K2000 Audio;Power Board Theory - Part 7; Muting, Switching Matrix, and Mix Output Amplifiers Theory

Kurzweil K2000
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SECTION 5
AUDPWRJ.DOC
12122/92
5.1.3.5
MUTING
CIRCUITS
During
power-up
and
power-down,
muting
circuits
on
all
audio
outputs
are
asserted
to
prevent
transient
noises
and
damage
to
speakers
and
human
ears.
The
muting
circuit
description
provided
here
applies
to
all
other
outputs
as
well.
Using
the
Qll
circuitry
as
an
example
(I-I),
this
JI09
N-channel
JFET
is
either
ON
or
OFF.
When
OFF,
it
is
not
conducting,
thus
any
audio
present
passes
through
R163
and
out
Jll.
When
ON
this
JFET
shorts
the
output
line
to
ground,
thus
muting
the
circuit.
To
control
this
function
take
note
-UNMUTE
arriving
in
the
upper
left
corner
of
the
schematic
(A-I),
being
delivered
from
the
collector
of
Q23
(H-7)
on
schematic
page
2.
This
signal,
when
high,
cuts
off
Q11,
and
all
other
JFETs
so
connected
and
allows
the
passage
of
audio.
When
low,
the
JFETS
are
ON
and
the
mutes
are
asserted.
5.1.3.6
AUDIO SWITCHING
MATRIX
The
scanner
microprocessor
has
direct,
individual
control
over
the
6
analog
switches
and
pairwise
control
of
the
4
variable
gain
amplifiers.
Five
of
the
analog
switches
(ASW
1,
ASW
3 -
ASW
6)
are
in
audio
signal
paths
while
the
sixth
is
in
a
gain
control
signal
path
(AsW_2).-
-
Note:
ASW
1
and
ASW
2
audio
switching
controls
are
drawn
on
schematic
page
4.
Refer
to
fIgure
5.1.3.1
below
for
a
simplified
detail
of
the
Audio
Switching
Matrix.
""
...
---~------l
fA"'"
.
~
~l
.......
_..J...l!t..
__
-+
___
...oj.
(
......
,
(:'.::rl-----------..;...---''----'
Figure
5.1.3.1
-
Audio
switching
matrix
In
all
of
the
audio
path
switching
cases,
the
switch
is
connected
to
the
summing
junction
of
a n
inverting
op-amp
with
the
input
resistor
at
the
input
of
the
switch.
Signal
current
through
the
input
resistor
either
enters
the
summing
junction
when
the
switch
is
on
or
is
diverted
to
ground
when
the
switch
is
off.
This
makes
each
switch
a
current-mode
switch
which
allows
it
to
block
large
voltages
when
off
and
avoid
large
voltage
swings
when
on.
This
minimizes
distortion
of
the
signal
and
crosstalk.
It
also
allows
the
switch
to
handle
signal
voltages
greater
than
its
own
power
voltage.
A
simple
detail
of
the
Analog
Switch
follows
in
figure
5.1.3.2.
'NPVT
CONTR~L
ll"'AlO~
SWITCH
15K
1---
---,
IS'K
:
:NO
~
IN: I .NC
••
____
,~I-J
,
,------'
>---
.. OVTPVT
•
Figure
5.1.3.2
-
Analog
switch
connection
for
audio
signals
Note
that
the
IN
and
NC
terminals
are
connected
when
the
control
voltage
is
low
(switch
off).
IN
and
NO
are
connected
when
the
control
voltage
is
high
(switch
on).
5.1.3.7
MIX
OUTPUT
AMPLIFIERS
The
two
sections
of
U1
(C-6
to
C-7)
do
the
final
mixing
of
audio
signals
from
the
switching
matrix
and
Effects
Unit.
Each
sums
5
signals
from
various
sources
into
one.
A
low
noise
amplifier
is
used
here
because
the
"noise
gain"
of
the
circuit
is
5
even
though
the
effective
gain
for
anyone
signal
is
unity.
The
100pF
capacitors
across
the
feed
back
resistors
compensate
for
stray
capacitance
at
the
large
summing
junction
and
also
the
finite
load
capacitance
presented
by
the
cable
to
the
volume
slider.
The
signal
level
going
into
the
volume
slider
is
6
volts
peak
if
one
of
the
mixed
sources
should
reach
peaks
near
6
volts
simultaneously.
8

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