Protection
of
Compression
Drivers
Whenever
a midrange or high frequency com-
pression
driver is connected directly to a power
amplifier, it is recommended that a capacitor be
wired in
series
with
the driver in order to attenu-
ated unwanted low frequency and switching
transient
signals,
and to block DC components,
any
of which can damage the driver.
This
applies
to most biamplified and triamplified
systems
employing compression drivers. (In conventional
systems
with
passive,
high-level dividing networks,
the network comes between the power amplifier
and
the driver(s) so the protection is already
built-in.)
NOTES:
1.
In all JBL
monitors which
are
designed
for
biamplifi-
cation,
the
dividing networks already incorporate
the
required
attenuation
capacitors
between
the
inputfs)
and
the
compression
driver(s).
2.
Series
capacitors
are not
required
for low
frequency
and
midrange
cone
or
dome type
transducers.
CAUTION:
It is
imperative that
each
output
of the
5234A
electronic
frequency dividing network
be
properly
con-
nected.
Inadvertent exchange
of low
and high frequency
output connections
(at the
5234A,
the
power amplifiers,
or
the
transducers)
may
result
in
severe
damage
to
midrange
or
high frequency loudspeaker
system
components.
If in
doubt, always test
the low
frequency loudspeaker(s) first,
since
the
presence
of
high frequencies there will
be
less
apt
to
cause
damage than
the
converse.
The
low frequency
series
protection capacitors
should be non-polarized electrolytics, either mylar,
metallized polyester or polyester type
with
working
voltages
of at least 50 V. DO NOT USE
POLAR-
IZED ELECTROLYTIC CAPACITORS.
Maximum
capacitance
depends not only on the crossover
frequency used for the driver, but also on the
driver's
impedance, as listed in Table 1. Suitable
10%
tolerance capacitors are available directly
from
a
JBL professional products dealer or JBL directly;
the recommended
values
for specific
cutoff
fre-
quencies,
and their JBL part numbers, are listed
in the Table 2.
Crossover
Frequency
Capacitor Values
(Driver
Impedance)
Crossover
Frequency
sn
4a
250
Hz to 500 Hz
40M
F
80M
F
150MF
500
Hz to
5000
Hz
20M
F
40M
F
80M
F
Above
5000
Hz
2M
F
5u
F
8uF
Table
1: Maximum
Values
for Low
Frequency
Attenuating
Protection
Capacitors
Capacitor
Value
JBL
Part
Number
For
Optimum
Results
at
These
Crossover Frequencies
(Based
on
Driver
Impedance)
Capacitor
Value
JBL
Part
Number
8n
4a
72
MF
52938
275
Hz
550
H/
1100
Hz
52
MF
52939 400
Hz
750
Hz
1
500
Hz
20
MF
53881
1000
Hz
2000
Hz
4000
Hz
16.5MF
1
0358
1200
Hz
2500
Hz
5000
Hz
13.5MF
1035Q'
1500
Hz
3000
Hz
6000
Hz
12
MF
10434
1700
Hz
3500
Hz
7000
Hz
8
MF
10391 2500
Hz
5000
Hz
10000
Hz
7
MF
57529
3000
Hz
6000
Hz
11000
Hz
6
MF
10296 3500
Hz
7000
Hz
13000
Hz
4
MF
41040 5000
Hz
10000
Hz
3
MF
11937
7000
Hz
13000
Hz
Table
2: JBL
Protection
Capacitors
for
Compression Drivers
NOTES:
1.
The
optimum value cited
in
Table
2
assumes
that
the
capacitor
is
active
at
approximately
one
octave
below
the
listed
crossover
frequency,
assuring
minimum acoustical
interference
with
crossover
region performance while
maintaining
a
good
degree
of
protection.
2.
Somewhat
smaller capacitor
values
may be
required
for
additional protection
in
high-power
sound
reinforcement
systems.
3.
To
shunt
the
reactive component
of the
compression
driver's
impedance below horn
cutoff,
a 50 W
resistor
having
a
value
of two to
three times
the
rated impedance
of the
compression
driver should
be
connected
across
the
driver's
terminals.
CAUTION:
Below
the
cutoff frequency
of the
capacitor,
the power amplifier will
be
unterminated.
If the
power
amplifier has
an
output transformer (typical
of
vacuum
tube amps
and
some solid state amps designed
to
drive
constant
voltage
lines),
a 20 W
resistor
equal
to ten
times
the
compression
driver impedance should
be
installed
across
the
amplifier output terminals.
8
To Next Page
Loading...
Need help?
General
