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APPLICATION
NOTE
NO.
2
WARNING
POWER
AMPLIFIER
PROTECTION
SYSTEMS
PRO
·
TECT
AMPLIFIERS
BUT
NOT
SPEAKERS!
YOU
,
AND/OR
YOUR
SPEAKER
MANUFACTURER
ARE
RESPONSIBLE
FOR
SPEAKER
PROTECTION
!
Except in unusual sets
of
circumstances, the cir-
cuitry included in power amplifiers designated
as
"protection", whether made
up
of
transistor limiters,
circuit
breakers, fuses,
SCR's,
or
whatever, is design-
ed primarily
to
protect the amplifier from damage
and will only protect speakers incidentally. There are
some exceptions
to
this rule, which allow the user
to
vary the output power I
im
its
of
his amplifier
to
match
the power rating
of
his speakers.
Even
this, however,
is
a far from perfect speaker protection scheme. The
reason why
this
holds true is simple: each make
or
model speaker has its own unique operating
capabilities.
As
a rule, a speaker system (including
all electronic parts such as crossover parts and
electrostatic element transformers, as well as voice
coils) will be able
to
dissipate a certain amount
of
power continuously without burning
up
. This de-
pends on the size, quality, configuration, etc
.,
of
those parts. The same speaker will be able
to
handle
a somewhat higher power level for a short period
of
time
(in the millisecond range) without being
destroyed by heat,
but
may then be endangered by
such phenomena as extending moving elements
beyond their normal range
of
travel (bottoming, torn
diaphragm, etc.), overvoltaging electrostatic panels
(arcing),
or
other such suddenly disabling events.
Since the points
at
which these disastrous
happenings
will
occur,
differ
for different speakers, a
speaker protection system
must
be completely ad-
justable
if
it
is
to
be useful for more than a narrow
range
of
speakers. Furthermore,
if
the user is
to
be
able to do adequate adjustment on a protection sys-
tem external
to
his speakers, he
must
receive ac-
curate information about the speaker power han-
dling capabilities from the speaker manufacturer.
Speaker protection systems embodied in
amp
lifiers
are comparatively useless, therefore, unless the
following things are true:
(1) The protection system can be accurately ad-
justed by the user.
(2) The power handling capabilities
of
the speak-
ers are clearly stated by the manufacturer.
(3) The information about the speaker protection
and speaker power handling are stated in
common terms
so
that the user can adjust the
protection properly for the speaker.
These three things are rarely true simultaneously.
It
is
for this reason
that
CROWN
takes the approach
that speaker protection is the responsibility
of
the
speaker manufacturer. Amplifier manufacturers
have long been required
to
provide protection in their
products for any faul
ts
occurring externally
to
the
amplifier outputs, such as speaker short circuits,
open circuits, etc.
In
order
to
fairly share the respon-
sibility, and since the protection cannot
be
ex-
haustively provided any other way,
we
feel that
speaker manufacturers should provide protection
tailored to
their
individual products. At
CROWN
we
have done this with our own speakers, simply be-
cause
it
is the only comprehensive means
of
pro-
viding worthwhile protection for all
of
our speakers.
Increasing power available from modern amplifiers
increases the danger
of
speaker damage. Although
occasionally an internal amplifier malfunction can
contribute
to
speaker failure,
it
is much more likely
that
speakers wil I be overpowered by inadvertent use
of
too
much
power. In the absence
of
internal protec-
tion in most speaker systems,
CROWN
re
commends
very strongly
that
in-line speaker fuses be employed
in all systems. The fuse selection nomograph re-
printed in each
CROWN
amplifier manual can be an
invaluable help in choosing fuses
of
the proper size.
However, the effectiveness
of
fuses in protecting
speakers is limited in two important
ways
:
(1)
Fuses
as a rule protect only against prolonged
overpowering, and can only prevent speakers
from being driven with more than their
RMS
or
average power rating.
Fuses
cannot protect
against sudden high level transients
of
short
duration. The use
of
high
-s
peed instrument
fuses will give the best protection available
from fuses,
but
musical transients can have
an effect before the fastest fuses blow.
(2)
An
appropriate fuse can only be selected
if
the
manufacturer's specifications for his speaker
are accurately and clearly stated.
If
a speaker
can handle 100 watt peaks and
30
watts con-
tinuously,
but
is advertised, and therefore
fused, as a 100 watt speaker, then the speaker
wil I destroy itself before the fuse can protect
it
under a continuous power level above
30
watts.
25
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