Specifications
16
Two
Ohm or Not
Two
Ohn
6.
Two
Ohm
or
Not
Two
Ohm
NOTE:
Ensure
adequate
ventilation
and monitor
the
FAU
LT
indicators
to
guard
against
thermal
shutdown
when
driving two
ohm
loads.
A Preamble.
The
load that
a
loudspeaker
presents
to
an amplifier
is very
complex
and
at different
frequencies
can
be
inductive,
capacitive,
resistive,
or a
combination
of
these
(reactive).
With the
complex
interaclion
of
these
attributes,
which
alter
from
loudspeaker
to
loudspeaker,
a definitive
load
for
an
amplifier
does
not really
exist.
Loudspeakers
operating
within
an enclosure
are
specified
with
a nominal
impedance.
This
nominal
impedance
is
only
a
rough
guide
to the load
it
presents
to
an amplifier.
As an example,
a loudspeakerwith
a nominal
imped-
ance
of say 8
ohms,
may
have
an impedance
of over
50 ohms
at
resonance (bass
frequencies),
drop to
less
than
6 ohms
after
the resonance
peak
(through
its
mid
band area)
and then
increaseto
over
16ohms
for higher
frequencies.
A 4
ohm load
makes
an
amplifier
work
"harde/'
than
an 8
ohm load
at the
same
voltage,
as double
the
cunent
is required.
Though
various
loudspeakers
may
be
marked with
the same
nominalimpedance,
some
loads
are more
difficult
than
others.
Bass frequencies
usually
exhibit
higher
impedances
and require
higher
voltages
to achieve
the
desired
result.
They
also reflect
higher
energy
back to the
amplifier simply
due
to the
amount
of cone
excursion
involved
at lower
frequencies.
The Mid frequency
band
usually
offers
the lowest
impedances
and
the
highest
duty cycles
requiring
both
high
voltage
and
high
cunent.
The
High
frequency
region
usually
offers
a moderate
impedance
and
usually
does
not
need
much
voltage
but
the instantaneous
current
demand
can
be much
greater
than
you
think.
As
well as this
burden
on
the
amplifier,
the
transient
waveforms
found
in actual
use
can demand
a lot
more
current
than the
"steady-state"
sinewaves
used
in most
amplifier
bench
tests.
The
power
output
of
your
PimlPhzmquoted
on the
specification
sheet is
derived from
a voltage exanrsion
into
a resistive
load
for
a sine wave at
a
given
frequency.
Though
this
method is in line
with the
various
standards
that exist,
it only
gives
an
indication
to
the
maximum
voltage
s,wing
(before
clipping) for
a
given
load.
Th is
method
of rating
power
does not
give
an indication
of the
current
(Ampere)
capability
of the
amplifier,
nor does
it show
the amplifier's
ability
to
sustain
high
energy waveforms.
Your
P4au/P,'hzmamplifier
is designed
to be
able to
deliver
more
than
twice the
cunent
than that
shown
on the
specification
sheet
to cope with
difficult loads
andlor
high
energy
waveforms.
This extra
cunent
reserve
is the
result
of over engi-
neering and
is the
headroom
the
amplifier utilizes
to
control the
loudspeaker
and
dealwith
the
"reactive
energy"
from the
loudspeaker
load that
has to
be
dissipated
within
the
amplifier.
Your Pfu/Ptltzm
amplifier is
able
to drive
2
ohm
loads
or
operate
in
BRIDGE
mode
into
4
ohms. The
operator
must
be aware
that when
driving
2 ohm
loads
or bridged
4
ohm loads
that
the currents
running
in the
output
stage
are
very large
and will
€use
greater
heat
build
up within
the
amplifier than
higher
impedance
loads.
The Front
Panel
FAULT
lndicators
can be
used to
provide
an indication
of the
"difficulty"
of the load
and
will
give
the
operator
an indication
of
the heat build
up in
the
output
stage.
lf the fault
indicators
flash
with
the
"clip"
LED or do
not illuminate
until well
into clippingthen
the load can
be
considered
as normal
or easy.
lf the fault
indicator
starts
to flash
before the "clip"
LED
then the
load
should
be considered
complex
and/or difficult.
For the
more complex
and/or
difficult
loads, the
illumination
of
the'Yault'
LED on
programme
peaks
should
be
interpreted
as
the
output level limit.
Driving
the output
continuously
past
this
point
could
result in
muting
of the
output
stage, fuse's
blowing
or
premature
thermal
shutdown.
Thefault
detection
circuit
is
also thermally compen-
sated, and fault
indication
will occur
earlierwhen
the
unit is hot.
lf the'fault"
LED continually
lights earlier
than normal,
then
the
unit is heating
up.
lf the signal
level
is not
reduced
to compensateforthe
heating
of
the unit
then thermal
shutdorrn
may occur.
/tu