Australian Monitor KA1500 - Manual (Page 16)

6. Two Ohm or
Not Two Ohm
NOTE: Ensure adequate ventilation and monitor the
FAULT 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 interaction 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 loudspeaker with 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 increase to over 16 ohms
for higher frequencies.
A 4 ohm load makes an amplifier work "harder" than
an 8 ohm load at the same voltage, as double the
current is required.
Though various loudspeakers may be marked with
the same nominal impedance, 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 current.
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 KA1500 KA1500 KA1500 KA1500 KA1500 amplifier quoted on
the specification sheet is derived from a voltage
excursion 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 swing (before
clipping) for a given load. This 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 KA1500 KA1500 KA1500 KA1500 KA1500 amplifier is designed to be able to deliver
more than twice the current than that shown on the
specification sheet to cope with difficult loads and/or
high energy waveforms.
This extra current reserve is the result of over
engineering and is the headroom the amplifier utilizes
to control the loudspeaker and deal with the “reactive
energy” from the loudspeaker load that has to be
dissipated within the amplifier.
Your KA1500 KA1500 KA1500 KA1500 KA1500 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 cause greater
heat build up within the amplifier than higher
impedance loads.
The Front Panel FAULT Indicators 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.
If the fault indicators flash with the "clip" LED or do
not illuminate until well into clipping then the load can
be considered as normal or easy.
If 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 "fault" 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.
The fault detection circuit is also thermally compen-
sated, and fault indication will occur earlier when the
unit is hot. If the "fault" LED continually lights earlier
than normal, then the unit is heating up. If the signal
level is not reduced to compensate for the heating of
the unit then thermal shutdown may occur.
16 Two Ohm or Not Two Ohm