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 impedance
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
AM1200
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
AM1200
amplifier has been specifically 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
AM1200
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" LEDs 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"
LEDs 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, breakers tripping or
premature thermal shutdown.
The fault detection circuit is also thermally
compensated, 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