AUSTRALIAN MONITOR AM1200 Specifications download pdf (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 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.

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AUSTRALIAN MONITOR AM1200 Specifications Page 16

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